ANALYSIS OF FIRE SCENE ACCELERANT RESIDUES BY GC
GC is almost universally employed in crime laboratories for the analysis of fire scene accelerant residues. Today, most laboratories use capillary GC columns for increased sensitivity and efficiency. Increasingly, mass spectrometry has been employed as the detector for the GC to identify certain components of the residues. All of the hydro carbon accelerants, including gasoline, kerosene-based materials, fuel oils, and other consumer products have many components. Gasoline, for example, is made up of more than 300 substances. In basic GC analysis of accelerant residues, the resulting chromatogram will be a pattern of peaks that is characteristic of the accelerant type. Thus, one can identify an accelerant as gasoline or as a kerosene-based product, for example, but it is generally not possible to identify a specific product or manufacturer by this method. The key to effective analysis of accelerants by GC is to have a comprehensive library of chromatograms that are obtained preferably on the same instrument as the analysis of unknowns, or at least taken under the same conditions. This library would include not only the various products, brands, and types of accelerants, but also their various forms. For instance, an accelerant may appear in its neat, unburnt form or partially burnt or almost totally consumed. The chromatograms of these materials will be quite different and it would be difficult to tell what is present unless there are good standards for comparison. Figure 1(a) and (b) shows chromatograms of pure gasoline and kerosene. Figure 2 shows the chromatogram of gasoline headspace. Mass spectrometry has added flexibility and refinement to GC analysis of fire scene evidence. Individual components of residues can be unequivocally identified. In addition, mass spectrometry provides some features that make it especially valuable in identifying ignitable liquids in the presence of contaminants or in mixtures containing multiple ignitable liquids. The first of these is called selective ion monitoring, whereby the mass spectrometer looks for particular ions that are characteristic of particular types of flammables. The other enhancement is called target compound analysis. In this technique, a profile of compounds that are present in each type of accelerant, such as gasoline, are monitored by the mass spectrometer. These compounds can be easily identified even in complex mixtures. The mass spectra of individual components of a material such as gasoline can be easily dis played. The mass spectrum of one of the compounds found in gasoline is shown in Figure 3.
FIGURE 1 Chromatograms of neat (pure liquid) gasoline and kerosene. Note how different the pat terns in the peaks of these chromatograms are. Gasoline is much more volatile as can be seen by the large number of peaks at the beginning of the run.
FIGURE 2 Chromatogram of gasoline headspace. In this chromatogram, most of the most-volatile peaks that come out early are missing, having been burned off by heat.
FIGURE 3 Mass spectrum of one of the components in a gasoline mixture. This peak pattern is characteristic of normal hydrocarbons.
