At most crime scenes some physical evidence is always present and such evidence often is a clue to the identity of the perpetrator. At a potential arson scene, however, the evidence is all burned up. Or is it? Despite the fact that what remains after a fire looks like just a tangled and charred mass of debris, clues still exist within the debris and ashes. An arson fire may be started for insurance proposes, for revenge against the building owner or occupants or in many cases to cover up another crime such as a robbery or even a murder.
One of the first steps in determining if a fire is deliberately set is to determine if some type of accelerant material has been used to start the fire. The common accelerants used to start an arson fire are volatile liquids which are readily available to the arsonist. Gasoline, diesel, or kerosene which are readily available fuels at service stations or Mineral Spirits or turpentine are flammable liquids that can be purchased at a hardware, paint, or home supply store. All of these accelerants are petroleum distillates and are complex mixtures of hydrocarbon molecules of varying lengths. While all of these complex mixtures have similar properties they vary in one property: their boiling point temperature. Their boiling points vary over a wide range of temperature values. Because of this, the individual accelerants will vary in composition during the evaporation process when they are used to start a fire. As a fire starts the more volatile components in the mixture evaporate faster then the higher boiling components and eventually leaving the heavier components in the debris as the accelerants becomes less volatile and less available after a period of time.
Physical Evidence of Accelerants
To the trained arson investigator there are several physical signs of accelerant use. Localized burn patterns on the floor of a structure or extensive overhead damage which is inconsistent with whatever natural fuel sources are available at the site indicate accelerant use. In cases where the fire damage is extensive this evidence may be destroyed or buried under debris as the building roof and ceilings collapse. If there are eyewitness or security camera recording indicating that the fire was fast moving or there was the odor of gasoline or a solvent in the air around the building these can also indicate accelerant use.
Detecting the Presence of Accelerants
Despite all the technological advances that have been achieved in the last decade, the human nose (of a trained investigator) is still one of the best detection devices and is quite capable of detecting the presence of accelerant vapors even in very small amounts. Since the heavier, less volatile components of an accelerant will still remain the fire debris it is only the more volatile, lower boiling components that the nose detects. These are present above the debris in the air that is generally referred to as the headspace. The nose is detecting the chemical composition of the headspace. Then the brain compares this to the known chemical fingerprint that it has stored from experience and from this the investigator makes a decision about the possible use of an accelerant.
In many investigations a trained arson dog is used to detect areas where accelerants may have been used. Investigators can then explore these areas in more detail. The dog's sense of smell is approximately one hundred times more sensitive than that of a human. The problem with fire dogs is that they must be able to determine accelerant vapors in the presence of a wide variety of other organic smells arising from a fire. Burning plastics, cloth, wood and paints all give off vapors in combustion and some of these are the same as produced from a potential accelerant. Extensive training is needed to train the dog's discriminatory powers to recognize the accelerant in the presence of other combinations of vapors. The human nose, while less sensitive, is attached to the eyes and a brain that can look at materials present in the debris and identify objects (burnt plastics, furniture, etc.) that might lead to the presence of compounds which might mimic or mask an accelerant.
Portable Sniffer Devices
A wide variety of portable, hand-held devices have been developed to detect volatile compounds and accelerant residue at a fire scene. While these are capable of detecting vapors at the scene, the major problem with these devices is they lack the discriminatory power of the nose to respond to accelerant vapors and not the other array of vapors present at the scene.
Laboratory Analysis
An assumption that many arsonists make is that with the intensity of a fire there will be no evidence of the accelerant that they have used left behind afterwards. This is a false assumption and the amount of an accelerant that remains after a fire depends on many factors. The amount and quality of the accelerant used, the materials that it is poured on (porous, non-porous etc.), the amount of time that has elapsed since the fire began and the severity of the fire all affect the amount of vapors left behind. While accelerants like gasoline are complex mixtures of chemical compounds, accelerants like acetone or alcohol are single chemicals and much harder to detect. In addition, these compounds are water soluble so may be washed away by the fire extinguishing process or by subsequent rain storms.
It falls to the forensic laboratory to make the critical determination of whether accelerant residue exists in debris from a fire. While methods and equipment may vary from laboratory to laboratory the general principle of the analysis remain the same. A sample of various types of debris from the suspected origin of the fire are transmitted to the lab with appropriate chain-of-custody requirements having been met. The sample is then extracted to obtain the organic compounds present. Distillation by steam or alcohols are sometimes use although most labs now use either static or dynamic headspace analysis. Remember that headspace refers to the area above the sample into which volatile compounds gather. Static headspace allows materials to volatilize at room temperature while dynamic headspace refers to that occurring at elevated temperatures usually above 150 degrees centigrade. Material in the headspace is then absorbed onto a column of material like activated charcoal, tenex, etc. The material on the column is then desorbed using a small volume of solvent and the resulting sample is then analyzed using one or more of a variety of analytical instruments.
The End Result
The end result of the analysis is the development of a chemical fingerprint or pattern of chemical components which are matched to the pattern of known accelerants contained in the instruments database. As analytical equipment has improved most notably with the introduction of Gas Chromatography-Mass Spectroscopy the levels of accelerant that can be detected have gotten smaller and smaller leading to a more reliable detection of accelerant use in suspicious fires. However, with this sophistication also comes problems, most notably that of sample contamination. Greater care must be taken in securing sample integrity. As an example, if debris is taken from a fire where gasoline is expected as an accelerant and the samples are placed in the trunk of a car or van where a gas can is also stored (or with a leaky gas tank) these samples can easily be contaminated by traces of gas from the vehicle. Such cases give the defense opportunities to call into question the validity of test results obtained by the lab and may negatively impact a subsequent conviction.
