Fire debris
The Fire Debris Section performs fire debris and chemical unknown analysis. The identification or characterization of these unknown materials is accomplished by the examination of their physical properties, wet chemistry techniques, and using analytical instrumentation.
Fire Debris Analysis
Fire debris analysis is the examination of evidentiary items to determine the presence or absence of an ignitable liquid. Ignitable liquids are any liquid, or the liquid phase of any material, that is capable of fueling a fire. These include a flammable liquid, combustible liquid or any other material that can be liquefied and burned. If an ignitable liquid is present, it is classified, and possible sources are determined. While most materials tested consist of debris from a fire, other items that can be tested are clothing from a victim or suspect, soil or vegetation from around building exteriors or empty containers that may have been used to carry an ignitable liquid. The analysis of ignitable liquids generally follows a three-step process:
Sample Preparation and Extraction
Most fire debris samples in the laboratory are prepared by a process known as heated passive headspace concentration. A piece of activated charcoal (c-strip) is placed inside the fire debris container. The sample is heated overnight. Volatile compounds from the debris enter the headspace of the container and stick to the c-strip, concentrating the sample. The c-strip is then removed from the container, placed into a vial, and eluted with a solvent, removing the volatile compounds. The solvent is then analyzed by gas chromatography-mass spectrometry (GC-MS).
Separation and Detection
Each sample is run through a gas chromatograph-mass spectrometer (GC-MS). The GC portion of this instrument separates the compounds from each other. After separation, the individual compounds enter the MS for detection. Fire debris sample analysis can be very complicated. The number of compounds in an ignitable liquid can vary from one (example toluene) to hundreds (example kerosene). Also, fire debris samples produce volatile compounds that are not components of ignitable liquids.
Data Analysis and Interpretation
Analysis consists of a visual comparison of the chromatograms from the unknown sample to those of reference ignitable liquids. Pattern matching, carbon number range determination, extracted ion profiling, target compound analysis, and mass spectra library matching is also performed. If an ignitable liquid is detected, it is placed into one of the following American Society for Testing and Materials (ASTM) classifications:
- Gasoline
- Petroleum Distillates
- Isoparaffinic Products
- Aromatic Products
- Naphthenic-Paraffinic Products
- N-alkane Products
- Oxygenated Solvents
- Miscellaneous
Chemical Unknown Analysis
The analysis of chemical unknowns involves the examination of materials related to fires (example, road flares). This type of evidence can be encountered at a variety of fire scenes. Evidence may be in the form of a solid, liquid, mixture or gas. Submissions may be for identification or comparison to a known sample.