Page 231 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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background, or that such small quantities of hydrocarbons could be detected
that virtually any sample would be positive. In fact, more sensitivity alone
is, not by itself, the solution to obtaining the additional information the
forensic scientist needs. What is needed is more specificity in the test. The
outcome of enhanced specificity is increased confidence in the result and a
natural by-product is some increase in sensitivity. More specific methods can
often reveal the organic characteristics necessary to conclusively report
whether the sample is positive or negative for the presence of an ignitable
liquid. The way that a scientist can improve specificity can be in the form of
new chromatographic methods, new processing software, new instrumental
hardware, or a combination of these.

5.2.2.1 Specificity Improvement — Chromatographic
To improve specificity, the scientist can turn to improving chromatographic
resolution, which usually requires longer analysis times, or to narrower inter-
nal diameter capillary columns that have higher resolution but less sample
capacity. The scientist can also use multidimensional or 2D chromatography
(GCxGC) which can be very complex, especially for this matrix. Improving
chromatography alone is not enough to adequately address these samples.
Instead of improving chromatography — or in addition to it — we turn to
detection systems to improve specificity.

5.2.2.2 Specificity Improvement — Detection
Detection using mass spectrometry has greatly improved specificity over past
detectors such as FID or PID. Today, GC/MS represents the benchmark in
specificity and thus the benchmark in confidence in the result. However, for
weak samples, highly complex samples, or highly weathered samples, forensic
scientists may still find themselves lacking some chemical characteristics
necessary to provide a clear positive or negative result. In these cases analysts
find that the sample comparability is not adequate relative to their library of
reference analyses. To obtain more information from GC/MS analysis, there
are ways of improving specificity using new software for data reduction.

5.2.2.3 Specificity Improvement — Data Processing Software
Newly developed algorithms, to deconvolute chromatographically coeluting
compounds, effectively enhance selectivity. This is a result of the resurgence
of Time-of-Flight (TOF) mass spectrometers which have showcased the abil-
ity to deconvolute target compounds from very difficult artificial concoctions
as well as real samples. Currently, this is not just a feature for TOF spectrom-
eters but can also be conducted on data acquired by more common ion trap
or quadrupole-based mass spectrometers. Software such as Ion Fingerprint
Detection (IFD) has been used by this author and has shown similarly

© 2004 by CRC Press LLC