Introduction to Arson Analysis
Updated 4-15-09 for an arson homicide trial
(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 the
AAFS and at Zeno’s Forensic Science Site est. 3-30-05/3-10-06
Arson Analysis by Gas
Chromatography/Mass Spectrometry
Denver Police Crime Laboratory
William D. McDougall II
Forensic Analyst (Retired)
Original Presentation
September 2004
wdmll@comcast.net
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 Mass
Spectrometer and a HP 5973 Mass Spectrometer in the Denver Police Crime Laboratory.
M. Jennifer Thomas, Forensic Chemist, generated most of the mass data displays obtained
from 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)
Injector >
Control Panel
Column Oven
Gas Chromatograph
Ion Trap
Mass Spectrometer
Chromatography
“chromato-graphy”
means color writing
It is a physical process of separating
complex mixtures
THIN LAYER CHROMATOGRAPHY
The liquid (solvent or solvents)
is the mobile phase
NON-POLAR
Silica gel
SiO2.H2O
POLAR STATIONARY
PHASE
(On plastic, glass
or foil backing)
MEDIUM
POLAR
POLAR
( NON-POLAR, MEDIUM POLAR
OR SOLVENT MIXTURE )
COLUMN CHROMATOGRAPHY
ADSORPTION CHROMATOGRAPHY
POLAR
POLAR
STATIONARY
PHASE
MEDIUM
POLAR
aluminum oxide Al2O3
(Less polar than silica)
Normal Phase Chromatography
Interaction of the
adsorptivity and
solubility of the
analytes relative
to the two phases
NONPOLAR
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.
MODERN HPLC SYSTEM
CAN HAVE ONE OR MORE MOBILE PHASES
CAN HAVE MORE
THAN ONE PUMP
API (ESI)
M+1
Mass Spectra
Controlled substances
including “steroids”
are excellent candidates
for HPLC
REVERSED PHASE HPLC
THE STATIONARY PHASE IS
HYDROPHOBIC (NON-POLAR).
MODERATELY POLAR AND
NON-POLAR COMPOUNDS START
TO MIGRATE THROUGH THE COLUMN
AS THE MOBILE PHASE CHANGES FROM
AQUEOUS TO LESS POLAR ORGANIC
SOLVENT.
(Solvent Programming)
High pKa (basic) compounds will have a large k’ (longer
retention time) in basic buffered mobile phases. Low pKa
(acidic) compounds will have a large k’ in acidic buffered
mobile phases.
http://www.shu.ac.uk/schools/sci/chem/tutorials/chrom/gaschrm.htm
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.
After passing through the heated column,
The sample components
have to be in the vapor
state in order to pass
through the 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 is
called a chromatogram.
< Pattern of peaks
(Separated Components)
(6)
The outside dimension
of a capillary column is
about the size of a heavy
fishing line.
The Column
The Heart Of The
Gas Chromatograph
Columns for arson analysis are normally 15 to 30 meters in length.
A .25mm ID column
is commonly use
for arson analysis
(Liquid stationary phase
.25um thickness)
HP-5
Analyte >
< Air
Non-retained
5% phenyl methylpolysiloxane
is a relatively non-polar general
purpose stationary liquid phase
Retained
Chromatogram
DYNAMIC PARTITIONING
TAKES PLACE IN GAS
(LIQUID) CHROMATOGRAPHY
Static Partitioning
< BASIC AQUEOUS PHASE
High pKa (basic) compounds such
as cocaine and methamphetamine
are found in higher concentrations
in the lower phase after equilibration.
< CHLOROFORM PHASE
K= C stationary phase
C mobile phase
DYNAMIC PARTITIONING
THE ANALYTES PARTITION BETWEEN
THE STATIONARY LIQUID PHASE AND
THE MOBILE GAS PHASE.
N= # of theoretical plates
The column is made
up of a large number
of partitioning (separatory)
zones
PACKED COLUMNS HAVE APPROX. 10,000 PLATES
Each partitioning
zone equals one
theoretical plate
CAPILLARY COLUMNS HAVE APPROX 100,000 PLATES
CAPILLARY COLUMNS HAVE MORE THEORETICAL PLATES THAN PACKED COLUMNS
MINIBORE COLUMNS HAVE EVEN MORE THEORETICAL PLATES PER METER
(Used for packed columns)
(CAPILLARY COLUMNS ARE NOT AFFECTED BY EDDY DIFFUSION)
(CAPILLARY COLUMNS MINIMIZE THIS FACTOR)
Carrier Gases
NITROGEN, HELIUM
AND HYDROGEN HAVE
DIFFERENT SLOPES
HYDROGEN IS USED AT
HIGHER FLOW RATES
THAN HELIUM. NITROGEN
IS THE LEAST FAVORABLE.
Resolution
Resolution is proportional to the
square root of (N), therefore the
length of the column
In order to double the
resolution, the column length
has to be increased by a factor
of four
“SELECTIVITY” IS DETERMINED BY THE STATIONARY PHASE CHEMISTRY
(K’) IS OPTIMIZED BY TEMPERATURE PROGRAMMING
Fire
Fire is a chemical reaction. It is rapid
oxidation with the release of heat and
Light (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).
METHANE
Basic building block
of hydrocarbons
Straight Chain
(6) Carbons
(14) Hydrogens
Normal Alkane
HEXANE
Cycloalkane
Branched Alkane
2-METHYLHEXANE (Isoparaffin)
METHYLCYCLOHEXANE
Found is gasohol and alcoholic drinks
1-HEXENE
Thermo-Decomposition
ETHANOL
Thermo-Decomposition
Oxygenated Solvent
ACETONE
STYRENE
< Found in Gasoline >
INDANE
Structures from NIST 98 version 1.6
1-METHYLINDAN
BENZENE
(6) Carbons
(6) Hydrogens
Basic building block of
aromatic hydrocarbon
compounds
TOLUENE
Benzene ring plus a methyl group
C3 ALKYLBENZENE
ETHYLBENZENE
Hydrocarbons are fuel
for arson fires and
(Reference Compound)
non-arson related
1,2,4-TRIMETHYLBENZENE
fires
NAPHTHALENE
C4 ALKYLBENZENE
HYDROCARBONS FOUND IN FIRE
DEBRIS (Some of these compounds are found in
both gasoline and thermo-decomposition).
Structures from NIST 98 version 1.6
1-METHYLNAPHTHALENE
< Toluene
Styrene >
THERMODECOMPOSITION
Styrene and o-xylene
have similar retention
times.
Each separated sample
component is burnt in the
flame causing an increase
in the electrical signal.
(PYROLYSIS PRODUCTS)
Ethylbenzene >
m/p
-xylene
Each peak represents a compound
from the original sample mixture.
< m/p
-xylene
Each time the signal is increased
it is recorded as a peak in the
chromatogram.
Time > > >
The analytes in these two
chromatograms including styrene
and o-xylene would superimpose
Using the same time axis.
< Toluene
Ethylbenzene >
IGNITABLE LIQUID
MIXTURE
Time > > >
< o-xylene
Toluene followed by the other analytes
enter the detector one after another,
after migrating through the column.
(Note in both chromatograms m/pxylene co-elution)
< Chromatograms
( Separated Components )
http://www.shu.ac.uk/schools/sci/chem/tutorials/chrom/gaschrm.htm
The passage of three compounds
through the column over time.
Chromatogram
( Separated Components )
Time >>
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 with
increasing time in the column.
http://www.uga.edu/srel/AACES/GCtutorial/page1.html
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.
(Time of Elution)
http://www.uga.edu/srel/AACES/GCtutorial/page1.html
tR
(Retention Time)
< Peak Apex
(Measured in Minutes)
The retention time is the time a compound
spends in the column from the time of
injection to the time of elution of the
compound (measured to the peak apex).
< Time of Injection
Time >>
< n-C17 elutes before n-C18
TIC
AMU
AMU
Additional factors determining elution order and selectivity
IN ORDER TO SEPARATE M-XYLENE AND P-XYLENE,
A POLAR STATIONARY PHASE WOULD BE NEEDED.
In addition to molecular weight,
molecular structure is a factor
determining the boiling points, thus
the order of elution.
Time >>
>
Note the different profiles or patterns of mountain peaks
Note the different profiles or patterns of peaks
Kerosene
Gasoline
Pattern recognition
Chromatography peaks
Diesel
Fuel
Summery of gas chromatography (FID) used in fire debris analysis
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.
The Mass Detector
The mass detector used in arson analysis is
typically a Quadrupole or Ion Trap Mass
Spectrometer.
Peak (component) Co-elution
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 coelution 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.
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).
Electron impact ionization (EI) produces
In order to measure the mass of
a compound it has to be ionized.
The mass to charge ratio is
actually determined. Normally
the charge is one. The molecular
ion is not stable using electron
impact ionization (EI) and
undergoes fragmentation. The
fragmentation pattern is a display
of intensity versus mass of the
remaining molecular ion and
the newly created fragment ions.
Pressurized
positive ions, negative ions and neutral
species. But only the positive ions are
analyzed in this mode of analysis.
The neutral species and molecules (not
ionized), are pumped away.
Vacuum
Quadrupole filter
Ion Trap
Ionization
&
Quadrupole Fragmentation
AC and DC
Varied voltages
The ions are scanned from the
low masses to the high masses
over time.
< Mass Spectra
Mixture
(3)
Molecular ion >
Separated Components
(3)
(3)
http://ull.chemistry.uakron.edu/gcms/MS_detector/index.html
http://www.jeolusa.com/ms/docs/whatisms.html
1=H+
16=O+
17=[OH]+
18=[H2O]+
Gas Chromatograph-Ion Trap Mass Spectrometer (MS/MS)
Extracted ion profiles of gasoline
Gasoline
Toluene
Toluene
Aromatics
Pattern of peaks
Indanes
Toluene
Naphthalenes
Chromatogram (TIC)
Aliphatics
Mass Spectrum
TIC
Sample syringe
Injector >
Control Panel
The ions are scanned (separated) from the
low masses to the high masses over
time. An increasing RF (AC)
voltage is used to filter
the mass ions
Column
Column Oven
Gas Chromatograph
Ion Trap
Mass Spectrometer
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.
Mass Spectrometry Vacuum
A high vacuum is needed to prevent unwanted collisions between
the analyte ions and gas molecules during the migration of the ions
from 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 to
stabilize 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 by
the detector.
(Electron impact ionization)
(End-cap)
(Ring)
Separated components enter here >
(End-cap)
http://ull.chemistry.uakron.edu/gcms/MS_detector/index.html
After the positive ions pass through the mass analyzer with
unit resolution, they are detected as an amplified electrical signal.
http://ull.chemistry.uakron.edu/gcms/MS_detector/index.html
For each positive ion that strikes the inner surface, a
gain of approximately one million electrons is produced.
http://ull.chemistry.uakron.edu/gcms/MS_detector/index.html
Ion Trap Analyzer
Showing the different electrical potentials applied to the ion trap
(Electron impact ionization)
With additional wave form manipulations applied to
the End-cap electrodes, MS/MS can be used to further
analyze fire debris containing matrices.
(Figure taken from www.ivv.fhg.de/ms/ms-analyzers.html#Ion_Trap
Special Note!
Advanced quadrupole mass spectrometry applications use
tandem mass spectrometry in space or time. Multiple quadrupole
mass filters in series or a single quadrupole ion trap mass
spectrometer (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 spectra
and mass chromatograms (minus background interference). This
application can be useful in arson analysis as a compliment to single
quadrupole mass spectrometry. This technique can remove extra
artifact peaks found in extracted ion profiles (mass chromatograms)
of ignitable liquids containing complex background interference.
Tandem mass spectrometry is used in a number of applications
including both gas chromatography/mass spectrometry and high
performance liquid chromatography/mass spectrometry.
(Ion Trap and Quadrupole Mass Filter)
(Ion trap)
Waters Corporation
These equations are used for both the
Quadrupole Mass Filter and The Ion Trap
(For the Ion Trap)
(DC=0V for the Ion Trap)
(The Quadrupole Mass Filter uses both DC and RF voltages)
The AC frequency is about one megahertz and is called
the fundamental RF. It is applied to the ring electrode.
az
DC = 0V
(RF)
qZ
^
||
The larger the mass
the smaller the q
A constant RF voltage is used to trap the ions
The instability along the (RF) axis is expressed as
DC
az
qz
q
z
z
Trajectory of a single ion >
.
> 0 9 (unstable)
q z RF
Resonance allows an ion to escape
Before q > 0.9
q zO RF
z
m
c
The larger the RF voltage
the larger the q
Z
(The RF voltage is ramped (> 6000 V) to destabilize the ion trajectories)
(The frequency of the AC applied to the end-caps is approximately ½ of the fundamental RF)
Also called axial modulation
(When the secular frequency of an ion is equal to the end-caps’ frequency it undergoes resonance)
(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
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
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 highvacuum system.
Schematic of a quadrupole filter
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://www.chem.vt.edu/chem-ed/ms/quadrupo.html
http://avogadro.chem.iastate.edu/CHEM577/CHEM577-C.pdf
RLC circuits are designed as high pass filters
(high frequencies) and as low pass filters (low
frequencies) in many types of applications.
DC and RF voltages
(+) Rods
RF
DC
(-) Rods
http://www.chm.uri.edu/chm412/Chap11.ppt
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/
http://www.chm.uri.edu/chm412/Chap11.ppt
RF >DC
+ DC Potential
Heavy ions pass
through the rods
RF Potential
( Light ions are
destabilized and lost.)
Positive Ions
DC >RF
- DC Potential
Heavy ions drift in
to the negative rods.
http://avogadro.chem.iastate.edu/CHEM577/CHEM577-C.pdf
RF Potential
(Light ions pass
through the rods)
The proper ratio of DC
and RF potentials results
in unit resolution.
Band Filter
The AC voltage has a frequency in the radio frequency range (RF)
RF>DC (+) Rods
Small ion
High pass filter
Large ion DC>RF
Low pass filter
(-) Rods
High and low pass filters combined
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 filtering
rods 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
(RF) mass to high mass (with unit resolution), to pass through the
rods to the detector over time. One scan in approx ½ second.
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
(RF)
The ratio of the DC and RF voltages is adjusted to produce unit mass
resolution. The magnitudes of the two voltages (in a fixed ratio)
are ramped through the entire mass range.
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 unit
resolved mass peaks are assigned masses from a calibration table.
Unit
< resolution
v
Tune and Calibration
In addition to unit resolution
and mass calibration, relative
intensities are obtained. Library
mass spectral searches of unknown
mass spectra are now possible.
From NIST 98 version 1.6
Electron Impact Ionization (70 electron volts)
The relative intensity of the molecular ion found in different c lasses of hydrocarbons.
The below ion fragmentation patterns are a function of molecular structure
http://194.94.42.12/licensed_materials/00897/papers/0007004/7402 26ww.htm
The energy needed to break the bonds in the above classes of hydrocarbons is much less than
70 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.
(Approximately 5 eV will break a carbon, carbon single bond)
~
Next the components are ionized by electron impact (EI)
and undergo fragmentation.
Separated Components
Filament
< Toluene
Mass Filter
>
^
Separated by chromatography
Lenses accelerate and focus the ions
Toluene
Mass Spectrum
http://chipo.chem.uic.edu/web1/ocol/spec/MS1.htm
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 the
positive set of rods. The negative set
of rods have an equal negative DC
slope and a RF 180 degrees out of
phase.
http://ull.chemistry.uakron.edu/gcms/MS_detect
or/index.html
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).
Toluene
http://chipo.chem.uic.edu/web1/ocol/spec/MS1.htm
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
Ethylbenzene
Ethylbenzene
http://science.csustan.edu/tutorial/mass/ethyben5.htm
Decane is a straight chain hydrocarbon
with no ring structure. The molecular ion
has minimal relative intensity and the base
peak is of comparatively low mass.
n-C10 (Decane)
TIC
The next set of slides show different types of non-aromatic
hydrocarbons found in a Light Petroleum Distillate. Note
that the Light Petroleum Distillate is compared to a gasoline
reference.
( Light De-Aromatized Distillate )
TIC
TIC
Note the ions 43, 57 and 71
Normal Alkane
Heptane n-C7
Note the ions 39, 55, 67(& 69)
and 83
Methylcyclohexane
Cycloalkane
Note the ions 43, 57, 71 and 85
Normal Alkane
Octane (n-C8)
Note the ions 39, 55, 67 and 83
Cycloalkane
Ethylcyclohexane
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 liquids
and the alkenes are normally associated with background
interference 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 is
important. Are there any normal alkanes present (intense equally
spaced 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, the
following slides are highlighted.
No Apparent Equally Spaced Intense Peaks
Not a normal alkane mixture?
Unknown Mixture
Unknown Mixture
Based on the position of the unknown
mixture to the reference peak (1,2,4Trimethylbenzene) found in gasoline,
the mixture is a medium range product.
< (n-C10 co-elutes)
Ion averaging of all the ions in the TIC
Peak ? >
Virtually no aromatic ions (91,105,119,134)
Unknown Mixture
(Medium Range Product)
No dominant cycloalkane ions (55,69,83,97)
TIC
Not a Naphthenic Paraffinic Product
(Isoparaffin Product?)
(Normal Alkanes??)
^||
Note Ratio
||
v
< Ratio of ion 57 to ion 43 is found in isoparaffins
The mixture is composed of compounds containing
ions 43, 57, 71, 85, 99 and 113. These ions are
found in both isoparaffins and normal alkanes.
< Normal Alkane ?
Unknown Mixture
Retention Time of 7.97 min
(Straight Chain Hydrocarbon)
(Medium Range Product)
TIC
Mass Spectrum of Peak With Retention Time of 7.97 min
Note Ratio
Has the ion ratio of a branched alkane (isoparaffin) and
< has a molecular ion of 156 Daltons
The normal alkane “Undecane” (n-C11) also has a
molecular ion of 156 Daltons
Mass Spectrum Of Peak From the Unknown Mixture
(Retention Time of 7.97 min)
Appears to be an Isoparaffin !
Library Search Results Of The Above Mass Spectrum
Branched Alkane (Isoparaffin)
(molecular ion of 156 Daltons)
Note! equally spaced peaks >
Reference
Mixture contains n-C11
< n-C10
< n-C11
Medium Petroleum Distillate
(Normal alkane mixture plus
smaller amounts of aromatics,
isoparaffins and cycloalkanes)
Note Ratio
TIC
Comparison of the unknown
mixture in the previous
n-C12 > slides to a mixture
containing n-alkanes
Retention Time 10.334 min
<
The normal alkane n-C11 (Undecane)
has a molecular ion of 156 Daltons
n-C11 (Undecane)
Unknown
Peak ? >
Questionable peak and Undecane have different
retention times.
TIC
Medium Isoparaffin
Product?
Retention Time 7.97 min
Ion 57
Ion 41
Branched Alkane (Isoparaffin)
71
85
Molecular ion
99
Ion 156
Reference
< n-C11
Medium Petroleum Distillate
Ion 43
TIC
Ion 57
71
Retention Time 10.334 min
85
Undecane (n-C11)
99
Molecular ion
Ion 156
Reference
Medium Petroleum Distillate
Unknown
In summary, the unknown is a
Medium Isoparaffinic Product.
The following set of slides illustrate additional ways of
displaying an ignitable liquid using a mass spectrometer.
Since gasoline is often encountered in arson cases it has
been used for this demonstration.
Ion Trap Mass Spectrometer
TIC ( 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
Library Search Results
1,2,4-TRIMETHYLBENZENE
Weathered Gasoline
The C3, C4 and C5 alkylbenzenes should be present in
an unknown sample to be considered gasoline. Weathering
can alter their relative concentrations. Additionally, the
naphthalenes, indanes, aliphatics and cycloalkanes found in
gasoline should also be present. Their relative concentrations
are also modified by weathering.
70% Evaporated Gasoline
TIC
TIC
90% Evaporated Gasoline
TIC
TIC
99% Evaporated Gasoline
TIC
TIC
< 1,2,4-TRIMETHYLBENZENE
Weathered Patterns of Ignitable Liquids
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.
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.
In the following ignitable liquid classifications, the various ignitable liquids are
compared 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 (nC10). 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.
Light Petroleum Distillate
Classification
There are many commercial solvents and solvent mixtures available to
the 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 will
further determine the overall classification (see the classification scheme
at the end of the presentation).
( Small concentration of aromatics )
( Light De-Aromatized Distillate )
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.
Gasoline
Classification
THE MOST USED AND ABUSED SOLVENT MIXTURE
TIC (Total Ion Chromatogram)
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.
Medium Petroleum Distillate
Classification
TIC
TIC
< 1,2,4-TRIMETHYLBENZENE
TIC
< 1,2,4-TRIMETHYLBENZENE
TIC
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.
Heavy Petroleum Distillate
Classification
TIC
TIC
FUEL
TIC
TIC
Note!
Biodiesel (B20) contains
petroleum diesel and
20 % fatty acid methyl
esters. These esters
appear primarily after nC18. (B20) may be found
in some arson fires as the
product is used in more
trucks and cars.
(see slide 112)
Diesel Fuel
Note the shift to the right
when compared to
gasoline
Gasoline
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.
Isoparaffinic
Classification
Medium Isoparaffinic Product
A Medium Naphthenic Paraffinic Product is basically a Medium Petroleum Distillate
with most of the aromatics and normal alkanes removed. The normal alkanes may be
present in diminished amounts. The remaining components are cycloalkanes and
isoparaffins.
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.
Naphthenic Paraffinic
Classification
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.
Kerosene std
Heavy Naphthenic Paraffinic Product
Normal alkanes are absent or in low concentrations
Aromatics are present only in minimal amounts
<
Note the Cycloalkanes
and Isoparaffins
Weathered Gasoline std
Background Interference
Matrices
The n-aldehydes have been found in a number of suspected
arson cases. The n-aldehydes can be confused for n-alkanes
under the right conditions.
Toluene
Styrene
m/p
-xylene
Ethylbenzene
http://people.uncw.edu/tyrellj/CHM585/c5.ppt
Polystyrene thermally decomposes into the above chromatogram
< Trimer
Alkenes
Alkenes
This triplet elutes close to the triplet
Of n-C13 and the Methylnaphthalenes
found in gasoline.
http://www.psrc.usm.edu/macrog/pp.htm
Polypropylene thermally decomposes into the above chromatogram
Polystyrene and Polypropylene Thermo-Decomposition
V
.
IN ANOTHER CASE THE
SAME PATTERN WITH
LESS OF THE TRIMER
AND STYRENE (MINUS
NAPHTHALENE) WAS
IDENTIFIED AS
GASOLINE.
THIS MIXTURE OF POLYSTYRENE AND
POLYPROPYLENE DECOMPOSITION
CONTAINS GASOLINE
(ALSO CONTAINS TERPENES)
#######
< terpene
UNK1
(ION SUMMED) EXTRACTED ION PROFILES
GASOLINE
Ions: 91+105+119+133
Ions: 117+132
GASOLINE
Ions: 128+142+156
GASOLINE
POLYMER
CONTAMINATION
POLYMER CONTAMINATION
POLYMER CONTAMINTION
AND TRACE GASOLINE (HIDDEN)
Ions: 55+69+83+97+111
Ions: 57+71+85+99
TIC
UNK1
Gasoline std
Extracted Ion 119
Terpene
Extracted Ion 119
Extracted Ion 134
Extracted Ion 134
Light
Medium
Heavy
C4-C9
C8-C13
C8-C20+
<---------------------------------Petroleum Distillates ------------------------------------>
<------------------------------De-Aromatized Distillates--------------------------------->
<--------------------------------Isoparaffinic Products----------------------------------->
<--------------------------Naphthenic Paraffinic Products----------------------------->
<-------------------------------- Aromatic Products------------------------------------- >
<----------------------------------N-Alkanes Products----------------------------------- >
<---------------------------------Oxygenated Solvents
<---------------------------------Others-Miscellaneous---------------------------------- >
Flammables
Combustibles
< 1,2,4-trimethylbenzene (& n-C10)
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 should
be built with chromatograms (TIC), extracted ion profiles and mass spectra.
.
What if an ignitable liquid is found in a Prestone container? Assuming that the laboratory
tests show that the contents are an ignitable liquid, could the residual contents
(ethylene glycol) have altered the ignitable liquid pattern? Is ethylene glycol an ignitable
liquid?
Prestone-ANTIFREEZE/COOLANT (ethylene glycol) MSDS ID: MSDSP149 Ethylene glycol is not a flammable or combustible liquid. It has a flash
point 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 classified
as 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 flash
point of 111 degrees C (232 F).
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.
CH3OH
(methanol)
Molecular formula of ethylene glycol
C2H6O2
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.
Biodiesel
B20
contains
both Diesel
fuel and
Biodiesel
Diesel fuel
Note
C17
&
C18
below
Biodiesel B20
Biodiesel B100