Page 232 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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impressive capabilities to the deconveluting algorithms associated with TOF.
IFD has also impressed the U.S. Environmental Protection Agency for the
analysis of target compounds in difficult environmental samples.
Another software approach to improve specificity is to use pattern rec-
ognition to search for the presence of certain chemical compounds and
1
classes. Macro methods and identification stratagems have been published 2
that aid the scientist in the interpretation of samples primarily using mass
ion profiles. A consistent approach to identification is required but it is often
very time-consuming. Thermo-Finnigan’s Xaminer program can be used in
a stand-alone fashion or is well integrated into their Xcalibur data processing
software. Xaminer can process the contents of a GC/MS sample analysis and
isolate target compounds. It then formulates these target compounds into
a spectragram comprised of compounds of many classes. The unknown
spectragram is then compared against the spectragrams developed with the
laboratory’s library of reference substance analyses and provides a list of
probable results. The results are listed in order of a fit score that uses the
same Biller–Biemann algorithms that scientists have been using for years
for NIST mass spectral library searches. The Xaminer algorithm uses an 8-
peak presearch and the 50 most significant spectragram peaks with propor-
tional mass weighting are removed. The simple intensity vs. compound
identification data arrays allows for very rapid searching against the labo-
ratory’s library of spectrograms developed from reference ignitable liquid
analyses. This gives confidence to the result as it adds the specificity of
pattern recognition across the chromatogram. Also of significant impor-
tance is that with increasing numbers of ignitable liquid substances, it
becomes more and more difficult for the scientist to remember every char-
acteristic for all the substances in the laboratory’s growing reference library.
It is thus a very valuable tool for the scientist, who can then ensure a
complete, reproducible, and unbiased review of the sample against the ref-
erence library. The scientist can then quickly double-check the results, know-
ing that the sample characteristics have also been compared to not only
common ignitable liquids in the reference library but also to the more
obscure substances that are rarely seen. Some scientists at the Miami–Dade
County Crime Laboratory are currently exploring the use of Xaminer in
conjunction with GC/TOF. 3

5.2.2.4 Specificity Improvement — Hardware
Specificity, beyond full scan mass spectrometry (GC/MS), can be improved
by adding additional hardware to the method. This can be done by adding
another gas chromatograph with the mass spectrometer as detector
(GCxGC/MS), using emerging technology such as GC/isotope ratio mass
spectrometry (GC/IRMS), improving the mass resolution of the spectrometer

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