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Chemical profiling of explosives
Brust GMH
Link to publication
Citation for published version (APA)Brust G M H (2014) Chemical profiling of explosives
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Download date 13 Mar 2020
Chapter 3Pentaerythritol tetranitrate (PETN) profiling in post-explosion residues
to constitute evidence of crime-scene presence
This chapter has been published as
H Brust A van Asten M Koeberg A van der Heijden CJ Kuijpers
P Schoenmakers
Forensic Sci Int 230 (2013) 37-45
38 Chapter 3
Cha
pter
3
Abstract
Pentaerythritol tetranitrate (PETN) and its degradation products are analyzed to
discriminate between residues originating from PETN explosions and residues
obtained under other circumstances such as natural degradation on textile or after
handling intact PETN The degradation products observed in post-explosion samples
were identified using liquid chromatographyndashmass spectrometry as the less-nitrated
analogues of PETN pentaerythritol trinitrate (PETriN) pentaerythritol dinitrate
(PEDiN) and pentaerythritol mononitrate (PEMN) Significant levels of these
degradation products were observed in post-explosion samples whereas only very low
levels were detected in a variety of intact PETN samples and naturally degraded PETN
Based on the peak areas of PETriN PEDiN and PEMN relative to PETN it was possible
to fully distinguish the post-explosion profiles from the profiles obtained from intact
PETN or after (accelerated) natural degradation Although more data are required to
accurately assess the strength of the evidence this work illustrates that PETN profiling
may yield valuable evidence when investigating a possible link between a suspect and
post-explosion PETN found on a crime scene
PETN profiling in post-explosion residues 39
Chapter 3
31 Introduction
Recently a series of safe crackings occurred in the Netherlands in which the explosive
PETN (pentaerythritol tetranitrate) was used A suspect was apprehended and his
clothing was sampled and analyzed by the NFI (Netherlands Forensic Institute)
Analysis by liquid chromatographyndashmass spectrometry (LCndashMS) led to the detection
of PETN but also relatively high amounts of its degradation products pentaerythritol
trinitrate (PETriN) the dinitrate (PEDiN) and the mononitrate (PEMN) This triggered
the question whether the identification of these degradation products could be explained
by the suspectrsquos presence at the crime scene The suspect stated that the residues
found on his clothing had originated from handling intact PETN which was also
found at his home The degradation products could then be explained by the presence
of impurities in the material or by natural degradation of the intact PETN as such or
on the clothing In this context differences between PETN chemical profiles obtained
by natural degradation and explosion were studied This chapter reports on the study
that was undertaken to investigate whether post-explosion PETN residue profiles can
be differentiated from PETN profiles arising from other processes such as natural
degradation or during synthesis
PETN is a powerful high explosive prepared by nitration of pentaerythritol (PE) It
was brought into use during World War II after formaldehyde and acetaldehyde
(precursors of PE) had become industrially available [1] PETN is relatively stable ndash
both chemically and physically ndash in comparison with other nitrate-ester explosives
such as ethylene glycol dinitrate (EGDN) and nitroglycerin (NG) [23] This has been
attributed to the symmetrical molecular structure of PETN [1] Major applications of
PETN as an explosive are as main charge in detonation cords and blasting caps [12] and
in formulations [4-6] (eg Semtex pentolite PEP 500)
In addition to its use as an explosive PETN also acts as a coronary vasodilator It is used
as the active ingredient in heart medicines for the treatment of angina pectoris [27] The
40 Chapter 3
Cha
pter
3
amounts of PETN used for medical purposes are very small compared to the amounts of
material required to cause explosions
311 Decomposition of PETNTraditional explosive-residues analysis only considers identification of the explosive used
and therefore limited information is available on the presence of degradation products
of PETN in post-explosion samples Moreover levels of explosives and degradation
products in post-explosion residues tend to be low making their detection challenging
Although detonation of PETN mainly results in the formation of gaseous products [1]
solid decomposition products are usually also formed because of incomplete detonation
Thin-layer chromatographic (TLC) analysis of post-explosion extracts showed
additional spots apart from PETN which were later identified by chemical-ionization
mass spectrometry (CIndashMS) and nuclear-magnetic-resonance (NMR) spectroscopy as
pentaerythritol trinitrate (PETriN) and pentaerythritol dinitrate (PEDiN) Another spot
could not be identified although it was suggested that this spot could be attributed to
pentaerythritol mononitrate (PEMN) [8] Fig 31 shows the chemical structure of PETN
and its less-nitrated analogues
O
O
O O
NOO
NO
O
N
O
O
NOO
O2NO
ONO2
O2NO OH
HO
ONO2
O2NO OH
HO
ONO2
HO OH
PETN PETriN
PEDiN PEMN
Fig 31 Chemical structure of PETN and its degradation products PETriN PEDiN and PEMN
More literature is available on chemical and environmental degradation of PETN The
degradation of PETN is influenced by a variety of parameters such as temperature the
PETN profiling in post-explosion residues 41
Chapter 3
presence of microorganisms and humidity There is consensus in literature that the first
and rate-determining step in the decomposition of PETN is the scission of the O-NO2
bond resulting in the release of nitrogen dioxide (NO2) [2-4910] This was observed for
nitrate esters in general [9] Several mechanisms for the following decomposition steps
have been postulated depending on the physical and chemical environment
PETN is stable compared to other organic explosives [4] and therefore the majority of
research into thermal decomposition of PETN has involved elevated temperatures (ie
above 100degC) However decomposition mechanisms are different at higher temperatures
than under ambient conditions [241112] Thus the results of accelerated-degradation
studies may not accurately reflect the natural degradation occurring on for instance the
clothing of a suspect However previous research on high-temperature decomposition
of PETN yields useful information on the identity of degradation products formed
As no condensed-phase decomposition products were detected after analysis of
naturally aged PETN at ambient temperatures it was suggested that at low temperatures
only gaseous decomposition products are formed [2] Decomposition of PETN at 53degC
was studied by monitoring the released NOx (mainly NO2) using a chemiluminescence
analyzer [12] PETN was found to have an NOx evolution rate that was roughly 1000
times lower than the evolution rate for nitrocellulose By extrapolating the NOx-emission
data the half-life time of PETN was estimated to be 12 million years confirming
its stability [12] Chambers et al [4] reported on the possible formation of peroxide
[(O2NOCH2)3C-CH2OO] nitrate [(O2NOCH2)3C-NO2] and aldehyde [(O2NOCH2)3C-
CHO] products at ambient temperatures but the presence of these products has not
been experimentally confirmed It was also suggested that the alkoxy radical formed by
scission of the O-NO2 bond could attack PETN resulting in polymer-like side products
such as dipentaerythritol hexanitrate (DiPEHN) and tripentaerythritol octanitrate
(TriPEON) [24]
Thermal ageing studies of PETN at 80degC [1] and 100degC [2] did not show significant
degradation but continued heating to temperatures above the melting point of PETN
42 Chapter 3
Cha
pter
3
(1413degC) resulted in gradual decomposition [1] Decomposition of PETN at higher
temperatures yielded a greater variety of ndash primarily gaseous ndash decomposition products
resulting from further breakdown of PETN [2-410] It was also reported that the
second step in the decomposition of PETN (after cleavage of the O-NO2 bond) is the
loss of a formaldehyde molecule [39-11] Infrared analysis of the residual material
after degradation of PETN in benzene at 185degC resulted in the identification of a
polyketo oxetane [9] This led to a proposed decomposition mechanism involving cyclic
intermediates Shepodd et al [11] identified several decomposition products (including
PETriN PEDiN and DiPEHN) using LCndashMS and capillary electrochromatographyndash
mass spectrometry (CECndashMS) after heating PETN under vacuum at temperatures up
to 135degC PETriN formation during high-temperature decomposition of PETN was also
suggested by Makashir and Kurian [3]
312 Other factors influencing PETN decompositionSeveral environmental factors have been found to accelerate PETN decomposition or to
result in different decomposition pathways These include the presence of water soil or
microorganisms
The presence of water has a detrimental effect on the stability of PETN Moisture results
in sequential hydrolysis of the O-NO2 bonds resulting in hydroxyl end groups [2413]
Several studies showed the formation of PETriN PEDiN and PEMN [1413] Hydrolysis
proceeded more rapidly under acidic or basic conditions [111]
Microbial degradation of PETN also resulted in the formation of PETriN PEDiN
and PEMN [1415] This behavior was also observed for other nitrate esters such as
nitroglycerin EGDN and nitrocellulose [1516] and it was suggested that biodegradation
of nitrate esters generally follows a hydrolytic pathway [17] Binks et al [14] isolated
a microbial culture (Enterobacter cloacae PB2) from explosive-contaminated soil
Several metabolites of PETN were detected including PEDiN The enzyme PETN
reductase was also isolated from the culture showing conversion of PETN to PETriN
and PEDiN In another study PETN was buried in soil and after 20 years 90 of the
PETN profiling in post-explosion residues 43
Chapter 3
PETN was found to be remaining From these results the half-life time of PETN in soil
was estimated to be 92 years [18]
Although PETN is relatively resistant to chemical reagents [1] several compounds can
accelerate its decomposition such as carbamite (13-diethyl-13-diphenylurea) calcium
carbonate magnesium oxide [3] ferrous chloride [1] and granular iron [5] When
analyzing degraded PETN it should be considered that some of the proposed degradation
products may also have originated as side products during synthesis Yasuda [19] used
TLC to identify PETriN DiPEHN and TriPEON in PETN samples Other commonly
encountered impurities are pentaerythritol (PE) PEMN and PEDiN [4]
313 Case assessmentIn the present study the possibility to discriminate between PETN degradation during
explosion and other scenarios is investigated This is important in assessing the evidential
value of an observed PETN chemical profile in cases as the example described above
The probability of the evidence should then be considered under different hypotheses
that may be postulated by the prosecution (Hp) or the defense (Hd) in line with the
Bayesian framework for evidence interpretation [2021] To discriminate between post-
explosion samples and other scenarios the following hypotheses were formulated
Hp The observed PETN degradation products on the suspectrsquos clothing originate from
a PETN explosion
Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as
impurities in the intact PETN handled by the suspect
Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed
by chemical and environmental degradation of PETN
To determine the specificity of post-explosion PETN profiles it should be investigated
whether similar profiles can be generated by other processes than PETN detonation
In this study PETN-detonation experiments were conducted Samples were taken and
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
Chapter 3Pentaerythritol tetranitrate (PETN) profiling in post-explosion residues
to constitute evidence of crime-scene presence
This chapter has been published as
H Brust A van Asten M Koeberg A van der Heijden CJ Kuijpers
P Schoenmakers
Forensic Sci Int 230 (2013) 37-45
38 Chapter 3
Cha
pter
3
Abstract
Pentaerythritol tetranitrate (PETN) and its degradation products are analyzed to
discriminate between residues originating from PETN explosions and residues
obtained under other circumstances such as natural degradation on textile or after
handling intact PETN The degradation products observed in post-explosion samples
were identified using liquid chromatographyndashmass spectrometry as the less-nitrated
analogues of PETN pentaerythritol trinitrate (PETriN) pentaerythritol dinitrate
(PEDiN) and pentaerythritol mononitrate (PEMN) Significant levels of these
degradation products were observed in post-explosion samples whereas only very low
levels were detected in a variety of intact PETN samples and naturally degraded PETN
Based on the peak areas of PETriN PEDiN and PEMN relative to PETN it was possible
to fully distinguish the post-explosion profiles from the profiles obtained from intact
PETN or after (accelerated) natural degradation Although more data are required to
accurately assess the strength of the evidence this work illustrates that PETN profiling
may yield valuable evidence when investigating a possible link between a suspect and
post-explosion PETN found on a crime scene
PETN profiling in post-explosion residues 39
Chapter 3
31 Introduction
Recently a series of safe crackings occurred in the Netherlands in which the explosive
PETN (pentaerythritol tetranitrate) was used A suspect was apprehended and his
clothing was sampled and analyzed by the NFI (Netherlands Forensic Institute)
Analysis by liquid chromatographyndashmass spectrometry (LCndashMS) led to the detection
of PETN but also relatively high amounts of its degradation products pentaerythritol
trinitrate (PETriN) the dinitrate (PEDiN) and the mononitrate (PEMN) This triggered
the question whether the identification of these degradation products could be explained
by the suspectrsquos presence at the crime scene The suspect stated that the residues
found on his clothing had originated from handling intact PETN which was also
found at his home The degradation products could then be explained by the presence
of impurities in the material or by natural degradation of the intact PETN as such or
on the clothing In this context differences between PETN chemical profiles obtained
by natural degradation and explosion were studied This chapter reports on the study
that was undertaken to investigate whether post-explosion PETN residue profiles can
be differentiated from PETN profiles arising from other processes such as natural
degradation or during synthesis
PETN is a powerful high explosive prepared by nitration of pentaerythritol (PE) It
was brought into use during World War II after formaldehyde and acetaldehyde
(precursors of PE) had become industrially available [1] PETN is relatively stable ndash
both chemically and physically ndash in comparison with other nitrate-ester explosives
such as ethylene glycol dinitrate (EGDN) and nitroglycerin (NG) [23] This has been
attributed to the symmetrical molecular structure of PETN [1] Major applications of
PETN as an explosive are as main charge in detonation cords and blasting caps [12] and
in formulations [4-6] (eg Semtex pentolite PEP 500)
In addition to its use as an explosive PETN also acts as a coronary vasodilator It is used
as the active ingredient in heart medicines for the treatment of angina pectoris [27] The
40 Chapter 3
Cha
pter
3
amounts of PETN used for medical purposes are very small compared to the amounts of
material required to cause explosions
311 Decomposition of PETNTraditional explosive-residues analysis only considers identification of the explosive used
and therefore limited information is available on the presence of degradation products
of PETN in post-explosion samples Moreover levels of explosives and degradation
products in post-explosion residues tend to be low making their detection challenging
Although detonation of PETN mainly results in the formation of gaseous products [1]
solid decomposition products are usually also formed because of incomplete detonation
Thin-layer chromatographic (TLC) analysis of post-explosion extracts showed
additional spots apart from PETN which were later identified by chemical-ionization
mass spectrometry (CIndashMS) and nuclear-magnetic-resonance (NMR) spectroscopy as
pentaerythritol trinitrate (PETriN) and pentaerythritol dinitrate (PEDiN) Another spot
could not be identified although it was suggested that this spot could be attributed to
pentaerythritol mononitrate (PEMN) [8] Fig 31 shows the chemical structure of PETN
and its less-nitrated analogues
O
O
O O
NOO
NO
O
N
O
O
NOO
O2NO
ONO2
O2NO OH
HO
ONO2
O2NO OH
HO
ONO2
HO OH
PETN PETriN
PEDiN PEMN
Fig 31 Chemical structure of PETN and its degradation products PETriN PEDiN and PEMN
More literature is available on chemical and environmental degradation of PETN The
degradation of PETN is influenced by a variety of parameters such as temperature the
PETN profiling in post-explosion residues 41
Chapter 3
presence of microorganisms and humidity There is consensus in literature that the first
and rate-determining step in the decomposition of PETN is the scission of the O-NO2
bond resulting in the release of nitrogen dioxide (NO2) [2-4910] This was observed for
nitrate esters in general [9] Several mechanisms for the following decomposition steps
have been postulated depending on the physical and chemical environment
PETN is stable compared to other organic explosives [4] and therefore the majority of
research into thermal decomposition of PETN has involved elevated temperatures (ie
above 100degC) However decomposition mechanisms are different at higher temperatures
than under ambient conditions [241112] Thus the results of accelerated-degradation
studies may not accurately reflect the natural degradation occurring on for instance the
clothing of a suspect However previous research on high-temperature decomposition
of PETN yields useful information on the identity of degradation products formed
As no condensed-phase decomposition products were detected after analysis of
naturally aged PETN at ambient temperatures it was suggested that at low temperatures
only gaseous decomposition products are formed [2] Decomposition of PETN at 53degC
was studied by monitoring the released NOx (mainly NO2) using a chemiluminescence
analyzer [12] PETN was found to have an NOx evolution rate that was roughly 1000
times lower than the evolution rate for nitrocellulose By extrapolating the NOx-emission
data the half-life time of PETN was estimated to be 12 million years confirming
its stability [12] Chambers et al [4] reported on the possible formation of peroxide
[(O2NOCH2)3C-CH2OO] nitrate [(O2NOCH2)3C-NO2] and aldehyde [(O2NOCH2)3C-
CHO] products at ambient temperatures but the presence of these products has not
been experimentally confirmed It was also suggested that the alkoxy radical formed by
scission of the O-NO2 bond could attack PETN resulting in polymer-like side products
such as dipentaerythritol hexanitrate (DiPEHN) and tripentaerythritol octanitrate
(TriPEON) [24]
Thermal ageing studies of PETN at 80degC [1] and 100degC [2] did not show significant
degradation but continued heating to temperatures above the melting point of PETN
42 Chapter 3
Cha
pter
3
(1413degC) resulted in gradual decomposition [1] Decomposition of PETN at higher
temperatures yielded a greater variety of ndash primarily gaseous ndash decomposition products
resulting from further breakdown of PETN [2-410] It was also reported that the
second step in the decomposition of PETN (after cleavage of the O-NO2 bond) is the
loss of a formaldehyde molecule [39-11] Infrared analysis of the residual material
after degradation of PETN in benzene at 185degC resulted in the identification of a
polyketo oxetane [9] This led to a proposed decomposition mechanism involving cyclic
intermediates Shepodd et al [11] identified several decomposition products (including
PETriN PEDiN and DiPEHN) using LCndashMS and capillary electrochromatographyndash
mass spectrometry (CECndashMS) after heating PETN under vacuum at temperatures up
to 135degC PETriN formation during high-temperature decomposition of PETN was also
suggested by Makashir and Kurian [3]
312 Other factors influencing PETN decompositionSeveral environmental factors have been found to accelerate PETN decomposition or to
result in different decomposition pathways These include the presence of water soil or
microorganisms
The presence of water has a detrimental effect on the stability of PETN Moisture results
in sequential hydrolysis of the O-NO2 bonds resulting in hydroxyl end groups [2413]
Several studies showed the formation of PETriN PEDiN and PEMN [1413] Hydrolysis
proceeded more rapidly under acidic or basic conditions [111]
Microbial degradation of PETN also resulted in the formation of PETriN PEDiN
and PEMN [1415] This behavior was also observed for other nitrate esters such as
nitroglycerin EGDN and nitrocellulose [1516] and it was suggested that biodegradation
of nitrate esters generally follows a hydrolytic pathway [17] Binks et al [14] isolated
a microbial culture (Enterobacter cloacae PB2) from explosive-contaminated soil
Several metabolites of PETN were detected including PEDiN The enzyme PETN
reductase was also isolated from the culture showing conversion of PETN to PETriN
and PEDiN In another study PETN was buried in soil and after 20 years 90 of the
PETN profiling in post-explosion residues 43
Chapter 3
PETN was found to be remaining From these results the half-life time of PETN in soil
was estimated to be 92 years [18]
Although PETN is relatively resistant to chemical reagents [1] several compounds can
accelerate its decomposition such as carbamite (13-diethyl-13-diphenylurea) calcium
carbonate magnesium oxide [3] ferrous chloride [1] and granular iron [5] When
analyzing degraded PETN it should be considered that some of the proposed degradation
products may also have originated as side products during synthesis Yasuda [19] used
TLC to identify PETriN DiPEHN and TriPEON in PETN samples Other commonly
encountered impurities are pentaerythritol (PE) PEMN and PEDiN [4]
313 Case assessmentIn the present study the possibility to discriminate between PETN degradation during
explosion and other scenarios is investigated This is important in assessing the evidential
value of an observed PETN chemical profile in cases as the example described above
The probability of the evidence should then be considered under different hypotheses
that may be postulated by the prosecution (Hp) or the defense (Hd) in line with the
Bayesian framework for evidence interpretation [2021] To discriminate between post-
explosion samples and other scenarios the following hypotheses were formulated
Hp The observed PETN degradation products on the suspectrsquos clothing originate from
a PETN explosion
Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as
impurities in the intact PETN handled by the suspect
Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed
by chemical and environmental degradation of PETN
To determine the specificity of post-explosion PETN profiles it should be investigated
whether similar profiles can be generated by other processes than PETN detonation
In this study PETN-detonation experiments were conducted Samples were taken and
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
38 Chapter 3
Cha
pter
3
Abstract
Pentaerythritol tetranitrate (PETN) and its degradation products are analyzed to
discriminate between residues originating from PETN explosions and residues
obtained under other circumstances such as natural degradation on textile or after
handling intact PETN The degradation products observed in post-explosion samples
were identified using liquid chromatographyndashmass spectrometry as the less-nitrated
analogues of PETN pentaerythritol trinitrate (PETriN) pentaerythritol dinitrate
(PEDiN) and pentaerythritol mononitrate (PEMN) Significant levels of these
degradation products were observed in post-explosion samples whereas only very low
levels were detected in a variety of intact PETN samples and naturally degraded PETN
Based on the peak areas of PETriN PEDiN and PEMN relative to PETN it was possible
to fully distinguish the post-explosion profiles from the profiles obtained from intact
PETN or after (accelerated) natural degradation Although more data are required to
accurately assess the strength of the evidence this work illustrates that PETN profiling
may yield valuable evidence when investigating a possible link between a suspect and
post-explosion PETN found on a crime scene
PETN profiling in post-explosion residues 39
Chapter 3
31 Introduction
Recently a series of safe crackings occurred in the Netherlands in which the explosive
PETN (pentaerythritol tetranitrate) was used A suspect was apprehended and his
clothing was sampled and analyzed by the NFI (Netherlands Forensic Institute)
Analysis by liquid chromatographyndashmass spectrometry (LCndashMS) led to the detection
of PETN but also relatively high amounts of its degradation products pentaerythritol
trinitrate (PETriN) the dinitrate (PEDiN) and the mononitrate (PEMN) This triggered
the question whether the identification of these degradation products could be explained
by the suspectrsquos presence at the crime scene The suspect stated that the residues
found on his clothing had originated from handling intact PETN which was also
found at his home The degradation products could then be explained by the presence
of impurities in the material or by natural degradation of the intact PETN as such or
on the clothing In this context differences between PETN chemical profiles obtained
by natural degradation and explosion were studied This chapter reports on the study
that was undertaken to investigate whether post-explosion PETN residue profiles can
be differentiated from PETN profiles arising from other processes such as natural
degradation or during synthesis
PETN is a powerful high explosive prepared by nitration of pentaerythritol (PE) It
was brought into use during World War II after formaldehyde and acetaldehyde
(precursors of PE) had become industrially available [1] PETN is relatively stable ndash
both chemically and physically ndash in comparison with other nitrate-ester explosives
such as ethylene glycol dinitrate (EGDN) and nitroglycerin (NG) [23] This has been
attributed to the symmetrical molecular structure of PETN [1] Major applications of
PETN as an explosive are as main charge in detonation cords and blasting caps [12] and
in formulations [4-6] (eg Semtex pentolite PEP 500)
In addition to its use as an explosive PETN also acts as a coronary vasodilator It is used
as the active ingredient in heart medicines for the treatment of angina pectoris [27] The
40 Chapter 3
Cha
pter
3
amounts of PETN used for medical purposes are very small compared to the amounts of
material required to cause explosions
311 Decomposition of PETNTraditional explosive-residues analysis only considers identification of the explosive used
and therefore limited information is available on the presence of degradation products
of PETN in post-explosion samples Moreover levels of explosives and degradation
products in post-explosion residues tend to be low making their detection challenging
Although detonation of PETN mainly results in the formation of gaseous products [1]
solid decomposition products are usually also formed because of incomplete detonation
Thin-layer chromatographic (TLC) analysis of post-explosion extracts showed
additional spots apart from PETN which were later identified by chemical-ionization
mass spectrometry (CIndashMS) and nuclear-magnetic-resonance (NMR) spectroscopy as
pentaerythritol trinitrate (PETriN) and pentaerythritol dinitrate (PEDiN) Another spot
could not be identified although it was suggested that this spot could be attributed to
pentaerythritol mononitrate (PEMN) [8] Fig 31 shows the chemical structure of PETN
and its less-nitrated analogues
O
O
O O
NOO
NO
O
N
O
O
NOO
O2NO
ONO2
O2NO OH
HO
ONO2
O2NO OH
HO
ONO2
HO OH
PETN PETriN
PEDiN PEMN
Fig 31 Chemical structure of PETN and its degradation products PETriN PEDiN and PEMN
More literature is available on chemical and environmental degradation of PETN The
degradation of PETN is influenced by a variety of parameters such as temperature the
PETN profiling in post-explosion residues 41
Chapter 3
presence of microorganisms and humidity There is consensus in literature that the first
and rate-determining step in the decomposition of PETN is the scission of the O-NO2
bond resulting in the release of nitrogen dioxide (NO2) [2-4910] This was observed for
nitrate esters in general [9] Several mechanisms for the following decomposition steps
have been postulated depending on the physical and chemical environment
PETN is stable compared to other organic explosives [4] and therefore the majority of
research into thermal decomposition of PETN has involved elevated temperatures (ie
above 100degC) However decomposition mechanisms are different at higher temperatures
than under ambient conditions [241112] Thus the results of accelerated-degradation
studies may not accurately reflect the natural degradation occurring on for instance the
clothing of a suspect However previous research on high-temperature decomposition
of PETN yields useful information on the identity of degradation products formed
As no condensed-phase decomposition products were detected after analysis of
naturally aged PETN at ambient temperatures it was suggested that at low temperatures
only gaseous decomposition products are formed [2] Decomposition of PETN at 53degC
was studied by monitoring the released NOx (mainly NO2) using a chemiluminescence
analyzer [12] PETN was found to have an NOx evolution rate that was roughly 1000
times lower than the evolution rate for nitrocellulose By extrapolating the NOx-emission
data the half-life time of PETN was estimated to be 12 million years confirming
its stability [12] Chambers et al [4] reported on the possible formation of peroxide
[(O2NOCH2)3C-CH2OO] nitrate [(O2NOCH2)3C-NO2] and aldehyde [(O2NOCH2)3C-
CHO] products at ambient temperatures but the presence of these products has not
been experimentally confirmed It was also suggested that the alkoxy radical formed by
scission of the O-NO2 bond could attack PETN resulting in polymer-like side products
such as dipentaerythritol hexanitrate (DiPEHN) and tripentaerythritol octanitrate
(TriPEON) [24]
Thermal ageing studies of PETN at 80degC [1] and 100degC [2] did not show significant
degradation but continued heating to temperatures above the melting point of PETN
42 Chapter 3
Cha
pter
3
(1413degC) resulted in gradual decomposition [1] Decomposition of PETN at higher
temperatures yielded a greater variety of ndash primarily gaseous ndash decomposition products
resulting from further breakdown of PETN [2-410] It was also reported that the
second step in the decomposition of PETN (after cleavage of the O-NO2 bond) is the
loss of a formaldehyde molecule [39-11] Infrared analysis of the residual material
after degradation of PETN in benzene at 185degC resulted in the identification of a
polyketo oxetane [9] This led to a proposed decomposition mechanism involving cyclic
intermediates Shepodd et al [11] identified several decomposition products (including
PETriN PEDiN and DiPEHN) using LCndashMS and capillary electrochromatographyndash
mass spectrometry (CECndashMS) after heating PETN under vacuum at temperatures up
to 135degC PETriN formation during high-temperature decomposition of PETN was also
suggested by Makashir and Kurian [3]
312 Other factors influencing PETN decompositionSeveral environmental factors have been found to accelerate PETN decomposition or to
result in different decomposition pathways These include the presence of water soil or
microorganisms
The presence of water has a detrimental effect on the stability of PETN Moisture results
in sequential hydrolysis of the O-NO2 bonds resulting in hydroxyl end groups [2413]
Several studies showed the formation of PETriN PEDiN and PEMN [1413] Hydrolysis
proceeded more rapidly under acidic or basic conditions [111]
Microbial degradation of PETN also resulted in the formation of PETriN PEDiN
and PEMN [1415] This behavior was also observed for other nitrate esters such as
nitroglycerin EGDN and nitrocellulose [1516] and it was suggested that biodegradation
of nitrate esters generally follows a hydrolytic pathway [17] Binks et al [14] isolated
a microbial culture (Enterobacter cloacae PB2) from explosive-contaminated soil
Several metabolites of PETN were detected including PEDiN The enzyme PETN
reductase was also isolated from the culture showing conversion of PETN to PETriN
and PEDiN In another study PETN was buried in soil and after 20 years 90 of the
PETN profiling in post-explosion residues 43
Chapter 3
PETN was found to be remaining From these results the half-life time of PETN in soil
was estimated to be 92 years [18]
Although PETN is relatively resistant to chemical reagents [1] several compounds can
accelerate its decomposition such as carbamite (13-diethyl-13-diphenylurea) calcium
carbonate magnesium oxide [3] ferrous chloride [1] and granular iron [5] When
analyzing degraded PETN it should be considered that some of the proposed degradation
products may also have originated as side products during synthesis Yasuda [19] used
TLC to identify PETriN DiPEHN and TriPEON in PETN samples Other commonly
encountered impurities are pentaerythritol (PE) PEMN and PEDiN [4]
313 Case assessmentIn the present study the possibility to discriminate between PETN degradation during
explosion and other scenarios is investigated This is important in assessing the evidential
value of an observed PETN chemical profile in cases as the example described above
The probability of the evidence should then be considered under different hypotheses
that may be postulated by the prosecution (Hp) or the defense (Hd) in line with the
Bayesian framework for evidence interpretation [2021] To discriminate between post-
explosion samples and other scenarios the following hypotheses were formulated
Hp The observed PETN degradation products on the suspectrsquos clothing originate from
a PETN explosion
Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as
impurities in the intact PETN handled by the suspect
Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed
by chemical and environmental degradation of PETN
To determine the specificity of post-explosion PETN profiles it should be investigated
whether similar profiles can be generated by other processes than PETN detonation
In this study PETN-detonation experiments were conducted Samples were taken and
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 39
Chapter 3
31 Introduction
Recently a series of safe crackings occurred in the Netherlands in which the explosive
PETN (pentaerythritol tetranitrate) was used A suspect was apprehended and his
clothing was sampled and analyzed by the NFI (Netherlands Forensic Institute)
Analysis by liquid chromatographyndashmass spectrometry (LCndashMS) led to the detection
of PETN but also relatively high amounts of its degradation products pentaerythritol
trinitrate (PETriN) the dinitrate (PEDiN) and the mononitrate (PEMN) This triggered
the question whether the identification of these degradation products could be explained
by the suspectrsquos presence at the crime scene The suspect stated that the residues
found on his clothing had originated from handling intact PETN which was also
found at his home The degradation products could then be explained by the presence
of impurities in the material or by natural degradation of the intact PETN as such or
on the clothing In this context differences between PETN chemical profiles obtained
by natural degradation and explosion were studied This chapter reports on the study
that was undertaken to investigate whether post-explosion PETN residue profiles can
be differentiated from PETN profiles arising from other processes such as natural
degradation or during synthesis
PETN is a powerful high explosive prepared by nitration of pentaerythritol (PE) It
was brought into use during World War II after formaldehyde and acetaldehyde
(precursors of PE) had become industrially available [1] PETN is relatively stable ndash
both chemically and physically ndash in comparison with other nitrate-ester explosives
such as ethylene glycol dinitrate (EGDN) and nitroglycerin (NG) [23] This has been
attributed to the symmetrical molecular structure of PETN [1] Major applications of
PETN as an explosive are as main charge in detonation cords and blasting caps [12] and
in formulations [4-6] (eg Semtex pentolite PEP 500)
In addition to its use as an explosive PETN also acts as a coronary vasodilator It is used
as the active ingredient in heart medicines for the treatment of angina pectoris [27] The
40 Chapter 3
Cha
pter
3
amounts of PETN used for medical purposes are very small compared to the amounts of
material required to cause explosions
311 Decomposition of PETNTraditional explosive-residues analysis only considers identification of the explosive used
and therefore limited information is available on the presence of degradation products
of PETN in post-explosion samples Moreover levels of explosives and degradation
products in post-explosion residues tend to be low making their detection challenging
Although detonation of PETN mainly results in the formation of gaseous products [1]
solid decomposition products are usually also formed because of incomplete detonation
Thin-layer chromatographic (TLC) analysis of post-explosion extracts showed
additional spots apart from PETN which were later identified by chemical-ionization
mass spectrometry (CIndashMS) and nuclear-magnetic-resonance (NMR) spectroscopy as
pentaerythritol trinitrate (PETriN) and pentaerythritol dinitrate (PEDiN) Another spot
could not be identified although it was suggested that this spot could be attributed to
pentaerythritol mononitrate (PEMN) [8] Fig 31 shows the chemical structure of PETN
and its less-nitrated analogues
O
O
O O
NOO
NO
O
N
O
O
NOO
O2NO
ONO2
O2NO OH
HO
ONO2
O2NO OH
HO
ONO2
HO OH
PETN PETriN
PEDiN PEMN
Fig 31 Chemical structure of PETN and its degradation products PETriN PEDiN and PEMN
More literature is available on chemical and environmental degradation of PETN The
degradation of PETN is influenced by a variety of parameters such as temperature the
PETN profiling in post-explosion residues 41
Chapter 3
presence of microorganisms and humidity There is consensus in literature that the first
and rate-determining step in the decomposition of PETN is the scission of the O-NO2
bond resulting in the release of nitrogen dioxide (NO2) [2-4910] This was observed for
nitrate esters in general [9] Several mechanisms for the following decomposition steps
have been postulated depending on the physical and chemical environment
PETN is stable compared to other organic explosives [4] and therefore the majority of
research into thermal decomposition of PETN has involved elevated temperatures (ie
above 100degC) However decomposition mechanisms are different at higher temperatures
than under ambient conditions [241112] Thus the results of accelerated-degradation
studies may not accurately reflect the natural degradation occurring on for instance the
clothing of a suspect However previous research on high-temperature decomposition
of PETN yields useful information on the identity of degradation products formed
As no condensed-phase decomposition products were detected after analysis of
naturally aged PETN at ambient temperatures it was suggested that at low temperatures
only gaseous decomposition products are formed [2] Decomposition of PETN at 53degC
was studied by monitoring the released NOx (mainly NO2) using a chemiluminescence
analyzer [12] PETN was found to have an NOx evolution rate that was roughly 1000
times lower than the evolution rate for nitrocellulose By extrapolating the NOx-emission
data the half-life time of PETN was estimated to be 12 million years confirming
its stability [12] Chambers et al [4] reported on the possible formation of peroxide
[(O2NOCH2)3C-CH2OO] nitrate [(O2NOCH2)3C-NO2] and aldehyde [(O2NOCH2)3C-
CHO] products at ambient temperatures but the presence of these products has not
been experimentally confirmed It was also suggested that the alkoxy radical formed by
scission of the O-NO2 bond could attack PETN resulting in polymer-like side products
such as dipentaerythritol hexanitrate (DiPEHN) and tripentaerythritol octanitrate
(TriPEON) [24]
Thermal ageing studies of PETN at 80degC [1] and 100degC [2] did not show significant
degradation but continued heating to temperatures above the melting point of PETN
42 Chapter 3
Cha
pter
3
(1413degC) resulted in gradual decomposition [1] Decomposition of PETN at higher
temperatures yielded a greater variety of ndash primarily gaseous ndash decomposition products
resulting from further breakdown of PETN [2-410] It was also reported that the
second step in the decomposition of PETN (after cleavage of the O-NO2 bond) is the
loss of a formaldehyde molecule [39-11] Infrared analysis of the residual material
after degradation of PETN in benzene at 185degC resulted in the identification of a
polyketo oxetane [9] This led to a proposed decomposition mechanism involving cyclic
intermediates Shepodd et al [11] identified several decomposition products (including
PETriN PEDiN and DiPEHN) using LCndashMS and capillary electrochromatographyndash
mass spectrometry (CECndashMS) after heating PETN under vacuum at temperatures up
to 135degC PETriN formation during high-temperature decomposition of PETN was also
suggested by Makashir and Kurian [3]
312 Other factors influencing PETN decompositionSeveral environmental factors have been found to accelerate PETN decomposition or to
result in different decomposition pathways These include the presence of water soil or
microorganisms
The presence of water has a detrimental effect on the stability of PETN Moisture results
in sequential hydrolysis of the O-NO2 bonds resulting in hydroxyl end groups [2413]
Several studies showed the formation of PETriN PEDiN and PEMN [1413] Hydrolysis
proceeded more rapidly under acidic or basic conditions [111]
Microbial degradation of PETN also resulted in the formation of PETriN PEDiN
and PEMN [1415] This behavior was also observed for other nitrate esters such as
nitroglycerin EGDN and nitrocellulose [1516] and it was suggested that biodegradation
of nitrate esters generally follows a hydrolytic pathway [17] Binks et al [14] isolated
a microbial culture (Enterobacter cloacae PB2) from explosive-contaminated soil
Several metabolites of PETN were detected including PEDiN The enzyme PETN
reductase was also isolated from the culture showing conversion of PETN to PETriN
and PEDiN In another study PETN was buried in soil and after 20 years 90 of the
PETN profiling in post-explosion residues 43
Chapter 3
PETN was found to be remaining From these results the half-life time of PETN in soil
was estimated to be 92 years [18]
Although PETN is relatively resistant to chemical reagents [1] several compounds can
accelerate its decomposition such as carbamite (13-diethyl-13-diphenylurea) calcium
carbonate magnesium oxide [3] ferrous chloride [1] and granular iron [5] When
analyzing degraded PETN it should be considered that some of the proposed degradation
products may also have originated as side products during synthesis Yasuda [19] used
TLC to identify PETriN DiPEHN and TriPEON in PETN samples Other commonly
encountered impurities are pentaerythritol (PE) PEMN and PEDiN [4]
313 Case assessmentIn the present study the possibility to discriminate between PETN degradation during
explosion and other scenarios is investigated This is important in assessing the evidential
value of an observed PETN chemical profile in cases as the example described above
The probability of the evidence should then be considered under different hypotheses
that may be postulated by the prosecution (Hp) or the defense (Hd) in line with the
Bayesian framework for evidence interpretation [2021] To discriminate between post-
explosion samples and other scenarios the following hypotheses were formulated
Hp The observed PETN degradation products on the suspectrsquos clothing originate from
a PETN explosion
Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as
impurities in the intact PETN handled by the suspect
Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed
by chemical and environmental degradation of PETN
To determine the specificity of post-explosion PETN profiles it should be investigated
whether similar profiles can be generated by other processes than PETN detonation
In this study PETN-detonation experiments were conducted Samples were taken and
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
40 Chapter 3
Cha
pter
3
amounts of PETN used for medical purposes are very small compared to the amounts of
material required to cause explosions
311 Decomposition of PETNTraditional explosive-residues analysis only considers identification of the explosive used
and therefore limited information is available on the presence of degradation products
of PETN in post-explosion samples Moreover levels of explosives and degradation
products in post-explosion residues tend to be low making their detection challenging
Although detonation of PETN mainly results in the formation of gaseous products [1]
solid decomposition products are usually also formed because of incomplete detonation
Thin-layer chromatographic (TLC) analysis of post-explosion extracts showed
additional spots apart from PETN which were later identified by chemical-ionization
mass spectrometry (CIndashMS) and nuclear-magnetic-resonance (NMR) spectroscopy as
pentaerythritol trinitrate (PETriN) and pentaerythritol dinitrate (PEDiN) Another spot
could not be identified although it was suggested that this spot could be attributed to
pentaerythritol mononitrate (PEMN) [8] Fig 31 shows the chemical structure of PETN
and its less-nitrated analogues
O
O
O O
NOO
NO
O
N
O
O
NOO
O2NO
ONO2
O2NO OH
HO
ONO2
O2NO OH
HO
ONO2
HO OH
PETN PETriN
PEDiN PEMN
Fig 31 Chemical structure of PETN and its degradation products PETriN PEDiN and PEMN
More literature is available on chemical and environmental degradation of PETN The
degradation of PETN is influenced by a variety of parameters such as temperature the
PETN profiling in post-explosion residues 41
Chapter 3
presence of microorganisms and humidity There is consensus in literature that the first
and rate-determining step in the decomposition of PETN is the scission of the O-NO2
bond resulting in the release of nitrogen dioxide (NO2) [2-4910] This was observed for
nitrate esters in general [9] Several mechanisms for the following decomposition steps
have been postulated depending on the physical and chemical environment
PETN is stable compared to other organic explosives [4] and therefore the majority of
research into thermal decomposition of PETN has involved elevated temperatures (ie
above 100degC) However decomposition mechanisms are different at higher temperatures
than under ambient conditions [241112] Thus the results of accelerated-degradation
studies may not accurately reflect the natural degradation occurring on for instance the
clothing of a suspect However previous research on high-temperature decomposition
of PETN yields useful information on the identity of degradation products formed
As no condensed-phase decomposition products were detected after analysis of
naturally aged PETN at ambient temperatures it was suggested that at low temperatures
only gaseous decomposition products are formed [2] Decomposition of PETN at 53degC
was studied by monitoring the released NOx (mainly NO2) using a chemiluminescence
analyzer [12] PETN was found to have an NOx evolution rate that was roughly 1000
times lower than the evolution rate for nitrocellulose By extrapolating the NOx-emission
data the half-life time of PETN was estimated to be 12 million years confirming
its stability [12] Chambers et al [4] reported on the possible formation of peroxide
[(O2NOCH2)3C-CH2OO] nitrate [(O2NOCH2)3C-NO2] and aldehyde [(O2NOCH2)3C-
CHO] products at ambient temperatures but the presence of these products has not
been experimentally confirmed It was also suggested that the alkoxy radical formed by
scission of the O-NO2 bond could attack PETN resulting in polymer-like side products
such as dipentaerythritol hexanitrate (DiPEHN) and tripentaerythritol octanitrate
(TriPEON) [24]
Thermal ageing studies of PETN at 80degC [1] and 100degC [2] did not show significant
degradation but continued heating to temperatures above the melting point of PETN
42 Chapter 3
Cha
pter
3
(1413degC) resulted in gradual decomposition [1] Decomposition of PETN at higher
temperatures yielded a greater variety of ndash primarily gaseous ndash decomposition products
resulting from further breakdown of PETN [2-410] It was also reported that the
second step in the decomposition of PETN (after cleavage of the O-NO2 bond) is the
loss of a formaldehyde molecule [39-11] Infrared analysis of the residual material
after degradation of PETN in benzene at 185degC resulted in the identification of a
polyketo oxetane [9] This led to a proposed decomposition mechanism involving cyclic
intermediates Shepodd et al [11] identified several decomposition products (including
PETriN PEDiN and DiPEHN) using LCndashMS and capillary electrochromatographyndash
mass spectrometry (CECndashMS) after heating PETN under vacuum at temperatures up
to 135degC PETriN formation during high-temperature decomposition of PETN was also
suggested by Makashir and Kurian [3]
312 Other factors influencing PETN decompositionSeveral environmental factors have been found to accelerate PETN decomposition or to
result in different decomposition pathways These include the presence of water soil or
microorganisms
The presence of water has a detrimental effect on the stability of PETN Moisture results
in sequential hydrolysis of the O-NO2 bonds resulting in hydroxyl end groups [2413]
Several studies showed the formation of PETriN PEDiN and PEMN [1413] Hydrolysis
proceeded more rapidly under acidic or basic conditions [111]
Microbial degradation of PETN also resulted in the formation of PETriN PEDiN
and PEMN [1415] This behavior was also observed for other nitrate esters such as
nitroglycerin EGDN and nitrocellulose [1516] and it was suggested that biodegradation
of nitrate esters generally follows a hydrolytic pathway [17] Binks et al [14] isolated
a microbial culture (Enterobacter cloacae PB2) from explosive-contaminated soil
Several metabolites of PETN were detected including PEDiN The enzyme PETN
reductase was also isolated from the culture showing conversion of PETN to PETriN
and PEDiN In another study PETN was buried in soil and after 20 years 90 of the
PETN profiling in post-explosion residues 43
Chapter 3
PETN was found to be remaining From these results the half-life time of PETN in soil
was estimated to be 92 years [18]
Although PETN is relatively resistant to chemical reagents [1] several compounds can
accelerate its decomposition such as carbamite (13-diethyl-13-diphenylurea) calcium
carbonate magnesium oxide [3] ferrous chloride [1] and granular iron [5] When
analyzing degraded PETN it should be considered that some of the proposed degradation
products may also have originated as side products during synthesis Yasuda [19] used
TLC to identify PETriN DiPEHN and TriPEON in PETN samples Other commonly
encountered impurities are pentaerythritol (PE) PEMN and PEDiN [4]
313 Case assessmentIn the present study the possibility to discriminate between PETN degradation during
explosion and other scenarios is investigated This is important in assessing the evidential
value of an observed PETN chemical profile in cases as the example described above
The probability of the evidence should then be considered under different hypotheses
that may be postulated by the prosecution (Hp) or the defense (Hd) in line with the
Bayesian framework for evidence interpretation [2021] To discriminate between post-
explosion samples and other scenarios the following hypotheses were formulated
Hp The observed PETN degradation products on the suspectrsquos clothing originate from
a PETN explosion
Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as
impurities in the intact PETN handled by the suspect
Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed
by chemical and environmental degradation of PETN
To determine the specificity of post-explosion PETN profiles it should be investigated
whether similar profiles can be generated by other processes than PETN detonation
In this study PETN-detonation experiments were conducted Samples were taken and
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 41
Chapter 3
presence of microorganisms and humidity There is consensus in literature that the first
and rate-determining step in the decomposition of PETN is the scission of the O-NO2
bond resulting in the release of nitrogen dioxide (NO2) [2-4910] This was observed for
nitrate esters in general [9] Several mechanisms for the following decomposition steps
have been postulated depending on the physical and chemical environment
PETN is stable compared to other organic explosives [4] and therefore the majority of
research into thermal decomposition of PETN has involved elevated temperatures (ie
above 100degC) However decomposition mechanisms are different at higher temperatures
than under ambient conditions [241112] Thus the results of accelerated-degradation
studies may not accurately reflect the natural degradation occurring on for instance the
clothing of a suspect However previous research on high-temperature decomposition
of PETN yields useful information on the identity of degradation products formed
As no condensed-phase decomposition products were detected after analysis of
naturally aged PETN at ambient temperatures it was suggested that at low temperatures
only gaseous decomposition products are formed [2] Decomposition of PETN at 53degC
was studied by monitoring the released NOx (mainly NO2) using a chemiluminescence
analyzer [12] PETN was found to have an NOx evolution rate that was roughly 1000
times lower than the evolution rate for nitrocellulose By extrapolating the NOx-emission
data the half-life time of PETN was estimated to be 12 million years confirming
its stability [12] Chambers et al [4] reported on the possible formation of peroxide
[(O2NOCH2)3C-CH2OO] nitrate [(O2NOCH2)3C-NO2] and aldehyde [(O2NOCH2)3C-
CHO] products at ambient temperatures but the presence of these products has not
been experimentally confirmed It was also suggested that the alkoxy radical formed by
scission of the O-NO2 bond could attack PETN resulting in polymer-like side products
such as dipentaerythritol hexanitrate (DiPEHN) and tripentaerythritol octanitrate
(TriPEON) [24]
Thermal ageing studies of PETN at 80degC [1] and 100degC [2] did not show significant
degradation but continued heating to temperatures above the melting point of PETN
42 Chapter 3
Cha
pter
3
(1413degC) resulted in gradual decomposition [1] Decomposition of PETN at higher
temperatures yielded a greater variety of ndash primarily gaseous ndash decomposition products
resulting from further breakdown of PETN [2-410] It was also reported that the
second step in the decomposition of PETN (after cleavage of the O-NO2 bond) is the
loss of a formaldehyde molecule [39-11] Infrared analysis of the residual material
after degradation of PETN in benzene at 185degC resulted in the identification of a
polyketo oxetane [9] This led to a proposed decomposition mechanism involving cyclic
intermediates Shepodd et al [11] identified several decomposition products (including
PETriN PEDiN and DiPEHN) using LCndashMS and capillary electrochromatographyndash
mass spectrometry (CECndashMS) after heating PETN under vacuum at temperatures up
to 135degC PETriN formation during high-temperature decomposition of PETN was also
suggested by Makashir and Kurian [3]
312 Other factors influencing PETN decompositionSeveral environmental factors have been found to accelerate PETN decomposition or to
result in different decomposition pathways These include the presence of water soil or
microorganisms
The presence of water has a detrimental effect on the stability of PETN Moisture results
in sequential hydrolysis of the O-NO2 bonds resulting in hydroxyl end groups [2413]
Several studies showed the formation of PETriN PEDiN and PEMN [1413] Hydrolysis
proceeded more rapidly under acidic or basic conditions [111]
Microbial degradation of PETN also resulted in the formation of PETriN PEDiN
and PEMN [1415] This behavior was also observed for other nitrate esters such as
nitroglycerin EGDN and nitrocellulose [1516] and it was suggested that biodegradation
of nitrate esters generally follows a hydrolytic pathway [17] Binks et al [14] isolated
a microbial culture (Enterobacter cloacae PB2) from explosive-contaminated soil
Several metabolites of PETN were detected including PEDiN The enzyme PETN
reductase was also isolated from the culture showing conversion of PETN to PETriN
and PEDiN In another study PETN was buried in soil and after 20 years 90 of the
PETN profiling in post-explosion residues 43
Chapter 3
PETN was found to be remaining From these results the half-life time of PETN in soil
was estimated to be 92 years [18]
Although PETN is relatively resistant to chemical reagents [1] several compounds can
accelerate its decomposition such as carbamite (13-diethyl-13-diphenylurea) calcium
carbonate magnesium oxide [3] ferrous chloride [1] and granular iron [5] When
analyzing degraded PETN it should be considered that some of the proposed degradation
products may also have originated as side products during synthesis Yasuda [19] used
TLC to identify PETriN DiPEHN and TriPEON in PETN samples Other commonly
encountered impurities are pentaerythritol (PE) PEMN and PEDiN [4]
313 Case assessmentIn the present study the possibility to discriminate between PETN degradation during
explosion and other scenarios is investigated This is important in assessing the evidential
value of an observed PETN chemical profile in cases as the example described above
The probability of the evidence should then be considered under different hypotheses
that may be postulated by the prosecution (Hp) or the defense (Hd) in line with the
Bayesian framework for evidence interpretation [2021] To discriminate between post-
explosion samples and other scenarios the following hypotheses were formulated
Hp The observed PETN degradation products on the suspectrsquos clothing originate from
a PETN explosion
Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as
impurities in the intact PETN handled by the suspect
Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed
by chemical and environmental degradation of PETN
To determine the specificity of post-explosion PETN profiles it should be investigated
whether similar profiles can be generated by other processes than PETN detonation
In this study PETN-detonation experiments were conducted Samples were taken and
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
42 Chapter 3
Cha
pter
3
(1413degC) resulted in gradual decomposition [1] Decomposition of PETN at higher
temperatures yielded a greater variety of ndash primarily gaseous ndash decomposition products
resulting from further breakdown of PETN [2-410] It was also reported that the
second step in the decomposition of PETN (after cleavage of the O-NO2 bond) is the
loss of a formaldehyde molecule [39-11] Infrared analysis of the residual material
after degradation of PETN in benzene at 185degC resulted in the identification of a
polyketo oxetane [9] This led to a proposed decomposition mechanism involving cyclic
intermediates Shepodd et al [11] identified several decomposition products (including
PETriN PEDiN and DiPEHN) using LCndashMS and capillary electrochromatographyndash
mass spectrometry (CECndashMS) after heating PETN under vacuum at temperatures up
to 135degC PETriN formation during high-temperature decomposition of PETN was also
suggested by Makashir and Kurian [3]
312 Other factors influencing PETN decompositionSeveral environmental factors have been found to accelerate PETN decomposition or to
result in different decomposition pathways These include the presence of water soil or
microorganisms
The presence of water has a detrimental effect on the stability of PETN Moisture results
in sequential hydrolysis of the O-NO2 bonds resulting in hydroxyl end groups [2413]
Several studies showed the formation of PETriN PEDiN and PEMN [1413] Hydrolysis
proceeded more rapidly under acidic or basic conditions [111]
Microbial degradation of PETN also resulted in the formation of PETriN PEDiN
and PEMN [1415] This behavior was also observed for other nitrate esters such as
nitroglycerin EGDN and nitrocellulose [1516] and it was suggested that biodegradation
of nitrate esters generally follows a hydrolytic pathway [17] Binks et al [14] isolated
a microbial culture (Enterobacter cloacae PB2) from explosive-contaminated soil
Several metabolites of PETN were detected including PEDiN The enzyme PETN
reductase was also isolated from the culture showing conversion of PETN to PETriN
and PEDiN In another study PETN was buried in soil and after 20 years 90 of the
PETN profiling in post-explosion residues 43
Chapter 3
PETN was found to be remaining From these results the half-life time of PETN in soil
was estimated to be 92 years [18]
Although PETN is relatively resistant to chemical reagents [1] several compounds can
accelerate its decomposition such as carbamite (13-diethyl-13-diphenylurea) calcium
carbonate magnesium oxide [3] ferrous chloride [1] and granular iron [5] When
analyzing degraded PETN it should be considered that some of the proposed degradation
products may also have originated as side products during synthesis Yasuda [19] used
TLC to identify PETriN DiPEHN and TriPEON in PETN samples Other commonly
encountered impurities are pentaerythritol (PE) PEMN and PEDiN [4]
313 Case assessmentIn the present study the possibility to discriminate between PETN degradation during
explosion and other scenarios is investigated This is important in assessing the evidential
value of an observed PETN chemical profile in cases as the example described above
The probability of the evidence should then be considered under different hypotheses
that may be postulated by the prosecution (Hp) or the defense (Hd) in line with the
Bayesian framework for evidence interpretation [2021] To discriminate between post-
explosion samples and other scenarios the following hypotheses were formulated
Hp The observed PETN degradation products on the suspectrsquos clothing originate from
a PETN explosion
Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as
impurities in the intact PETN handled by the suspect
Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed
by chemical and environmental degradation of PETN
To determine the specificity of post-explosion PETN profiles it should be investigated
whether similar profiles can be generated by other processes than PETN detonation
In this study PETN-detonation experiments were conducted Samples were taken and
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 43
Chapter 3
PETN was found to be remaining From these results the half-life time of PETN in soil
was estimated to be 92 years [18]
Although PETN is relatively resistant to chemical reagents [1] several compounds can
accelerate its decomposition such as carbamite (13-diethyl-13-diphenylurea) calcium
carbonate magnesium oxide [3] ferrous chloride [1] and granular iron [5] When
analyzing degraded PETN it should be considered that some of the proposed degradation
products may also have originated as side products during synthesis Yasuda [19] used
TLC to identify PETriN DiPEHN and TriPEON in PETN samples Other commonly
encountered impurities are pentaerythritol (PE) PEMN and PEDiN [4]
313 Case assessmentIn the present study the possibility to discriminate between PETN degradation during
explosion and other scenarios is investigated This is important in assessing the evidential
value of an observed PETN chemical profile in cases as the example described above
The probability of the evidence should then be considered under different hypotheses
that may be postulated by the prosecution (Hp) or the defense (Hd) in line with the
Bayesian framework for evidence interpretation [2021] To discriminate between post-
explosion samples and other scenarios the following hypotheses were formulated
Hp The observed PETN degradation products on the suspectrsquos clothing originate from
a PETN explosion
Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as
impurities in the intact PETN handled by the suspect
Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed
by chemical and environmental degradation of PETN
To determine the specificity of post-explosion PETN profiles it should be investigated
whether similar profiles can be generated by other processes than PETN detonation
In this study PETN-detonation experiments were conducted Samples were taken and
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
44 Chapter 3
Cha
pter
3
analyzed using LCndashMS to establish the PETN chemical profile and to observe the
variation therein In addition a selection of PETN samples of different origins were
analyzed to determine the impurity profile that might have been expected if intact
PETN material were present on the suspectrsquos clothing Finally numerous experiments
were conducted to effectuate PETN degradation through chemical and environmental
processes These experiments included various textile matrices and variation in
parameters such as temperature and humidity The LCndashMS profiles of all experiments
were compared to establish to what extent observed PETN profiles can provide support
for the hypothesis that PETN residues originate from an explosion
32 Experimental
321 Chemicals and materialsHigh-purity PETN (containing a low level of PETriN as a minor impurity) was
provided by TNO Technical Sciences department of Energetic Materials (Rijswijk The
Netherlands) Rathburn (Walkerburn UK) HPLC grade methanol was used for both
sample preparation and LCndashMS analysis Ultra-pure water prepared using a Milli-Q
(Millipore Bedford MA USA) or a PureLab Ultra (Elga High Wycombe UK)
system was used both for sample preparation and LCndashMS analysis For approximate
quantification a PETN analytical standard from AccuStandard (New Haven CT USA)
was used (01 mgmL in methanol)
322 Explosion experimentsExplosion experiments were performed by detonating 15 g of PETN (no confinement) in
a cylindrical (oslash = 450 mm h = 300 mm) set-up with stainless steel witness plates to collect
de detonation products (Fig 32) The witness plates were replaced after each explosion
To initiate the explosion a detonation cord was used that also contained PETN Two
experiments were conducted using only the detonation cord and five experiments with
15 g of PETN each
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 45
Chapter 3
Witness plates
15 g PETN
Detonation cord
ba
Fig 32 Setup (oslash = 400 mm h = 300 mm) of explosion experiments before (a) and after the explosion (b)
323 Sampling of post-explosion residuesThe witness plates were swabbed using sterile gauzes (Klinion NW Compres 5 times 5
cm Medeco Oud-Beijerland The Netherlands) wetted with methanol After each
experiment the witness plates were swabbed at three locations one swab for the bottom
plate and one for each half of the ring plate Post-explosion swabs were extracted with 10
mL of methanol and the extracts were filtered through a 045 microm regenerated cellulose
(RC) filter (Whatman Dassel Germany) Because of the low analyte levels the samples
were concentrated by solvent evaporation under nitrogen down to a volume of about 1
mL
324 Degradation experimentsTo simulate natural degradation PETN was applied to different types of fabric
resembling the clothing of a suspect The effect of three different parameters was
investigated matrix type temperature and humidity PETN was applied to different
types of fabric both in its solid form and in solution In casework it is more likely to
encounter PETN as a solid on the clothing of a suspect Applying a solution of PETN
was done to achieve a more homogeneous distribution of the PETN and to increase
the contact area between the PETN and the textile In solid form 3 mg of PETN were
deposited on fabric pieces of ca 40 times 40 mm For application in solution 150 microL from
a 20 mgmL solution of PETN were added to the textile and left to dry All experiments
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
46 Chapter 3
Cha
pter
3
were performed in duplicate The textile types used were cotton acrylic and polyester as
these are three of the most frequently encountered types in forensic casework All three
matrix types were tested at two different temperatures room temperature (20degC) and
60degC Samples were stored in the dark without humidity control at both temperatures
Measured relative-humidity values ranged from 30 to 55 Samples stored at room
temperature were analyzed after 12 weeks and samples aged at 60degC were collected
after 2 4 8 and 12 weeks Samples were stored at 60degC in an electrical oven with a 75degC
safety limit (auto shut-off) to accelerate degradation The influence of temperature on
the degradation rate of PETN is expected to follow the Arrhenius equation
k AeEART=minus (31)
Where k is the reaction-rate constant A the pre-exponential (or frequency) factor EA
the activation energy R the gas constant and T the temperature As a rule of thumb
the reaction rate doubles with 10degC increase in temperature [22] This would imply
that storage at 60degC for 12 weeks resembles storage at room temperature for 4 years
The exact increase of the reaction rate depends on the activation energy The factor 2
mentioned above would correspond to an activation energy of about 45 kJmol Published
values for the activation energy of the degradation of PETN vary from 125 kJmol to
293 kJmol [349-11] indicating that 12 weeks of storage at 60degC simulates storage
at room temperature for at least 118 years To examine the influence of the humidity
samples were stored at room temperature in a desiccator where the relative humidity was
controlled at 90 using a water-glycerol mixture Water-glycerol mixtures have been
demonstrated to provide a stable humidity level in a closed environment [23] Samples
were analyzed after 8 and 12 weeks of storage
The natural-degradation samples were extracted using 10 mL of methanol and the extract
was filtered through a 045 microm RC filter Because of the high PETN concentrations
a separate LCndashMS analysis was performed on each sample after an additional 50-
fold dilution in methanol to accurately determine the PETN content At these lower
concentrations the PETN peaks were within the linear range of the LCndashMS system
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 47
Chapter 3
325 LCndashMS analysisSamples were analyzed on a Thermo Fisher Scientific (Waltham MA USA) Surveyor
HPLC Plus system with a PDA detector connected to a Thermo Scientific LTQ
Orbitrap XL mass analyzer Separations were performed on a LiChrospher RP18
analytical column (Merck Darmstadt Germany 2 times 250 mm dp 5 microm) equipped with
a Phenomenex (Torrence CA USA) Securityguard C18 guard column (4 times 2 mm dp 5
microm) The column temperature was maintained at 35degC The mobile phase consisted of
water-methanol using a gradient at a flow rate of 200 microLmin The following gradient
was used 0ndash2 min 45 MeOH 2ndash15 min 45ndash90 MeOH linear 15ndash20 min 90
MeOH 20ndash25 min 45 MeOH Four percent chloroform in methanol were added post
column (2 microLmin) to enhance negative-ion yield by the formation of stable chloride
adducts The injection volume was 10 microL (partial-loop injection mode) All samples
were diluted 11 with ultra-pure water prior to injection in order to match the starting
mobile phase composition and to reduce peak broadening The MS was operated in
negative mode using an APCI ion source (atmospheric-pressure chemical ionization)
Table 31 Mass-spectrometric conditions
Source APCIPolarity NegativeVaporizer temperature 160ordmCCapillary temperature 125ordmCCorona discharge current 20 μASheath gas 100 (arb)Auxiliary gas 5 (arb)Capillary voltage -22 VTube lens -9231 VMass resolution 60000 (at mz 400)Scan range mz 163ndash1000Lock masses 255232954 [C16H32O2ndashH]-
291209631 [C16H32O2+35Cl]-
The method used has been described in more detail by Xu et al [24] In this work the
method was adapted by using a water-methanol gradient for optimum separation of the
degradation products of PETN instead of running in isocratic mode Also the mass scan
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
48 Chapter 3
Cha
pter
3
range was extended to 163ndash1000 mz MS instrument settings are summarized in Table
31 For undiluted1 natural-degradation samples the LC eluent was directed to waste
during elution of PETN (135ndash18 min) using a six-port valve to prevent contamination
of the ion source
Fig 33 Mass spectrum of PETN The main peaks at mz 216 261 306 315 and 351 are attributed to the ions [Mndash3NO2+3H+35Cl]- [Mndash2NO2+2H+35Cl]- [MndashNO2+H+35Cl]- [MndashH]- and [M+35Cl]- with M being C5H8N4O12 the molecular formula of PETN
33 Results
331 Analytical methodWhen applying the method described in the previous section PETN was found to elute
at 1410 min (SD = 020 min n = 199) Its mass spectrum is shown in Fig 33 The
base peak in the mass spectrum was found at mz 351 corresponding to the chloride
adduct of PETN [C5H8N4O12+35Cl]- In the ion source PETN partially loses its nitro
groups resulting in the ions also representing the base peaks for the degradation
compounds [25] viz [MndashNO2+H+35Cl]- (mz 306) [Mndash2NO2+2H+35Cl]- (mz 261)
[Mndash3NO2+3H+35Cl]- (mz 216) and [Mndash4NO2+4H+35Cl]- (mz 171) In addition the
[MndashH]- ion is also formed (mz 315) and nitrate (mz 378) formate and acetate adducts
were observed This indicates that direct mass spectrometry is not suitable for PETN
1 Undiluted means here apart from the twofold dilution that is part of the sample-preparation procedure
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 49
Chapter 3
profiling as the fragmentation would interfere with the detection of the degradation
products Therefore prior to MS detection separation of the different components using
liquid chromatography is required
With the LCndashMS method presented in this chapter base-line separation of PETN and
its degradation products was realized as shown in Fig 34 This figure shows a typical
extracted-ion chromatogram obtained for a post-explosion extract and illustrates how
PETriN PEDiN and PEMN can be identified in post-explosion and natural-degradation
samples The conditions used to record Fig 34 can be used for PETN profiling PE was
also detected but it was not included in the profile because it was frequently observed
in background samples (blank textile extracts blank swabs etc)
Fig 34 Extracted-ion chromatogram (mz 3509833 3059982 2610131 and 2160281) of a post-explosion extract The peak at tR 1411 represents PETN and the peaks at 1138 578 and 331 its degradation products (PETriN PEDiN and PEMN respectively)
Identification of the degradation products of PETN was based on the accurate masses
obtained using the Orbitrap mass spectrometer in combination with the observed
fragmentation patterns Similar to PETN the less-nitrated analogues also lose their
nitro group(s) in the ion source The exact masses and ions used for identification of
PETN and its degradation products are listed in Table 32
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
50 Chapter 3
Cha
pter
3
Table 32 Retention times and ions used for identification of PETN and its degradation products
Compound Retention time (tR in min) Major peaks in mass spectrum (mz)
Ion
PETN 1410 (SD = 020 n = 206) 3509833 [C5H8N4O12+35Cl]-
3150066 [MndashH]-
3059982 [MndashNO2+H+35Cl]-
2610131 [Mndash2NO2+2H+35Cl]-
2160281 [Mndash3NO2+3H+35Cl]-
1710430 [Mndash4NO2+4H+35Cl]-
PETriN 1137 (SD = 025 n = 236) 3059982 [C5H9N3O10+35Cl]-
2610131 [MndashNO2+H+35Cl]-
2160281 [Mndash2NO2+2H+35Cl]-
1710430 [Mndash3NO2+3H+35Cl]-
PEDiN 585 (SD = 016 n = 232) 2610131 [C5H10N2O8+35Cl]-
2160281 [MndashNO2+H+35Cl]-
1710430 [Mndash2NO2+2H+35Cl]-
PEMN 336 (SD = 009 n = 223) 2160281 [C5H11NO6+35Cl]-
1710430 [MndashNO2+H+35Cl]-
PE 281 (SD = 008 n = 30) 1710430 [C5H12O4+35Cl]-
Because standards of the degradation products were not commercially available at the
time this research was conducted2 the profile was constructed using peak-area ratios of
the degradation products relative to PETN (extracted-ion peak areas for the base peak for
each compound) In this way the chemical profile was normalized to the amount of PETN
present in the sample This approach corrects for the overall amount of PETN residue
sampled and for compound-independent sources of variation in the LCndashMS analysis It
should however be noted that the peak-area ratio does not reflect the relative amount of
the degradation product in the sample This would only be true if the sensitivity would
be identical for the base peaks of all compounds in the profile Because MS sensitivity
depends on ionization efficiency and on in-source fragmentation determination of the
relative and absolute amounts of the degradation products in the samples is only possible
when standards are available
2 AccuStandard recently introduced a PETriN analytical standard but it was not included in this research because it was not available at that time
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 51
Chapter 3
In case of intact PETN and controlled natural degradation substantial differences in
peak areas were measured for the degradation products versus PETN The PETN content
was brought in the linear range through dilution PETN showed linear response in the
range of 001ndash5 ppm with correlation coefficients higher than 0997 Diluted samples
were correlated to undiluted samples using calibration curves to correctly determine
peak-area ratios This allowed the determination of the peak area of the PETN as if the
linear range was extended to the original PETN concentration in the undiluted sample
In the absence of suitable standards the use of peak-area ratios requires stability of the
response of the degradation products relative to PETN across all measurements This
was studied using a naturally-degraded (60degC) PETN sample Within a measurement
series peak-area repeatability (given as RSD) was shown to be 098 for PETN and
123 for PETriN and variation in the PETriNPETN peak-area ratio was 097 (n = 5)
However day-to-day variations in the PETriNPETN peak-area ratio within a period of
5 months were as high as 10 This can be explained either by changes in the sample
or by compound-specific variation in the PETriN and PETN response Significant
variations in PETN sensitivity were observed likely caused by variations in ionization
efficiency and fragmentation ratios The degree of contamination of the ion source and
the heated capillary may also have added to compound-specific variations in the MS
response This contamination is more severe due to the relatively low vaporizer and
capillary temperatures necessary for ionization of explosive compounds than usually
encountered in LCndashAPCIndashMS Variations in relative responses of the degradation
products of PETN and absolute calibration using custom-made PETriN PEDiN and
PEMN standards will be discussed in chapter 4
332 Explosion experimentsAnalysis of the post-explosion extracts consistently showed the presence of PETN
PETriN PEDiN and PEMN (Fig 34) in line with the observations of Basch et al [8] The
absolute amount of PETN recovered from the post-explosion swabs varied substantially
ranging from 002 to 7 μg as can be expected from an uncontrolled process such as
an explosion This illustrates the importance of extensive and diverse sampling in post-
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
52 Chapter 3
Cha
pter
3
explosion crime-scene investigations Consequently dilution or further concentration of
the extract was often necessary to ensure a PETN content within the linear range and
sufficiently large peak areas for the degradation products Fig 35 shows the chemical
profiles for the post-explosion samples Substantial variations in the profiles are observed
between different explosions and even within different samplings of the same explosion
This is consistent with the examination of the witness plates after the explosions which
showed inhomogeneous distribution of the residues as illustrated in Fig 32b
0010203040506070809
1
a b c a b c a b c a b c a b
1 2 3 4 5
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 35 Degradation profiles of five explosion experiments sampled in triplicate (a bottom plate b c ring plate) showing the relative peak-area ratios of PETriN PEDiN and PEMN
Despite these variations which are intrinsic to the violent and uncontrolled processes
involved in an explosion significant relative peak areas for the PETN degradation
products PETriN en PEDiN were observed in all experiments and in all samples
Additionally it should be noted that the variations shown in Fig 35 due to the
uncontrolled nature of the explosions is substantially larger than the repeatability of
roughly 10 as mentioned in section 331 for the PETriNPETN peak-area ratio This
natural variation should however not be seen as a positive aspect as it indicates that
a substantial difference in the relative amounts of the PETN degradation products is
necessary to reliably discriminate PETN post-explosion profiles from profiles obtained
through other processes
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 53
Chapter 3
333 Impurity profiling of intact PETNSeveral intact PETN samples were analyzed to investigate variations in impurity profiles
of intact material and to determine whether profiles from residues arising from handling
intact PETN could be differentiated from post-explosion profiles Intact PETN samples
were obtained from improvised sources M75 and M93 hand grenades and PEP 500
plastic explosives M75 and M93 hand-grenade and PEP 500 samples have presumably
been produced at least 20 years ago3 All samples contained PETriN whereas PEDiN
was detected in 6 of the 17 samples Peak-area ratios of PETriN and PEDiN relative to
PETN are shown in Fig 36 In some of the samples DiPEHN and TriPEON were also
detected at very low levels By comparing Figs 34 and 35 it is clear that although
PETriN and PEDiN were observed in intact PETN samples the peak-area ratios were
much lower than those observed in post-explosion residues This was also the case for
the three home-made PETN samples for which a PETriNPETN peak-area ratio was
observed that was 20ndash100 times lower than that observed in samples from the explosion
experiments
0
0004
0008
0012
0016
002
1 2 3 4 5 6 7M
93 1a 1b 2a 2b 1 2hm
1hm
2hm
3
M75 PEP500 PETN
Peak
are
a re
lativ
e to
PET
N PETriNPEDiN
Fig 36 Presence of PETriN and PEDiN in different intact PETN samples originating from M75 or M93 hand grenades or PEP500 plastic explosive For PEP500 blocks samples were taken both from the inside of the block (a) as well as from the exterior surface (b)
3 All intact PETN samples were obtained from reference material from NFI casework The military explosives originate from former Yugoslavia and batch markings give indications on the original production date
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
54 Chapter 3
Cha
pter
3
RT 000 - 1800 SM 7B
0 5 10 15Time (min)
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
0
20
40
60
80
100
Rel
ativ
e Ab
unda
nce
143735098276
116230599774
59526101331
116130599768594
26101309
33321602777
Waste
a
b
Fig 37 Extracted-ion chromatograms (mz 3509833 3059982 2610131 and 2160281) of PETN stored on acrylic at 60degC for 12 weeks The diluted sample predominantly shows the presence of PETN (tR 1437 mz 351) and minor degradation peaks (a) The undiluted sample clearly shows the presence of PETriN (tR 1164 mz 306) PEDiN (tR
594 mz 261) and PEMN (tR 331 mz 216) (b) PETN is not detected in the latter example because the LC effluent was diverted to waste to prevent contamination of the mass spectrometer
334 Natural degradation of PETNTo simulate natural degradation PETN was applied on different matrices (acrylic
cotton and polyester) and these were stored under different conditions ie room
temperature (20degC) without humidity control room temperature with a high relative
humidity (90) and high temperature (60degC) without humidity control Samples stored
at room temperature and in the dark for 12 weeks did not show significant degradation
Only very low levels of PETriN were detected with a PETriNPETN peak-area ratio
of 00021 plusmn 00014 The detected PETriN was however not formed by degradation but
originated from a minor impurity in the original intact PETN Increased humidity to
promote hydrolysis of the ester-bonds showed the formation of PEDiN and PEMN but
only at very low levels However the PETriNPETN peak-area ratio did not increase
significantly in comparison with storage at room temperature as shown in Table 33
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 55
Chapter 3
As the storage temperature was increased to 60degC to accelerate PETN decomposition
substantial degradation was observed PETriN PEDiN and PEMN were detected in all
undiluted 60degC-samples although the PETN content had to be measured after dilution
because of the large difference between the PETN concentration and the concentrations
of the degradation products (Fig 37)
Table 33 Overview of the peak areas of the degradation products of PETN relative to PETN for different degradation environments
PETriN PEDiN PEMNPost-explosion 039 (SD = 019) 016 (SD = 018) 0052 (SD = 0098)Intact PETN 00081 (SD = 00046) 000068 (SD = 000031) -Natural degradation (20degC)
00021 (SD = 00014) - -
Natural degradation (60degC)
0014 (SD = 00051) 00092 (SD = 00091) 00015 (SD = 00019)
Natural degradation (humidity 90)
00025 (SD = 000099) 3110-4 (SD = 1510-4) 7610-5 (SD = 2810-5)
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a b a b a bs l s l s l s l2 weeks 4 weeks 8 weeks 12 weeks
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 38 Degradation of PETN on acrylic at 60degC sampled at different time intervals PETN was applied both as a solid and from solution for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
Sampling at different time intervals within a period of 12 weeks showed a gradual
increase in relative concentrations of degradation products (Fig 38) PETN applied
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
56 Chapter 3
Cha
pter
3
from solution shows an increased degradation rate in comparison with PETN applied
as a solid This can be explained by the increased contact area between PETN and the
acrylic matrix This effect was also observed for degradation on polyester and especially
on cotton fabric as illustrated in Fig 39 By including PETN reference samples (without
matrix in its solid form and from solution) it was shown that significant degradation
of PETN at elevated temperature only occurred in the presence of textile The extent
of degradation varies for the different types of textile and it is strongest on acrylic
Although significant PETN degradation on fabric is observed at elevated temperatures it
should be noted that the extent of degradation as expressed in the peak-area ratio versus
PETN was still much lower than the ratios observed in the explosion experiments By
comparing Fig 39 with Fig 35 it can be seen that the PETriNPETN peak-area ratio
was roughly 10ndash50 times lower than the ratio observed in the samples from the explosion
experiments
0000
0005
0010
0015
0020
0025
0030
a b a b a b a b a b a bs l s l s l s lRef Cotton Polyester Acrylic
Peak
are
a re
lativ
e to
PET
N PETriNPEDiNPEMN
Fig 39 Degradation of PETN on different matrix types at 60degC after 12 weeks of storage PETN was applied both as a solid (lsquosrsquo) and from solution (lsquolrsquo) for more homogeneous application lsquoarsquo and lsquobrsquo represent duplicate experiments
34 Discussion
Below the results are discussed in the light of the previously formulated hypotheses
Referring to the Bayesian framework to assess the evidential value the evidence is the
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 57
Chapter 3
observed PETN chemical profile and more specifically the measured peak-area ratios of
the degradation products versus PETN
341 Hp The observed PETN degradation products on the suspectrsquos clothing originate from a PETN explosionThe explosion experiments conducted in this study have shown that the relatively
high levels of PETN degradation products as observed in the actual case described
in the introduction are consistently found in post-explosion residues This creates the
possibility to use PETN profiling to constitute evidence for the presence of a suspect
andor an object at a PETN explosion site However the same experiments also indicate
that substantial variations in absolute amounts and peak-area ratios are observed
between different explosion experiments and even between the different samples taken
within one explosion experiment The reason for these variations lies in the intrinsically
uncontrolled nature of explosions It is clear that the presented methodology does not
allow discrimination between different explosion events Additionally it should be
considered that post-explosion profiles may change when other factors are varied such
as the confinement or the sampling distance Since especially in non-ideal detonations
changes in pressure and temperature profiles can be expected if the charge is confined
[2627] even higher ratios of degradation products could be created In addition
confinement can result in an explosion that is more complete yielding more gaseous
products and lower amounts of solid residue Because of the labour-intensive nature of
the explosion experiments and the need for controlled sampling conditions the effect of
the explosion configuration on the PETN profile was not investigated
342 Hd1 The observed PETN degradation products on the suspectrsquos clothing were present as impurities in the handled intact PETNAnalysis of a variety of intact PETN samples revealed only low levels of impurities
Although PETriN was detected in all samples the PETriNPETN peak-area ratios were
much lower than the values observed after the explosion experiments With the limited
number of PETN samples studied in this work it cannot be excluded that intact PETN
samples exist that contain higher levels of PETriN PEDiN and PEMN However the
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
58 Chapter 3
Cha
pter
3
low PETriN peak-area ratios in the three home-made PETN samples in combination
with existing knowledge on the limited formation of PETriN during PETN synthesis
[1] indicate that it is not likely to encounter PETN samples that contain such high levels
of impurities as to be comparable with the levels of degradation products observed in
post-explosion residues
343 Hd2 The observed PETN degradation products on the suspectrsquos clothing were formed by chemical and environmental degradation of PETNPETN did not show significant degradation on fabric at room temperature and even
at high humidity only low PETriN levels were detected The highest relative ratios of
PETriN PEDiN and PEMN in case of natural degradation were observed after storage at
an elevated temperature of 60degC Accelerated ageing at 60degC for 12 weeks corresponds
according to Eq (31) using the lowest reported activation energy of 125 kJmol to
118 years at room temperature indicating that this is an extreme condition to force
degradation The fact that under these conditions still only relatively low relative peak
areas were observed for the PETN degradation products confirms the reported stability
of PETN [1-4] Although this study indicates that it is impossible to achieve peak-area
ratios similar to those observed in the PETN explosion residues through (accelerated)
degradation effects of UV radiation pH strong oxidationreduction conditions
metal-catalyzed chemical degradation and biodegradation through various types of
microorganisms were not studied in this work Some of these conditions have shown to
enhance the degradation of PETN as described in section 31 but the conclusions from
those studies [13511141517] were not based on the area ratios of the degradation
products and therefore it cannot be excluded that certain conditions show fast and very
substantial degradation When the method is applied in a specific case it is therefore
recommended to establish the environmental conditions under which the PETN residue
was formed and to conduct a stability check under these conditions
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 59
Chapter 3
344 Comparison of the different hypothesesFig 310 shows that for typical post-explosion PETN profiles much higher relative
PETriN PEDiN and PEMN concentrations is found than in profiles of intact PETN
even after prolonged storage of residues on fabric at elevated temperature This effect
outweighs the variation observed in the explosion experiments The lowest peak-area
ratios obtained in the explosion experiments are still substantially higher than the highest
peak-area ratios measured for intact PETN and the PETN degradation experiments
This indicates that when a PETN profile is obtained with a peak-area ratio that falls in
the range reported for the explosion experiments this implies support for the hypothesis
that the residue originates from a PETN explosion
00
02
04
06
08
10
4b 2a M75 (2) PEP(2a) Acrylic CottonPost-explosion Intact PETN Degr (60degC)
Peak
are
a re
lativ
e to
PET
N
PETriN
PEDiN
PEMN
000
001
002
003
Fig 310 PETN chemical profiles observed under different hypotheses For each hypothesis the profile with the highest PETriNPETN response ratio is shown as well as the lowest The insert shows an enlargement of the profiles resulting from intact PETN and natural degradation at 60degC
As described in section 331 variations in measured peak-area ratios were observed
over time To examine whether these compound-specific variations would influence
the discrimination between post-explosion and natural-degradation (60degC) samples a
two-sample t-test was conducted PETriNPETN ratios measured in natural degradation
samples on different time intervals (n = 5) were incorporated in this test to account for
variations in peak-area ratios over time
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
60 Chapter 3
Cha
pter
3
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(32)
Where X1 and X2 are the mean PETriNPETN ratios for post-explosion PETN and
natural-degradation samples respectively sX12 is the variance of the post-explosion
samples sX a22 the pooled variance of the repeated measurements on natural degradation
samples and sX b22 the variance of the individual naturally degraded PETN samples The
number of post-explosion samples natural degradation samples that were repeatedly
analyzed and the number of individual natural degradation samples are represented by
n1 n2a and n2b respectively The tobserved collects both the sensitivity due to the difference
between the two groups as well as the extra variability introduced by analysis of natural
degradation samples on different time intervals A detailed explanation of the t-test
used here is provided in the appendix (section 36) The t-test showed that despite the
variations in peak-area ratios there is a significant difference between PETriNPETN
peak-area ratios for post-explosion samples (X1 = 039) and natural degradation (X2 =
0015) with tobserved (14) = 75 and p = 5middot10-6
To quantify the associated evidential value likelihood ratios (ie LR values) can be
obtained by establishing density distributions obtained under each hypothesis based
on the data generated in this study Such distributions based on the PETriNPETN
peak-area ratios are shown in Fig 311 This figure illustrates complete separation of
the distribution of Hp from Hd1 and Hd2 despite the very broad distribution obtained
for Hp due to the variation observed for the explosion experiments Statistical analysis
indicates that more data are required to accurately fit the distributions and reliably
calculate the associated likelihood ratios for a given PETriNPETN peak-area ratio
Contrary to for instance illicit drugs generating sufficient data is a cumbersome and
difficult task in the field of forensic explosives analysis The criminal use of organic
explosives such as PETN on a national and even international level is relatively rare
and hence forensic institutes generally have only limited sets of reference samples
Additionally explosion and degradation experiments are time consuming labour
intensive and require extensive preparations It is therefore recommended that samples
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 61
Chapter 3
from post-explosion PETN casework in the future will be analyzed with the method
described in this chapter to enlarge the PETN post-explosion profiling data set When
sufficient data are generated to allow the accurate modeling of the post-explosion profile
frequency distribution LR values for a given residue profile can be established Despite
the fact that this study does not allow accurate determination of likelihood ratios and
that additional data cannot easily be obtained it is still possible to assess the evidential
value in a qualitative manner by using a verbal scale4 The results of this study will thus
form the basis for future probabilistic conclusions in cases where the aforementioned
hypotheses are relevant
0
5
10
15
20
25
30
0
20
40
60
80
100
00 02 04 06 08 10
Rel
fre
q d
ensi
ty (p
ost-e
xpl)
Rel
fre
q d
ensi
ty (I
ntac
t 60
degC)
PETriN response normalized to PETN
Intact PETN
Natural degradation (60degC)
Post-explosion
0 005
Fig 311 Distribution of PETriNPETN peak-area ratios obtained under Hp (post-explosion) Hd1 (intact PETN) and Hd2 (natural degradation) For Hd2 results from degradation for 12 weeks at 60degC on textile were used because this showed most progressive degradation The relative-frequency density is the frequency normalized to the number of data points in the population (n = 14 for post-explosion n = 17 for intact PETN and n = 12 for natural degradation) divided by the bin size
4 The Bayesian verbal framework used at the NFI for reporting evidence for which a quantitative LR value cannot be established is based on the following scale [28]ldquoThe findings of the investigation areequally probableslightly more probablemore probablemuch more probablevery much more probablewhen Hypothesis 1 is true than when Hypothesis 2 is truerdquo
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
62 Chapter 3
Cha
pter
3
35 Conclusions
Overall it can be concluded that PETN chemical profiling using the LCndashMS method
presented in this chapter can yield convincing evidence with respect to the question
whether suspects or objects have been present at a PETN-explosion site This
differentiation can be made based on the ratios of the degradation products (PETriN
PEDiN and PEMN) relative to PETN This research shows that in post-explosion
PETN samples much higher relative concentrations of the degradation products can
be detected than in naturally degraded PETN samples or intact PETN Fully separated
relative-frequency-density distributions for the PETriNPETN ratio were obtained
under the different hypotheses (post-explosion vs natural degradation or intact PETN)
Even extreme conditions such as 12 weeks of storage at 60degC did not create an impurity
profile similar to the profile obtained after an explosion Therefore the impurity profile
obtained in casework can be used as valuable evidence when investigating a relationship
between a suspect and a PETN post-explosion site Because more data are required to
reliably calculate likelihood ratios it is recommended that samples from post-explosion
PETN casework are analyzed using the described method The method does not allow
differentiation of different PETN explosion events and also does not yield information on
the moment of presence (eg during or after the explosion) When applying the method
the conditions under which the residues have been created and maintained needs to be
carefully examined to ensure that minimal PETN degradation still applies in line with
the results of this study
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 63
Chapter 3
36 Appendix
Description of the two-sample t-test
The t-test was conducted on PETriNPETN ratios of 14 PETN post-explosion samples
(X1) and 12 natural degradation samples (X2) Three of the natural degradation samples
were analyzed multiple times (n2r = 5) to test the repeatability
tobservedX X
nsn
sn
sX X a
a
X b
b
= minus
+ +
1 2
12
1
22
2
22
2
(33)
This equation comes from applying error propagation on the variance of the difference
between 1 and 2
sX XX
sX XX
sX X X X1 2 1 2
2 1 2
1
2
2 1 2
2
2
minus( ) =part minus( )
part
+part minus( )
part
22
Where
part minus( )part
=partpart
= =
X XX
s XX
s snsX X X X
1 2
1
2
2 1
1
22 2
1
21 1 1 1
1
And
part minus( )part
=partpart
= = +
X XX
s XX
s snsX X X
aX a
1 2
2
2
2 2
2
22 2
2
22 2 2 2
1 112
22ns
bX b
The variances of the repeated measurements of the naturally-degraded PETN samples
were calculated and tested for their homogeneity using Levenersquos test F (212) = 115 and
p = 035 Because of their homogeneity the variances were pooled resulting in sX a22
The pooled variance was significantly different from variance of the individual natural
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
64 Chapter 3
Cha
pter
3
degradation samples (sX b22) with F (812) = 384 and p = 002 Therefore the variances
sX a22 and sX b2
2 cannot be pooled
X1 and X2 are the mean PETriNPETN ratios observed for post-explosion samples
and naturally degraded PETN respectively X2is composed of the 9 individual natural
degradation samples and the means of the 3 natural degradation samples
sX 22 is the variance in the natural degradation sample set and is composed of the variance
in the repeated measurements (sX a22) as well as the variance in the individual natural
degradation samples (sX b22)
The degrees of freedom associated with the t-test were calculated using the Welch-
Satterthwaite equation
νR
sn
sn
sn
snn
sn
X X a
a
X b
b
X X a
a
=+ +( )+
minus
12
1
22
2
22
2
12
1
2
1
22
2
1
minus+
2
22
2
2
2 1νpooled
X b
b
b
snn
(34)
Where νpooled represents the degrees of freedom in the data set with repeated measurements
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
PETN profiling in post-explosion residues 65
Chapter 3
References[1] T Urbanski Chemistry and Technology of Explosives Vol 2 Pergamon Press Oxford 1964
[2] MF Foltz Aging of Pentaerythritol Tetranitrate (PETN) LLNL-TR-415057 2009
[3] PS Makashir EM Kurian Propellants Explos Pyrotech 24 (1999) 260-265
[4] DM Chambers Perspectives on Pentaerythritol Tetranitrate (PETN) Decomposition URCL-ID-148956 2002
[5] L Zhuang L Gui RW Gillham Environ Sci Technol 42 (2008) 4534-4539
[6] J Yinon Toxicity and Metabolism of Explosives CRC Press Boca Raton 1990
[7] HI Russek Am J Med Sci 252 (1966) 9-20
[8] A Basch Y Margalit S Abramovich-Bar Y Bamberger D Daphna T Tamiri S Zitrin J Energ Mater 4 (1986) 77-91
[9] MA Hiskey KR Brower JC Oxley J Phys Chem 95 (1991) 3955-3960
[10] WL Ng JE Field HM Hauser J Chem Soc Perkin Trans 2 (1976) 637-639
[11] T Shepodd R Behrens D Anex D MillerK Anderson Degradation chemistry of PETN and its homologues SAND--97-8684C 1997
[12] HN Volltrauer J Hazard Mater 5 (1982) 353-357
[13] FJ DiCarlo JM Hartigan GE Phillips Anal Chem 36 (1964) 2301-2303
[14] PR Binks CE French S Nicklin NC Bruce Appl Environ Microbiol 62 (1996) 1214-1219
[15] GF White JR Snape J Gen Microbiol 139 (1993) 1947-1957
[16] TM Wendt JH Cornell AM Kaplan Appl Environ Microbiol 36 (1978) 693-699
[17] DL Kaplan Curr Opin Biotechnol 3 (1992) 253-260
[18] FW DuBoisJF Baytos Weathering of explosives for twenty years LA-11931 UC-741 1991
[19] SK Yasuda J Chromatogr A 51 (1970) 253-260
[20] C Aitken F Taroni Statistics and the Evaluation of Evidence for Forensic Scientists 2nd ed Wiley Chichester 2004
[21] B Robertson GA Vignaux Interpreting evidence evaluating forensic science in the courtroom Wiley Chichester 1995
[22] KA Connors Chemical Kinetics The Study of Reaction Rates in Solution VCH New York 1990
[23] CF Forney DG Brandl Horttechnology 2 (1992) 52-54
[24] X Xu M Koeberg C Kuijpers E Kok Sci Justice 54 (2014) 3-21
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx
66 Chapter 3
Cha
pter
3
[25] S Zitrin T Tamiri S Tamiri Analysis of Explosives by Infrared Spectrometry in Beveridge A (Ed) Forensic Investigation of Explosions CRC Press Boca Raton FL 2011
[26] J Akhavan The Chemistry of Explosives RSC Cambridge 2004
[27] MA Cook The Science of High Explosives Reinhold Pub Corp New York 1958
[28] The NFI series of verbal probability terms and the Bayesian framework for the interpretation of evidence 2008 Original title Vakbijlage De reeks waarschijnlijkheidstermen van het NFI en het Bayesiaanse model voor interpretatie van bewijs available on httpforensischinstituutnlkenniscentrumpublicatiesvakbijlagenindexaspx