Introduction to Arson Analysis

Introduction to Arson Analysis

Arson Analysis by Gas Chromatography/Mass Spectrometry

Denver Police Crime LaboratoryWilliam D. McDougall IIForensic Analyst (Retired)

Original PresentationSeptember 2004

[email protected]

(On the Web at www.denvergov.org/Crime_lab/ est. 1-?-05/ 3-10-06)

The original presentation was also found as a reference link at theAAFS and at Zeno’s Forensic Science Site est. 3-30-05/3-10-06

Updated 4-15-09 for an arson homicide trial

An abbreviated version of this PowerPoint presentation was given to a joint Denver Fire Department (Arson Bureau) and BATF Seminar for fire investigators (September 2004). This slide presentation has been expanded and will be upgraded as needed.

References and Credits

A number of the graphic slides and annotations were obtained from the Internet. Many of the annotations and graphic displays have been altered (and added to), to highlight the present topic. I have displayed these slides in a progressive manner of complexity and I have tried to bridge slides of similar material so that it is easy to compare the material on different slides.

Most of the mass data displays were obtained from a Varian Saturn 2000 Ion Trap MassSpectrometer and a HP 5973 Mass Spectrometer in the Denver Police Crime Laboratory.

M. Jennifer Thomas, Forensic Chemist, generated most of the mass data displays obtainedfrom the HP 5973 Mass Spectrometer.

This HP GC/MSD is comprised of a HP 6890 GC

interfaced to a HP 5972 Mass Selective Detector Quadrupole Mass

Spectrometer. The system uses electron impact (EI)

ionization and is capable of performing full mass scans or selective ion monitoring (SIM).

Gas Chromatograph-Ion Trap Mass Spectrometer (MS/MS)

Gas Chromatograph

Mass Spectrometer

Column Oven

Control Panel

Ion Trap

Injector >

Chromatography

“chromato-graphy” means color writing

It is a physical process of separating complex mixtures

THIN LAYER CHROMATOGRAPHY

COLUMN CHROMATOGRAPHY

THE MORE POLAR COMPOUNDS MIGRATE SLOWER THAN LESS POLAR COMPOUNDS. NON-POLAR LIQUID MOBILE PHASES ARE USED IN THE MIGRATION OF LESS POLAR COMPOUNDS AND MEDIUM POLAR LIQUID MOBILE PHASES ARE USED IN THE MIGRATION OF MORE POLAR COMPOUNDS. THE RIGHT MIX OF SOLVENTS ( NON-POLAR AND MEDIUM POLAR OR MODIFIER ) IS NEEDED FOR THE MIGRATION AND SPEPARATION OF ALL THE POLAR ANALYTES . NON-POLAR COMPOUNDS MIGRATE AHEAD ( ELUTE FIRST ) OF THE POLAR COMPOUNDS.

POLAR STATIONARYPHASE

POLAR STATIONARYPHASE

Interaction of theadsorptivity andsolubility of theanalytes relativeto the two phases

The liquid (solvent or solvents) is the mobile phase

(On plastic, glassor foil backing)

aluminum oxide Al2O3

(Less polar than silica)

Silica gel

SiO2.H2O

Normal Phase Chromatography

NON-POLAR

POLAR

MEDIUMPOLAR

POLAR

MEDIUMPOLAR

NON-POLAR

( NON-POLAR, MEDIUM POLAROR SOLVENT MIXTURE )

ADSORPTION CHROMATOGRAPHY

MODERN HPLC SYSTEM

THE STATIONARY PHASE ISHYDROPHOBIC (NON-POLAR).MODERATELY POLAR ANDNON-POLAR COMPOUNDS STARTTO MIGRATE THROUGH THE COLUMNAS THE MOBILE PHASE CHANGES FROMAQUEOUS TO LESS POLAR ORGANICSOLVENT.

REVERSED PHASE HPLC

CAN HAVE ONE OR MORE MOBILE PHASES

CAN HAVE MORE THAN ONE PUMP

High pKa (basic) compounds will have a large k’ (longerretention time) in basic buffered mobile phases. Low pKa (acidic) compounds will have a large k’ in acidic buffered mobile phases.(Solvent Programming)

Controlled substancesincluding “steroids” are excellent candidates for HPLC

API

M+1

(ESI)

Mass Spectra

Gas Chromatography –The sample mixture is injected and vaporized. Next the mixture is transported through the heated column by the flow of an inert, gaseous mobile phase. The column contains a thin coating of a liquid stationary phase which differentially retards the migration of the sample components.

(Separated Components)

After passing through the heated column, The separated sample components enter The detector causing an increase in the signal which is recorded as a peak for each component. The collection of peaks iscalled a chromatogram.The sample components

have to be in the vaporstate in order to passthrough the column.

http://www.shu.ac.uk/schools/sci/chem/tutorials/chrom/gaschrm.htm

(6)

< Pattern of peaks

Columns for arson analysis are normally 15 to 30 meters in length.

The outside dimensionof a capillary column isabout the size of a heavyfishing line.

The Heart Of The Gas Chromatograph

The Column

A .25mm ID columnis commonly usefor arson analysis

5% phenyl methylpolysiloxane is a relatively non-polar general purpose stationary liquid phase

HP-5

Non-retained Retained(Liquid stationary phase.25um thickness) Chromatogram

Analyte >

< Air

DYNAMIC PARTITIONING TAKES PLACE IN GAS(LIQUID) CHROMATOGRAPHY

High pKa (basic) compounds suchas cocaine and methamphetamineare found in higher concentrationsin the lower phase after equilibration.

< BASIC AQUEOUS PHASE

< CHLOROFORM PHASE

Static Partitioning

THE ANALYTES PARTITION BETWEENTHE STATIONARY LIQUID PHASE AND THE MOBILE GAS PHASE.

DYNAMIC PARTITIONING

C stationary phaseC mobile phaseK=

CAPILLARY COLUMNS HAVE MORE THEORETICAL PLATES THAN PACKED COLUMNS

MINIBORE COLUMNS HAVE EVEN MORE THEORETICAL PLATES PER METER

N= # of theoretical plates

The column is madeup of a large numberof partitioning (separatory)zones

Each partitioning zone equals onetheoretical plate

PACKED COLUMNS HAVE APPROX. 10,000 PLATES

CAPILLARY COLUMNS HAVE APPROX 100,000 PLATES

Carrier GasesNITROGEN, HELIUMAND HYDROGEN HAVEDIFFERENT SLOPES

(CAPILLARY COLUMNS ARE NOT AFFECTED BY EDDY DIFFUSION)

(CAPILLARY COLUMNS MINIMIZE THIS FACTOR)

HYDROGEN IS USED ATHIGHER FLOW RATES THAN HELIUM. NITROGENIS THE LEAST FAVORABLE.

(Used for packed columns)

“SELECTIVITY” IS DETERMINED BY THE STATIONARY PHASE CHEMISTRY (K’) IS OPTIMIZED BY TEMPERATURE PROGRAMMING

In order to double the resolution, the column length has to be increased by a factor of four

Resolution is proportional to the square root of (N), therefore thelength of the column

Resolution

Fire

Fire is a chemical reaction. It is rapidoxidation with the release of heat andLight (plus CO2 and water).

In order for the reaction (fire) to happen, there needs to be an ignition source(a spark or friction), oxygen (air) and a fuel source (such as ignitable liquids).

ACETONE

ETHANOL1-HEXENE

STYRENE

INDANE

METHANE HEXANE

METHYL-CYCLOHEXANE2-METHYLHEXANE

1-METHYLINDAN

Normal Alkane

Branched AlkaneCycloalkane

(Isoparaffin)

Straight Chain

Thermo-Decomposition

Thermo-DecompositionOxygenated Solvent

Found is gasohol and alcoholic drinks

Basic building blockof hydrocarbons

(6) Carbons(14) Hydrogens

< Found in Gasoline >

Structures from NIST 98 version 1.6

BENZENE TOLUENE

ETHYLBENZENE

1,2,4-TRIMETHYLBENZENE

C3 ALKYLBENZENE

C4 ALKYLBENZENE NAPHTHALENE

1-METHYLNAPHTHALENE

HYDROCARBONS FOUND IN FIRE DEBRIS (Some of these compounds are found in both gasoline and thermo-decomposition).

Basic building block of aromatic hydrocarboncompounds

(Reference Compound)

(6) Carbons(6) Hydrogens

Benzene ring plus a methyl group

Structures from NIST 98 version 1.6

Hydrocarbons are fuel for arson fires andnon-arson relatedfires

http://www.shu.ac.uk/schools/sci/chem/tutorials/chrom/gaschrm.htm

Each peak represents a compoundfrom the original sample mixture.

< Toluene

Ethylbenzene >

< o-xylene

< m/p-xylene

< Chromatograms

Time > > >

IGNITABLE LIQUIDMIXTURE ( Separated Components )

Styrene and o-xylenehave similar retentiontimes.

Each separated sample component is burnt in theflame causing an increasein the electrical signal.

Each time the signal is increasedit is recorded as a peak in thechromatogram.

Time > > >

< Toluene

THERMO-DECOMPOSITION

Ethylbenzene >

m/p-xylene

Styrene >

Toluene followed by the other analytes enter the detector one after another, after migrating through the column. (Note in both chromatograms m/p-xylene co-elution)

(PYROLYSIS PRODUCTS)

The analytes in these two chromatograms including styreneand o-xylene would superimposeUsing the same time axis.

Gas Chromatography is used for separating compounds in complex mixtures (gasoline, kerosene and etc). The sample mixture is introduced into the heated injection port and an inert gas flowing through the system carries the compounds into the column. The material within the column is called the stationary phase (non-polar stationary phases are best for arson analysis) and the various sample components interact with this material to a greater or lesser degree. The greater the interaction, the slower that particular compound will move through the column. As the various components begin to migrate through the column, they undergo a series of equilibrium steps between the stationary phase and the mobile phase (the carrier gas) so that the separation becomes more pronounced as compounds progress through the column. In the ideal situation (the column length, flow rate and temperature are appropriate for the sample mixture), the differences in interaction are sufficient to allow all the components in the sample to be completely separated (I.e., resolved) from each other. However, you should note that as the compounds migrate through the column, each chromatographic peak, representing a sample component, broadens withincreasing time in the column.

http://www.uga.edu/srel/AACES/GCtutorial/page1.html

Chromatogram

Time >>

( Separated Components )

The passage of three compounds through the column over time.

The retention (retention time) of a compound depends not only on the column length, the type of stationary phase and flow rate, but also on the column temperature. As the temperature increases, compounds move through the column faster. Thus, one can reduce the analysis time by increasing (temperature programming) the column temperature. In this run the column temperature is increased from 40 C to 240 C at 10 C/ min during the analysis. All of the normal alkane components are completely separated into narrow symmetrical peaks.

The retention time is the time a compoundspends in the column from the time of injection to the time of elution of thecompound (measured to the peak apex).

< Peak Apex

Time >>

< Time of Injection

(Time of Elution)

(Measured in Minutes)

Rt (Retention Time)http://www.uga.edu/srel/AACES/GCtutorial/page1.html

< n-C17 elutes before n-C18

TIC

AMU

AMU

Time >>

In addition to molecular weight,molecular structure is a factor determining the boiling points, thus the order of elution.

IN ORDER TO SEPARATE M-XYLENE AND P-XYLENE,A POLAR STATIONARY PHASE WOULD BE NEEDED. >

Additional factors determining elution order and selectivity

Note the different profiles or patterns of mountain peaks

Note the different profiles or patterns of peaks

Kerosene

Gasoline

Pattern recognition

Diesel Fuel

Chromatography peaks

Gas chromatography – A gas carries the mixture through a column. The column is coated with a thin layer of a semi-liquid phase. The liquid phase retards the mixture based on boiling points and molecular weight (mass). The lighter components pass through the column first and the heavier components pass through the column last. As the components leave the column, they are burnt and ionized in the flame of the detector. The increased ionization produces an electrical signal that is sent to a recorder and displayed as a profile of peaks or a chromatogram. Each peak in the chromatogram is identified by its retention time. The overall pattern (chromatogram) can be compared to standard chromatograms (gasoline, kerosene, diesel and etc.). If the unknown sample displays a strong signal without background interference then the above is all that is needed. But most of the time, the signal is weak and there is background interference. Furthermore, if the unknown is weathered (ignitable liquids evaporate) then the pattern is altered and more difficult to identify. Background interference at times, can be falsely reported as ignitable liquids. A more definitive detector is needed.

A mass spectrometer should be used to test fire debris cases. This type of detector produces mass spectra and extracted ion profiles in addition to chromatograms and retention times. The combination of all of these displays is a fingerprint for the identification of the ignitable liquid and the components in the ignitable liquid.

Summery of gas chromatography (FID) used in fire debris analysis

The Mass Detector

The mass detector used in arson analysis is typically a Quadrupole or Ion Trap Mass Spectrometer.

The preceding slides showed complex mixtures separated by chromatography. The separated components can be individually identified or class identified. But as shown in some of the previous slides, a number of the components were not separated. Peak co-elution is a frequent occurrence. There are a number of reasons including complexity of the mixture, the chemistry (chemical makeup of the stationary and /or mobile phases) of the separation process and the chemistry of the components in the mixture. The length of the column, temperature, the analysis time and other parameters affect the separation process. New advances in chromatography are improving the separation or isolation of components in complex mixtures resulting in reduced analysis time. Mass spectrometry carries out a second separation process. Mass spectrometry is used to ionize, fragment and filter the mixture component ions (after the mixture components pass through the column), producing mass spectra. Mass spectrometry computer algorithm techniques (Quadrupole and Ion Trap) use ion extraction algorithms to produce mass chromatograms. The algorithms isolate or extract ion information about the non-resolved and resolved (separated) components. Tandem mass spectrometry (MS/MS) uses multiple quadrupole mass filtering (MS/MS) or ion trapping (MS/MS in time). In addition to ionization and fragmentation, an ion pre-isolation process and collision-induced dissociation precedes a secondary ion separation (filtering) and mass spectra formulation. New computer algorithms coupled with fast scanning detectors, such as time of flight mass spectrometry (TOF) perform peak deconvolution. Peak deconvolution, peak find algorithms plus automated library search routines, are used to extract (isolate) mass spectra, identify and confirm the presence of multiple component co-eluting compounds, in complex mixtures.

Peak (component) Co-elution

http://ull.chemistry.uakron.edu/gcms/MS_detector/index.html

In addition to a profile of the separated components (chromatogram), the mass spectrometer produces a mass fragmentation pattern (mass spectrum) for each separated component (compound).

< Mass Spectra

Separated ComponentsMixture

(3)

(3)(3)

Pressurized Vacuum

Ionization &Fragmentation

In order to measure the mass ofa compound it has to be ionized. The mass to charge ratio is actually determined. Normally the charge is one. The molecularion is not stable using electronimpact ionization (EI) andundergoes fragmentation. Thefragmentation pattern is a displayof intensity versus mass of theremaining molecular ion andthe newly created fragment ions.

Ion Trap

Quadrupole

Electron impact ionization (EI) producespositive ions, negative ions and neutralspecies. But only the positive ions areanalyzed in this mode of analysis. The neutral species and molecules (notionized), are pumped away.

1 = H+ 16 = O+ 17 = [OH]+ 18 = [H2O]+

1=H+16=O+17=[OH]+18=[H2O]+

http://www.jeolusa.com/ms/docs/whatisms.html

Molecular ion >

The ions are scanned from the low masses to the high massesover time.

Quadrupole filter

AC and DCVaried voltages

Gas Chromatograph-Ion Trap Mass Spectrometer (MS/MS)

Gas Chromatograph

Mass SpectrometerColumn Oven

Control Panel

Ion Trap

Injector >

Column

The ions are scanned (separated) from the low masses to the high masses overtime. An increasing RF (AC)voltage is used to filterthe mass ions

Supplemental wave forms (end-cap electrodes) can be used for MS/MS enhanced CID of ions 91,105,134 & etc. found in trace gasoline containing large amounts background interference patterns.

Extracted ion profiles of gasoline

Toluene

Mass Spectrum

TolueneToluene

GasolinePattern of peaks

Chromatogram (TIC)

Aromatics

Indanes

Naphthalenes

AliphaticsTIC

Sample syringe

Mass Spectrometry Vacuum

A high vacuum is needed to prevent unwanted collisions between the analyte ions and gas molecules during the migration of the ionsfrom the source, through the analyzer and to the detector. The necessary “mean free path” is achieved with a vacuum of approximately 10

5 torr or less.

A mechanical low vacuum pump and a high vacuum pump (diffusion or turbo) are coupled to achieve the high vacuum.

The ion trap needs a partial vacuum (approximately 1 millitorr of helium) to dampen the kinetic energy of the ions in order tostabilize their trajectories. Collision-induced dissociation used in MS/MS also needs

increased gas pressure (a neutral gas).


The sample molecules are ionized, fragmented and trapped. The RF voltage is ramped and the ions from the lowest mass to the highest mass (with unit resolution), are ejected from the analyzer and detected bythe detector.

Separated components enter here >

(End-cap)

(Ring)

(End-cap)

(Electron impact ionization)


After the positive ions pass through the mass analyzer with unit resolution, they are detected as an amplified electrical signal.


For each positive ion that strikes the inner surface, a gain of approximately one million electrons is produced.

(Figure taken from www.ivv.fhg.de/ms/ms-analyzers.html#Ion_Trap

Ion Trap Analyzer

With additional wave form manipulations applied tothe End-cap electrodes, MS/MS can be used to furtheranalyze fire debris containing matrices.

(Electron impact ionization)

Showing the different electrical potentials applied to the ion trap

Advanced quadrupole mass spectrometry applications usetandem mass spectrometry in space or time. Multiple quadrupole mass filters in series or a single quadrupole ion trap massspectrometer (in time), are used to isolate one or more parent ions(MRM) which are subjected to “collision-induced dissociation”. The newly formed ions are called product ions. The product ions and remaining parent ions are then mass filtered to produce mass spectraand mass chromatograms (minus background interference). This application can be useful in arson analysis as a compliment to singlequadrupole mass spectrometry. This technique can remove extraartifact peaks found in extracted ion profiles (mass chromatograms)of ignitable liquids containing complex background interference.Tandem mass spectrometry is used in a number of applicationsincluding both gas chromatography/mass spectrometry and highperformance liquid chromatography/mass spectrometry.

Special Note!

(Ion Trap and Quadrupole Mass Filter)

(DC=0V for the Ion Trap)

(Ion trap)

(The Quadrupole Mass Filter uses both DC and RF voltages)

Waters Corporation

(For the Ion Trap)

These equations are used for both theQuadrupole Mass Filter and The Ion Trap

q z > 0.9

The AC frequency is about one megahertz and is calledthe fundamental RF. It is applied to the ring electrode.

The instability along the (RF) axis is expressed as q z

DC = 0V(RF)

(unstable)

(The frequency of the AC applied to the end-caps is approximately ½ of the fundamental RF)

q zOc RF m

q

a

q

Z

z

z

a z

The larger the RF voltagethe larger the

Z

The larger the massthe smaller the q z

q

Trajectory of a single ion >

|̂|

(When the secular frequency of an ion is equal to the end-caps’ frequency it undergoes resonance)

Resonance allows an ion to escape Before q z > 0.9

A constant RF voltage is used to trap the ions

DC

RF

(The RF voltage is ramped (> 6000 V) to destabilize the ion trajectories)

(The AC voltage applied to the end-caps is approximately 3 V) (Secular Frequency: frequency at which an ion oscillates in the trap)The secular frequency is dependent on the q z value

Also called axial modulation

Comparative diagram, roughly to scale, of a quadrupole mass filter and a cutaway view of a quadrupole ion trap mass analyzer.

http://www.abrf.org/ABRFNews/1996/September1996/sep96iontrap.html

Schematic of a quadrupole filter

Quadrupole mass spectrometers consist of an ion source, ion optics (lenses) to accelerate and focus the ions through an aperture into the quadrupole filter, the quadrupole filter itself with control voltage supplies, an exit aperture, an ion detector, detection electronics, and a high-vacuum system.

http://www.chem.vt.edu/chem-ed/ms/quadrupo.html

The ions are scanned (stabilized) from the lowest mass to the highest mass (with unit resolution) through the quadrupole filter and detected by the detector.

(RF voltage)

http://avogadro.chem.iastate.edu/CHEM577/CHEM577-C.pdf

RLC circuits are designed as high pass filters (high frequencies) and as low pass filters (lowfrequencies) in many types of applications.

The Quadrupole mass analyzer consists of four cylindrical rods onto which are applied both RF and DC electrical fields. These four rods are arranged in such a manner that they form one pair in the X plane, and one in the Y plane. As ions enter the Quadrupole, they begin to oscillate in both the X and Y planes, thus causing the lower m/e ions to be destabilized in the Quadrupole whenever the alternating (RF) component of the electric field exceeds the direct (DC) component. In this condition, the lower m/e ions will be thrown out of the Quadrupole and not reach the detector, thereby creating an effective high pass filter. If the direct component exceeds the alternating component then the high m/e ions become unstable, while the lower m/e will be stabilized by the presence of the alternating component making for an effective low pass filter. In the Quadrupole system, the mass analyzer is created by connecting the two pairs of rods in such a manner that the positive pair acts as a high pass filter and the negative pair acts as a low pass filter. By carefully matching the two fields, only ions of a particular mass are able to resonate at the correct frequency allowing them to pass through the Quadrupole at any time. In this regard, the Quadrupole mass analyzer is a very fast and efficient system.

http://www.elementalanalysis.com/icp/

DC and RF voltages

DC RF

http://www.chm.uri.edu/chm412/Chap11.ppt

(+) Rods

(-) Rods

+ DC Potential

RF Potential

- DC Potential

RF Potential(Light ions pass through the rods)

The proper ratio of DCand RF potentials resultsin unit resolution.

http://www.chm.uri.edu/chm412/Chap11.ppt

http://avogadro.chem.iastate.edu/CHEM577/CHEM577-C.pdf

Positive Ions

>

Heavy ions pass through the rods

( Light ions are destabilized and lost.)

DC >RF Heavy ions drift into the negative rods.

RF >DC

Band Filter

The AC voltage has a frequency in the radio frequency range (RF)

(RF)

RF>DC (+) Rods

DC>RF (-) Rods

Become a narrow band filter allowing ions of the same mass/charge to pass through to the detector.

The combination of the high pass filtering rods and the low pass filteringrods produce a narrow band filter. With the proper DC to RF ratio, unit mass resolution is achieved.

The fixed DC/RF ratio is ramped (DC and RF voltages are increased linearly), allowing all of the ions starting with low mass to high mass (with unit resolution), to pass through the rods to the detector over time. One scan in approx ½ second.

High pass filter

Low pass filter

High and low pass filters combined

Small ion

Large ion

(RF)

The ratio of the DC and RF voltages is adjusted to produce unit massresolution. The magnitudes of the two voltages (in a fixed ratio)are ramped through the entire mass range.

A RLC circuit is used as a tuner for old style radios. When the resonance frequency of a radio tuning circuit matches the broadcast frequency of a radio station, the circuit is “tuned in” to that radio station. Variable capacitors and inductors are used to make the adjustment to the circuit to maximize the signal.

http://www.kineticbooks.com/physics/17296/17329/sp.html

A calibration gas is used to tune and calibrate the mass spectrometer. Masses (69,219 & 502)are tuned by choosing the proper DC to RF ratio to achieve unit resolution (scan-line). Next the unitresolved mass peaks are assigned masses from a calibration table.

Tune and CalibrationIn addition to unit resolutionand mass calibration, relative intensities are obtained. Librarymass spectral searches of unknownmass spectra are now possible.

From NIST 98 version 1.6

Unitresolution<

v

The relative intensity of the molecular ion found in different classes of hydrocarbons.

http://194.94.42.12/licensed_materials/00897/papers/0007004/740226ww.htm

Electron Impact Ionization (70 electron volts)

The energy needed to break the bonds in the above classes of hydrocarbons is much less than70 electron volts. However, to maximize total ion current sensitivity and produce ion fragmentation patterns that are library searchable, 70 electron volts are the standard for electron impact ionization.

The below ion fragmentation patterns are a function of molecular structure

(Approximately 5 eV will break a carbon, carbon single bond)


Toluene

Mass Spectrum

Mass Filter

Next the components are ionized by electron impact (EI)and undergo fragmentation.

The ions are scanned (filtered) from the lowest mass to the highest mass with unit resolution. The ions strike the detector and are recorded as a mass spectrum.

The ramp display to the left is for thepositive set of rods. The negative setof rods have an equal negative DCslope and a RF 180 degrees out of phase.

~

< Toluene

Separated Components

http://chipo.chem.uic.edu/web1/ocol/spec/MS1.htm

>

Separated by chromatography

Filament

Lenses accelerate and focus the ions ^

The mass spectrum of toluene (methyl benzene) is shown below. The spectrum displays a strong 92 molecular ion, a 91 base peak and an assortment of minor peaks m/z = 65 and below (fragmentation ions).

http://chipo.chem.uic.edu/web1/ocol/spec/MS1.htm

Toluene

--------------------------------

The mass spectrum is a graph of intensity versus mass/charge (m/z). The most intense ion in the mass spectrum is called the base peak (displayed as 100%) and the other fragmentation ions are displayed relative to the base peak. The highest molecular weight ion in the mass spectrum normally represents the parent molecule (with an electron removed) and is called the molecular ion (M+). Note below, as the alkyl side chain increases, the molecular ion decreases in relative intensity (see toluene in the last slide). http://chipo.chem.uic.edu/web1/ocol/spec/MS1.htm

http://science.csustan.edu/tutorial/mass/ethyben5.htm

Ethylbenzene

Ethylbenzene

n-C10 (Decane)

Decane is a straight chain hydrocarbonwith no ring structure. The molecular ionhas minimal relative intensity and the base peak is of comparatively low mass.

TIC

The next set of slides show different types of non-aromatic hydrocarbons found in a Light Petroleum Distillate. Notethat the Light Petroleum Distillate is compared to a gasolinereference.

TIC

TIC

( Light De-Aromatized Distillate )

Heptane n-C7

Note the ions 43, 57 and 71

Normal Alkane

Methylcyclohexane

Note the ions 39, 55, 67(& 69)and 83 Cycloalkane

Octane (n-C8)

Note the ions 43, 57, 71 and 85

Normal Alkane

Ethylcyclohexane

Note the ions 39, 55, 67 and 83

Cycloalkane

The next slide is a mixture of Polystyrene and Polypropylene Decomposition

The ions found in an alkene are displayed. Note that most of the ions are the same as found in a cycloalkane. The cycloalkanes are associated with ignitable liquidsand the alkenes are normally associated with backgroundinterference matrices.

Alkene

Polystyrene and Polypropylene Decomposition

TIC

The next set of slides are used to identify an unknown mixture

Molecular weights, mass spectra, ion averaging, library searches, retention times and references are used in the identification of the unknown. In addition, the overall pattern of the mixture isimportant. Are there any normal alkanes present (intense equallyspaced peaks)? Is the mixture an Isoparaffin Product or a Naphthenic Paraffinic Product?

Ion summed extracted ion profiling is also routinely used. But to illustrate how to differentiate between branched alkanes (Isoparaffins) and normal alkanes, thefollowing slides are highlighted.

Unknown Mixture

No Apparent Equally Spaced Intense Peaks

Not a normal alkane mixture?

Unknown Mixture

Based on the position of the unknown mixture to the reference peak (1,2,4-Trimethylbenzene) found in gasoline, the mixture is a medium range product.

< (n-C10 co-elutes)

Unknown Mixture

Ion averaging of all the ions in the TIC

Note Ratio

Peak ? >

< Ratio of ion 57 to ion 43 is found in isoparaffins

TIC

Virtually no aromatic ions (91,105,119,134)

No dominant cycloalkane ions (55,69,83,97)

Not a Naphthenic Paraffinic Product

(Isoparaffin Product?) (Normal Alkanes??)

v

^

||

(Medium Range Product)

||The mixture is composed of compounds containingions 43, 57, 71, 85, 99 and 113. These ions arefound in both isoparaffins and normal alkanes.

Unknown Mixture

Retention Time of 7.97 min

Note Ratio

TIC

Has the ion ratio of a branched alkane (isoparaffin) andhas a molecular ion of 156 Daltons

< Normal Alkane ?

The normal alkane “Undecane” (n-C11) also has a molecular ion of 156 Daltons

(Medium Range Product)

Mass Spectrum of Peak With Retention Time of 7.97 min

<

(Straight Chain Hydrocarbon)

Branched Alkane (Isoparaffin) (molecular ion of 156 Daltons)

(Retention Time of 7.97 min)

Library Search Results Of The Above Mass Spectrum

Appears to be an Isoparaffin !

Mass Spectrum Of Peak From the Unknown Mixture

TIC

n-C11 (Undecane)

Retention Time 10.334 min

Medium Petroleum Distillate (Normal alkane mixture plus smaller amounts of aromatics, isoparaffins and cycloalkanes)

Note Ratio

< n-C11Mixture contains n-C11

Reference

The normal alkane n-C11 (Undecane) has a molecular ion of 156 Daltons

Comparison of the unknown mixture in the previous slides to a mixture containing n-alkanes

Note! equally spaced peaks >

< n-C10

n-C12 >

<

Unknown

Medium Isoparaffin Product?

Medium Petroleum Distillate

TIC

TIC

Undecane (n-C11)

Branched Alkane (Isoparaffin)Ion 41

Ion 57

Ion 43 Ion 57

Ion 156

Ion 156



Molecular ion

Molecular ion

< n-C11

Peak ? >

7185

99

71

85

99

Questionable peak and Undecane have different retention times.

Reference

Unknown

In summary, the unknown is a Medium Isoparaffinic Product.


Reference

The following set of slides illustrate additional ways ofdisplaying an ignitable liquid using a mass spectrometer.Since gasoline is often encountered in arson cases it hasbeen used for this demonstration.

Ion Trap Mass SpectrometerTIC ( Total Ion Chromatogram )

The gasoline pattern in the last slide represents a very strong signal with no background interference or extreme weathering. Mass Spectrometry or a Flame Ionization Detector (FID) could have been used to analyze this particular sample. Heated head space sampling of high concentration unknowns (light range mixtures, medium range mixtures and gasoline) with minimal background interference or extreme weathering can at times be analyzed by FID. However, mixtures that cannot be easily identified by pattern recognition should be analyzed by mass spectrometry. In addition, sampling by activated charcoal strips (passive adsorption) increases the possibility of detecting trace ignitable liquids but also increases background interference patterns. Therefore, the following mass data display techniques are strongly recommended for most fire debris samples.

TIC

Extracted ion profiles

Extracted ion profiles are electronically (by computer) simplified mass chromatograms. By using extracted ion profiles it is possible to isolate classes of organic compounds. The isolated classes are groups of isomers or chemically similar compounds found in ignitable liquids. Extracted ion profiles are paramount in isolating ignitable liquids from background interference.

Ions: 91+105+119+133

Ions: 117+132

Ions: 128+142+156

Ions: 55+69+83+97+111

Ions: 57+71+85+99

TIC

1,2,4-TRIMETHYLBENZENE

Library Search Results

Weathered Gasoline

The C3, C4 and C5 alkylbenzenes should be present inan unknown sample to be considered gasoline. Weatheringcan alter their relative concentrations. Additionally, thenaphthalenes, indanes, aliphatics and cycloalkanes found ingasoline should also be present. Their relative concentrationsare also modified by weathering.

TIC

TIC

70% Evaporated Gasoline

TIC

TIC


< 1,2,4-TRIMETHYLBENZENE

TIC

TIC


Because ignitable liquids are composed of light (flammable) and/or heavier combustible components, the composition can change when exposed to fire or air. The lighter components evaporate first, followed by the heavier components. In a chromatogram of weathered gasoline, the lighter components (peaks) in the front end (on the left side) are the first to dissipate or disappear. By the time gasoline is 99% evaporated, most of the components are missing.

According to the MSDS (MATERIAL SAFETY DATA SHEET) for gasoline, washing with soap and water will remove gasoline from skin exposed to gasoline.

Weathered Patterns of Ignitable Liquids

The following slides are simplified examples of the classification of the different types of Ignitable liquids. Ignitable liquids are liquid accelerants (they fuel and enhance the flame). There are literally hundreds of commercial products containing Ignitable liquids (flammable and combustible ranging from the light range to the heavy range). Ignitable liquids can be as simple as acetone (finger nail polish remover), wood alcohol(methanol) and ethanol ((found in beer, gasohol and 85% ethanol fuel (E85)). They can be found in lighter fluids (Zippo and Ronsonol), camp fuels (Colman fuel), charcoal starters, gasoline, kerosene (used in jet airliners), diesel (used in diesel trucks and cars) and home heating oils. They range in ability to ignite from the light range (flammable) through the middle range (combustible) to the heavy range.

Light Petroleum Distillate

ClassificationThere are many commercial solvents and solvent mixtures available tothe general public. Ignitable liquid classification schemes have been developed to group (classify) the various solvents and solvent mixtures. In addition, (if a mixture is light, medium or heavy) the composition willfurther determine the overall classification (see the classification scheme at the end of the presentation).

In the following ignitable liquid classifications, the various ignitable liquids arecompared to gasoline. 1,2,4-Trimethylbenzene found in gasoline is the reference peak.1,2,4-TRIMETHYLBENZENE co-elutes or has a retention time similar to decane (n-C10). A solvent or solvent mixture that is to the left of the reference peak is considered a light product or light mixture. A narrow range mixture that is centered relative to the reference peak (or is just to the right of) is a medium mixture. Any wide range mixture, whose center of peaks is to the right of the reference peak, is a heavy mixture.

( Small concentration of aromatics )

( Light De-Aromatized Distillate )

Gasoline

Classification

THE MOST USED AND ABUSED SOLVENT MIXTURE

A solvent or solvent mixture that is to the left of the reference peak is considered a light product or light mixture. A narrow range mixture that is centered relative to the reference peak (or is just to the right of) is a medium mixture. Any wide range mixture, whose center of peaks is to the right of the reference peak, is a heavy mixture. The reference peak is 1,2,4-TRIMETHYLBENZENE (n-C10 has a similar retention time or may co-elute) found in the gasoline reference.

TIC (Total Ion Chromatogram)


Classification



TIC

TIC


TIC

TIC

Heavy Petroleum Distillate

Classification


TIC

TIC

FUEL

TIC

TIC

Diesel Fuel

Note the shift to the rightwhen compared to gasoline

Gasoline

Note! Biodiesel (B20) contains petroleum diesel and 20 % fatty acid methyl esters. These esters appear primarily after n-C18. (B20) may be found in some arson fires as the product is used in more trucks and cars. (see slide 112)

Isoparaffinic

Classification


Medium Isoparaffinic Product

Naphthenic Paraffinic

Classification

A Medium Naphthenic Paraffinic Product is basically a Medium Petroleum Distillatewith most of the aromatics and normal alkanes removed. The normal alkanes may be present in diminished amounts. The remaining components are cycloalkanes and isoparaffins.

Kerosene has a normal alkane range from approximately C8 to C17. Diesel fuel has a normal alkane range from approximately C9 to C25. Both kerosene and Diesel Fuel are heavy petroleum products. Many lamp oils are light kerosene mixtures. A Heavy Naphthenic Paraffinic Product (lamp oil) is kerosene with most of the aromatics and normal alkanes removed. The normal alkanes may be present in diminished levels. The remaining components are cycloalkanes and isoparaffins.


Heavy Naphthenic Paraffinic Product

Kerosene std

Weathered Gasoline std

Normal alkanes are absent or in low concentrationsAromatics are present only in minimal amounts

Note the Cycloalkanes and Isoparaffins<

Background InterferenceMatrices

The n-aldehydes have been found in a number of suspectedarson cases. The n-aldehydes can be confused for n-alkanes under the right conditions.

Polystyrene thermally decomposes into the above chromatogram

http://people.uncw.edu/tyrellj/CHM585/c5.ppt

Ethylbenzene

Toluene Styrene

m/p-xylene

Polypropylene thermally decomposes into the above chromatogram

< Trimer

Alkenes

Alkenes

This triplet elutes close to the triplet Of n-C13 and the Methylnaphthalenesfound in gasoline.

http://www.psrc.usm.edu/macrog/pp.htm

IN ANOTHER CASE THESAME PATTERN WITH LESS OF THE TRIMER AND STYRENE (MINUSNAPHTHALENE) WASIDENTIFIED AS GASOLINE.

Polystyrene and Polypropylene Thermo-Decomposition

.

V

THIS MIXTURE OF POLYSTYRENE AND POLYPROPYLENE DECOMPOSITIONCONTAINS GASOLINE

(ALSO CONTAINS TERPENES)

#######

< terpene

(ION SUMMED) EXTRACTED ION PROFILESUNK1

Ions: 128+142+156

Ions: 117+132

Ions: 55+69+83+97+111

Ions: 91+105+119+133

Ions: 57+71+85+99

TIC

POLYMER CONTAMINATION

POLYMER CONTAMINATION

GASOLINE

GASOLINE

GASOLINE

POLYMER CONTAMINTIONAND TRACE GASOLINE (HIDDEN)

UNK1 Gasoline std

Extracted Ion 119Extracted Ion 119

Extracted Ion 134Extracted Ion 134

Terpene

LightC4-C9

MediumC8-C13

HeavyC8-C20+

<---------------------------------Petroleum Distillates ------------------------------------>

<--------------------------------Isoparaffinic Products-----------------------------------><--------------------------Naphthenic Paraffinic Products-----------------------------><-------------------------------- Aromatic Products------------------------------------- ><----------------------------------N-Alkanes Products----------------------------------- >

<------------------------------De-Aromatized Distillates--------------------------------->

<---------------------------------Oxygenated Solvents<---------------------------------Others-Miscellaneous---------------------------------- >

< 1,2,4-trimethylbenzene (& n-C10)

Flammables Combustibles

A number of the above classes of ignitable liquids are encountered in fire debris analysis (testing). An in-house library of ignitable liquids from the the various classes shouldbe built with chromatograms (TIC), extracted ion profiles and mass spectra.

What if an ignitable liquid is found in a Prestone container? Assuming that the laboratorytests show that the contents are an ignitable liquid, could the residual contents(ethylene glycol) have altered the ignitable liquid pattern? Is ethylene glycol an ignitableliquid?

Prestone-ANTIFREEZE/COOLANT (ethylene glycol) MSDS ID: MSDSP149 Ethylene glycol is not a flammable or combustible liquid. It has a flashpoint above 200 F. Slight to moderate fire hazard when exposed to heat or flame. The flash point is a gauge of how easy it is to ignite a liquid.

Flash point is the lowest temperature at which a liquid can form an ignitable mixture in air near the surface of the liquid. The lower the flash point, the easier it is to ignite the material. Liquids with a flash point of 100 F or less are classified as flammable liquids. Liquids with a flash point between 100 F and 200 F are classifiedas combustibles. For example, gasoline has a flash point of -40 degrees C (-40 F) and is more flammable than ethylene glycol (antifreeze) which has a flashpoint of 111 degrees C (232 F).

.

Above is a chromatogram showing ethylene glycol in addition to methanol and propylene glycol. All of the compounds in this chromatogram are polar compounds (soluble in water).These polar compounds are all alcohols. Knowing that methanol is a smaller polar compound than ethanol, it is apparent that ethylene glycol would come out at the beginning of a typical chromatogram (TIC) of gasoline and not compromise the gasoline pattern.

This chromatogram is from a more polar column than used for ignitable liquids. Therefore, the retention times are greater (longer) than had these compounds been analyzed on a typical column used for ignitable liquids.

Molecular formula of ethylene glycol C2H6O2

CH3OH(methanol)

Biodiesel B20

Diesel fuel

Biodiesel B100

Biodiesel B20 contains both Diesel fuel and Biodiesel

Note C17 & C18 below

Documents

Introduction to Arson Analysis