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The Analysis of Fire Accelerants
in Arson Suspected Crime Scenes


by: Michael Tidwell

Arson, as defined in the Encarta Encyclopedia, is the act of willfully and maliciously setting fire to a
building or other property. Arson is a very serious crime. According to statistics obtained
from the Federal Bureau of Investigation, FBI, 88,887 arson offenses were reported in 1996
resulting in over 913 million dollars in damage. In addition, the Bureau of Alcohol, Tobacco, and
Firearms, ATF, reported 383 deaths and over 3 billion dollars in damages resulting from arson
cases from 1993 to 1997. These figures only represent the cases in which arson was confirmed
as the cause of the fire. Several thousand more arson fires go unconfirmed every year. This paper
will address the laboratory detection and analysis of fire accelerants at arson suspected
crime scenes with an emphasis on the use of techniques in headspace gas chromatography.

The detection and anaylsis of fire accelerants has been thoroughly developed by the forensic community. Fire accelerants are typically a mixture of organic based solvents such as gasoline, paint thinner, and kerosene. Chemicals such as these are also often referred to as petroleum distillates. These distillates are
obtained from the distillation of crude oil. They are generally separated according to boiling
point. These carbon based compounds are usually highly volatile. The lighter fractions such as
gasoline or paint thinner will be more volatile than the heavier fractions such as kerosene or
diesel. Therefore, if an accelerant such as kerosene is used in an arson crime, it will burn
slower and not evaporate nearly as fast as one of the lighter fractions such as gasoline. The
analysis of many organic compounds that are used as fire accelerants is standard procedure in most
forensic science laboratories. The use of chromatography, specifically headspace gas
chromatography, has been very reliable in the analysis of fire accelerants. Gas chromatography
is defined as the partitioning of a species between an inert gaseous mobile phase and a solid
stationary phase. GC is most useful when analyzing volatile organic compounds. Typically, for
headspace analysis, solid debris samples are taken from a fire scene at the point where the fire is
suspected of starting. The debris samples are immediately placed in airtight containers to
prevent the volatile fire accelerants from evaporating. If the fire was accelerated by the
use of a fire accelerant, a small amount of that given accelerant will still be present in the
charred debris. Headspace gas chromatography takes advantage of the volatility of the
hydrocarbons to separate and detect them from the solid debris. In other words, the debris from the
arson suspected crime scene is heated inside the headspace gas chromatograph and if any volatile
hydrocarbons are present, they will vaporize and be present in the air or "headspace" directly
above the debris. For the actual analysis, the sample is placed inside a vial and sealed air-
tight by crimping a special cap onto the top of the sample vial. Once the heating has forced the
volatile hydrocarbons into the headspace above the sample inside the vial, an injection needle is
engaged which punctures a teflon septum at the top of the vial and injects the gaseous hydrocarbons
into the gas chromatograph for analysis. See Figure 1. The gas chromatograph will generally
separate the various hydrocarbons according to their boiling points. For example, kerosene has a
higher boiling point than gasoline. Therefore, these two accelerants can be successfully
detected using the GC.


Figure 1. A headspace gas chromatography vial.


To demonstrate the usefulness and validity of headspace gas chromatography in detecting fire
accelerants, an actual analysis was carried out. Two samples were analyzed using a Varian 3380 Gas
Chromatograph equipped with a Genesis Headspace Autosampler. The first sample was regular unleaded
gasoline. The second sample was kerosene. Approximately 1.0 ml of each of the two samples
was poured onto a small cloth and placed on a watchglass. The majority of the accelerant was
allowed to evaporate from each of the cloths. The lower boiling gasoline evaporated rather quickly
(~ 5 minutes), while the higher boiling kerosene took considerably longer(~ 2 hours). Next, each
cloth was placed inside of a headspace vial and sealed air-tight. See figure 1. The vial was then
placed in the headspace autosampler and the analysis performed. The chromatogram in figure 2
was given by gasoline. The chromatogram in figure 3 was given by kerosene. These two chromatograms
are obviously and visually very specific toward the complex mixtures of compounds which make up
gasoline and kerosene.


Figure 2. The headspace chromatogram of gasoline.


Figure 3. The headspace chromatogram of kerosene.

The subsequent chromatogram given by the unknown debris sample can then be compared to known
chromatograms of several chemicals that are commonly used as fire accelerants taking into
account the level of evaporation. The known accelerant chromatograms act as a fingerprint for
the comparison of the suspected accelerants in the debris. In some cases, the known accelerant may
be a sample of gasoline, diesel, or paint thinner taken from a suspect's home. The positive matching
of the accelerant at the crime scene with the accelerant from the suspect's home is very
powerful evidence towards an arson conviction. Headspace gas chromatography is the most popular
method of analysis for fire accelerants in use today.

In addition, portable gas chromatographs and portable infrared spectrometers have been
introduced that can be used at the crime scene. In addition, T-CAT has recently introduced a portable
mass spectrometer (MS). Samples of the air above the debris are sampled immediately after the
flames die using the MS. Portable instrumentation minimizes the need to bring the samples back to
the laboratory for tests. This type of crime scene analysis is also useful in minimizing sample
contamination. The question of sample contamination is always significant in arson
cases. Another type of analysis uses chemical tests. These tests rely on a color change when the
hydrocarbon reacts with the reagent. This type of analysis is not extremely popular due to the
inability to distinguish between hydrocarbons from fire accerlerants and hydrocarbons from burnt
plastics. These chemical tests are generally only used for preliminary or presumptive tests.
Another method is using canines to detect the accelerants. The ATF began training chemical
sniffing canines in 1989. The use of canines is often helpful in determining the source of the
fire, however, cannines have not been able to distinguish between different types of
accelerants. The final method of accelerant analysis involves the use of mechanical sniffers.
These cheap, reliable instruments are most commonly used to detect gases by measuring the
oxygen content in the air. The instrument cannot discriminate between different types of gases. Its
primary function is to merely detect the presence of a gas. Further methods of analysis can then be
used to identify the particular gas. Whether it is analyzed with the use of headspace GC, mechanical
sniffers, portable instruments, or chemical tests, the accelerants arsonists use will be detected.
With this arsenal of techniques, the analysis of fire accelerants in forensic chemistry has been
thoroughly developed.


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