Fire debris is a general term used to define the debris from a fire that is collected as evidence for laboratory examination. When a fire investigator suspects that a fire might have been deliberately set using accelerants such as ignitable liquids, it is possible to collect and analyze fire debris to see if such products are present.
When a person pours an ignitable liquid onto a substrate such as carpet, furniture, or clothing, that liquid gets adsorbed inside the substrate. When the liquid is set on fire, only the surface of the liquid burns and part of it is protected deep inside the substrate. If the fire department puts out the fire early enough, there are traces of this liquid left where the liquid was poured inside the burned debris. The fire investigator who collects the debris then sends it to the laboratory for analysis. The debris needs to be packaged in special containers that are sealed to prevent vapors of flammable or combustible liquids from escaping.
The forensic laboratory analyzes the debris using chemical techniques. First, the residues of the liquid are extracted from the debris, so they can be analyzed without the debris. For this step, different procedures can be used, but usually the sample is heated and the vapors are trapped onto a charcoal strip. This charcoal strip would then contain the residues of ignitable liquid. These residues are separated from the charcoal using a solvent. Once in the solvent, it is possible to analyze the residues with a gas chromatograph or gas chromatograph–mass spectrometer .
Once the analysis is done, it is important to interpret the results carefully. Modern furniture and clothing are composed of polymers that are based from petroleum products, the same petroleum products that are used to manufacture most of the flammable and combustible liquids such as gasoline, diesel fuel, charcoal starter fluid, and paint thinner. Thus, it is very important for the forensic scientist to be able to distinguish the presence of ignitable liquid from the chemicals that are produced by modern substrates. Only the proper collection, examination, analysis, and interpretation of the fire debris sample allow the forensic scientist to reach the proper conclusion.
see also Arson; Canine substance detection; Chromatography; Fire investigation.