The Arson Section examines fire debris for the presence of accelerants. Explosives and explosive residues are examined for chemical makeup. All fire and explosive materials are submitted to the laboratory by the fire service and law enforcement officials.
Packaging of Fire Debris:
Packaging of fire debris evidence is slightly different than many of the other sections of the laboratory. It is very important that fire debris evidence is packaged so that the sample is protected from both evaporation of volatile residues and from sample contamination after collection. This is best accomplished by placing the sample in an air-tight container. The most common are clean unused metal paint cans with a matching lid. Glass jars with tight fitting screw-on lids (using a Teflon type liner) may also be used. Nylon or polyamide "arson" evidence bags are a third option. If using these bags, be careful the debris placed inside does not puncture the bag. Nylon bags should be completely heat sealed or closed in an airtight fashion. In addition, never dry out evidence that is specifically for ignitable liquid analysis. Package and seal the fire debris right away and expedite delivery to the crime laboratory. Freeze or refrigerate samples containing soil or vegetation. Containers should never be filled more than 2/3 (66%) full, as the laboratory needs an adequate vapor space above the debris for testing. NFPA 921 should be referenced and used as a guide when collecting and packaging fire debris evidence.
Analysis of Fire Debris:
The most common technique the laboratory uses for ignitable liquid analysis is called "passive headspace concentration". This method extracts trapped ignitable liquid residues that are still possibly present in the fire debris. After being extracted from the evidence sample, the trapped ignitable liquid molecules are put into a liquid solution of carbon disulfide. The solution is then injected into a gas chromatograph/mass spectrometer. This instrument creates an electronic representation of the organic chemicals in the sample mixture. The instrument produces a "total ion chromatogram" which can then be further subdivided into specific ion fragment profiles and mass spectra to determine the presence or absence of characteristics specific to ignitable liquids.
| Ignitable Liquids | Percentage |
|---|---|
| Light Petroleum Distillates | 3% |
| Medium Petroleum Distillates | 7% |
| Heavy Petroleum Distillates | 4% |
| Miscellaneous | 4% |
| Gasoline | 24% |
| Mixtures | 3% |
| No Ignitable Liquids | 48% |
| Comparison Samples | 2% |
| Improperly Packaged | 4% |
| Total | 100% |
Explosive devices, powders or residues are analyzed for chemical composition. The most common types of explosives analyzed by the DCI Crime Laboratory include black powders and substitutes, smokeless powders and pyrotechnic compositions. All explosive devices should be rendered safe prior to submission to the laboratory.
For more information see the following web sites:
- International Association of Arson Investigators Iowa Chapter
http://www.iowaiaaichapter.org/
- International Association of Arson Investigators
http://www.firearson.com/
- National Institute of Justice
http://ncfs.ucf.edu/