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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

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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

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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

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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

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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

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[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