Accelerants are commonly used in arson fires because they provide additional fuel in areas where the items present may not burn easily. Arsonists often pour accelerants over the areas they want to burn to ensure that their fires spread as much as possible to maximize damage and destruction. Common accelerants are commercially available ignitable liquids-such as gasoline, lighter fluid, and kerosene-that are readily accessible to the arsonist. The identification of an accelerant is significant evidence in a fire investigation because it suggests that the fire was set intentionally.
Any substances, most commonly ignitable liquids, used intentionally to increase the rate and spread of fires.
The identification of accelerants at fire scenes is often a challenge for investigators. Examination of the fire debris by various techniques can be useful in identifying the origin of a fire and any areas of potential accelerant use. After the preliminary identification of a potential accelerant source, samples can be collected and taken back to the forensics laboratory for further analysis.
The types of fire debris most likely to contain sufficient accelerant residue for analysis are porous materials, such as wood and carpet, which can trap residual liquid. Accelerant residue can also pool in the cracks in floors, where it is somewhat protected from the fire. Investigators should collect and store any debris suspected to contain accelerant residue in airtight containers, preferably metal paint cans with friction lids, to eliminate the possibility of the loss of the volatile components within the samples.
Gas chromatography coupled with mass spectrometry is the most common technique used for the analysis of accelerants from fire debris. Before fire debris evidence can be analyzed using the dual instrument known as the gas chromatograph-mass spectrometer (GC-MS), however, the accelerant residue must first be extracted from the debris that was collected. Several techniques can be used to perform this extraction, and each has its own advantages and disadvantages.
In a solvent extraction, the fire debris is washed with a solvent that will dissolve the accelerant residue but not the debris, such as carbon disulfide. The extract can then be injected directly into the GC-MS. A drawback of solvent extraction is that large amounts of potentially hazardous solvents are required to perform an efficient extraction; in addition, this method does not concentrate the accelerant residue effectively. Although solvent extraction was at one time a popular method, it has generally been replaced by quicker, more efficient preconcentration techniques.
In passive headspace extraction, the metal paint can used to collect the debris is heated so that any accelerant present is vaporized and becomes saturated within the area above the debris in the can, which is known as the headspace. A small hole is made in the top of the can and a gastight syringe is used to draw up a sample of the vapor in the headspace, which can then be injected into the GC-MS. Passive headspace extraction is biased toward the more volatile components, but it minimizes the capacity for cross-contamination of the evidence because the accelerant residue is extracted from the same container in which the debris was collected.
A variation of the passive headspace extraction technique is adsorption/elution, in which the debris is heated in the can with a strip of activated charcoal suspended in the headspace. The accelerant vapor is trapped on the strip, from which it is dissolved by a solvent for injection into the GC-MS. Adsorption/elution is affected by the same volatility bias as the passive headspace method, but because the vapor is concentrated onto the charcoal strip, adsorption/elution greatly decreases the potential loss of low-volatility compounds.
The solid-phase microextraction (SPME) technique employs a coated fiber that is housed in a retractable apparatus. The can containing the debris is heated, and this fiber is subjected to the headspace of the can, where the accelerant vapor adsorbs onto the fiber. One advantage of SPME is that the fiber apparatus can be placed directly into the injection port of the GC-MS. The heat of the injection port causes the accelerant trapped on the fiber to desorb from the fiber so that it can be carried into the instrument for analysis. Another advantage of SPME is its potential use for on-site accelerant collection. An investigator can use the SPME fiber apparatus to adsorb accelerant vapor at the fire scene; with the fiber retracted into the apparatus, it is protected from the environment and can be transported directly to the laboratory for analysis.
Although many techniques have proven useful in accelerant identification, gas chromatography (GC) is by far the most commonly used technique in the forensics laboratory for fire debris analysis. GC is a separation technique that is capable of isolating the numerous individual compounds present in typically complex accelerants. The result of a GC analysis is a chromatogram, which is essentially a chart in which all the components are represented as individual peaks. The pattern of these peaks does not change for a substance and thus is characteristic of that substance. Therefore, when an accelerant residue is examined by GC, its peak pattern can be matched to the peak pattern of a known sample of the same accelerant analyzed for comparison.
When GC is coupled with mass spectrometry (MS), the chemical composition of a sample can be identified conclusively. The pairing of GC and MS allows the identification of individual peaks within the peak pattern and thus is the standard convention for accelerant identification. It should be noted that accelerant identification is considered class evidence because it cannot be individualized to one source. For example, if an accelerant is identified to be gasoline, the pump or even the service station from which it was purchased cannot be determined because of the inherent variation in the process of refining crude oil.
The American Society for Testing and Materials (ASTM), an organization that generates and maintains standards for procedures and materials in a wide array of fields, has developed standard accelerant classes for the identification of accelerants in court. The ASTM classification system for ignitable liquids provides a standardized method of accelerant description for forensic scientists. In this system, nine classes of ignitable liquids are subdivided into three boiling point ranges (light, medium, and heavy). The nine classes-gasoline, petroleum distillates, isoparaffinic products, aromatic products, naphthenic-paraffinic products, normal alkane products, dearomatized distillates, oxygenated solvents, and a final miscellaneous grouping-and their subdivisions provide standard guidelines for the identification of ignitable liquids based on chemical composition.
In a fire investigation, the primary goal is to identify whether the fire was accidental or intentional. The presence of an accelerant at a fire scene is often indicative of an intentional fire, or arson. Accelerants can be identified from fire debris through conventional forensic analysis.
Although chromatographic pattern matching is the convention for the identification of accelerants in a forensics laboratory, some factors can alter chromatographic patterns and make it difficult for investigators to identify conclusively any accelerant that may be present. Most common accelerants contain refined petroleum products, which are mixtures of hydrocarbons, and several of these hydrocarbons are found in everyday household products. For example, basic carpeting such as that found in many homes contains compounds similar to those found in common accelerants. This overlap presents a problem for a scientist attempting to identify an accelerant that soaked into a carpet before it was burned.
An efficient extraction technique, such as adsorption/elution or SPME, can separate an accelerant from the fire debris itself. Investigators can also use a data-processing technique called extracted ion chromatography (EIC)-in which specific characteristic peaks can be isolated from other peaks-to understand the data more fully. Because of the potential problem of interference, fire investigators should collect several debris samples, including samples in which no accelerant is expected to be found, in order to understand which chromatographic peaks correspond to the debris and which peaks correspond to an actual accelerant.