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versal carbon detector such as a flame- or photo-ionization detector. This
GC/FID or GC/PID approach is still being used today but is surpassed in
specificity — and thus in confidence in the result — as well as in its sensitivity,
by detection with a mass spectrometer (GC/MS). The mass spectrometer is
a much more flexible detector with effectively a universal response when
operated in a full scan mode. The result is a total ion chromatogram very
much like the chromatogram obtained from a GC/FID or GC/PID. The mass
spectrometer however can offer another dimension of specificity by obtaining
a mass spectrum for a response that can be used to obtain a tentative iden-
tification through computer library matching algorithms. Also, the scientist
can reconstruct specific-mass ions of interest that are characteristic for a
specific-class of organic compounds. For example an ion at m/z 120 can be
reconstructed to show an ion chromatogram that describes the profile for
dimethylbenzenes that may be present as a chemical class. This profile can
also be confirmed by similarly reconstructing (for a secondary identification)
the ion at m/z 105.
However, when samples are very complex, the interpretation may be
more difficult, and the scientist often would like to see more clear information
upon which to base the result. This is especially true when some character-
istics of an ignitable liquid may be observed in small quantities but may not
be comparable enough to the laboratory’s library of reference analyses to
document a positive result for the presence of an ignitable liquid. In some
complex, weak, or highly weathered samples, the characteristic ion profiles
do not compare well enough for the layman to understand, whether judge
or jury. More information is then needed to conclusively show whether an
ignitable liquid is present or not. Other methods of analysis may provide the
additional information that confidently documents a positive or negative
result in an unmistakable manner to both judge and jury.

5.2.2 Specificity — The Key to Difficult Samples
There are several ways to approach the analysis of samples that have a com-
plex makeup or matrix. Fire debris samples may be a significant challenge
when they have a complex matrix dominated by large quantities of pyroly-
sates, combined with compounds that are inherent to the material(s). These
can be many times more in quantity than any ignitable liquid residue that
may be present. To uncover the ignitable liquid characteristics, the method
has to either remove the unwanted pyrolysates and matrix or improve the
specificity of the test. Extract cleanup approaches have not been popular and
have not been routinely effective; some have recently been removed from lists
published by ASTM International.
It has been argued that more sensitivity is not required as this might lead
to false positive results that may really be a natural characteristic of the

© 2004 by CRC Press LLC