Page 202 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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5.1.3 Fire Debris Analysis
The investigation of a suspected arson includes the extraction, isolation, and
analysis of the debris resulting from the fire, in order to determine the
presence of any ignitable liquid residues (ILR) that may have been used as
an “accelerant” and could serve as evidence of an arson. Typical accelerants
used are automobile gasoline, diesel fuel, and other volatile mixtures that can
be acquired commercially. The literature describing the extraction of these
compounds from debris, the analysis of the extracts, and the interpretation
of the data is very mature. 17–24 Forensic scientists, through the ASTM, have
developed and published consensus standards for the analysis and interpre-
1–8
tation of fire debris evidence. These advances have led to what are now
fairly routine and standardized analytical methodologies for the analysis of
fire debris. In most cases, there is enough target compound concentration
in the sample so that the extraction and analysis is straightforward. In the
absence of methods for the extraction and analysis of very low concentrations
of the volatile organic compounds that make up ILRs, the sample may be
determined as negative, possibly ending the investigation process. Recent
reports of sensitive methods for the extraction and analysis of ILR target
compounds, including the use of solid phase microextraction (SPME), have
described the application of SPME as an improvement in the extraction and
preconcentration of the analytes of interest in a single, simple step prior to
GC/MS analysis. 25–35,12 These methods have been shown to improve the sen-
sitivity of the analysis by improving the extraction efficiency of very low
concentrations of the target compounds. Further complications during the
analysis of these kinds of samples include the presence of compounds from
the substrate background and product compounds formed during the com-
bustion and pyrolysis of substrate material. It has been reported that these
24
interferences could have an effect on the analysis of the ILRs, even when
GC/MS is used as the analytical technique. One possible solution to the
resolution of small quantities of target compounds in the presence of inter-
fering species is the use of GC/MS/MS, which isolates the target compounds
of interest in the first MS experiment (using an ion trap), followed by dis-
sociation of the “trapped” target compound, and finally, the identification
of the target compound, free of interferences. The following describes the
use of MS/MS in the isolation and analysis of ILRs in the presence of inter-
fering species when the sample contains ILRs in very low concentrations, and
GC/MS alone cannot solve the problem.

5.1.3.1 Ion Trap MS/MS in Fire Debris Analysis
Electron ionization (EI) is typically used as the source of ionization in the
mass spectral analysis of ignitable liquid residues. A voltage of 70 V imparts
enough energy in the ionization process to form positive ion fragments.

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