Detection of fire accelerants

Detection of fire accelerants is the process that a fire investigator uses to determine if fire accelerants were used at a fire scene. This process involves a combination of both field work and laboratory analysis by fire investigators and chemists.

In order for a positive identification of a fire accelerant to occur both field work and laboratory analysis must take place. This is because when a fire accelerant is used only ignitable liquid residues (ILRs) remain at the scene. It is the chemist's job to identify these ILRs and the investigators job to determine if they were used as fire accelerants or just present at the scene under normal circumstances.

It is common for the words fire accelerant and ignitable liquid to be used synonymously. An ignitable liquid is a liquid that will readily ignite when exposed to an ignition source, while a fire accelerant is a material that is used to increase the rate of combustion for materials that do not readily burn.

Ignitable liquids are not always fire accelerants, they may just be present at the scene under normal circumstances. Gasoline is the most common fire accelerant used, but it could also be present at a scene as an ignitable liquid due to gasoline being a common fuel. Although ignitable liquids are the most common fire accelerants, other chemicals such as propane or natural gas could also be used to accelerate a fire.

Detecting the use of a fire accelerant at a crime scene can be the difference between classifying a fire as accidental or as an arson. Once a case is determined to be an arson, the detection of fire accelerants will hold strong evidentiary value which the prosecutor can use during the trial should someone be charged.

Determining the origin of a fire is often one of the first tasks that a fire investigator must complete while at the scene. This is completed because the origin will have the highest probability of containing any ILRs left from the use of fire accelerant. This is logical because accelerants would be the first materials ignited as they have a lower ignition temperature than any other materials. Once the origin is determined the investigators must decide if fire accelerants were used at this scene. Often the first and most common way of determining if accelerants were used is by completing a visual inspection of the scene and specifically the origin. A trained investigator would look for cues like intense localized burning or pour patterns to indicate the use of accelerants.

Accelerant detecting canines can also be used to determine if accelerants were used at a scene and pinpoint the location of use. These canines have been trained to detect trace levels of ILRs and can lead an investigator to an area that will have a high probability of containing ILRs.

Detection with portable hydrocarbon sniffers is a recent method which is more readily being used by investigators. These are handheld electronic devices that sample the vapors at a scene and will give a reading for the concentration of hydrocarbons it is detecting. By comparing the concentration of hydrocarbons in the area to known levels of ILR free areas an investigator will be able to determine if ILRs are present at the scene. They will then take samples from the areas that are showing the highest concentrations.