Page 233 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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beyond unit mass (high resolution mass spectrometers [GC/HRMS]), or by
the addition of another mass spectrometer procedure (GC/MS/MS).
Very recently GCxGC/MS (or two-dimensional gas chromatogra-
phy/mass spectrometry, 2D GC/MS) has been used in experimentation on
4
fire debris. The objective was to address the extra and coeluting pyrolysate
peaks that complicate the interpretation of class ion profiles, thus adversely
affecting some compound ratios. A TOF instrument (Leco Pegasus) was used
due to its very fast scanning speed, essential for 2D GC/MS. An internal oven
modulator was used to sequentially heart-cut 4-second components that are
diverted to another more polar capillary column. The resulting 2D data plots,
similar in appearance to 2D Gel Proteomics data, illustrated the additional
separation of coeluting compounds. The data is displayed as a top–down
view of a now-2D total ion chromatogram. With real samples, these 2D plots,
which show a dot for the presence of each compound, are difficult to compare
even to the trained eye. This data has not been used in a court of law and in
this format would be incomprehensible to judge and jury alike. If the software
could be visually improved to more easily compare chemical class informa-
tion in a courtroom situation, this approach might have merit as it adds
additional specificity to the GC/MS analysis by chromatographically resolv-
ing some additional pyrolysates from compounds of interest. The 4-second
analysis on the more polar column does not appear to have a particularly
high level of chromatographic resolution. It is unknown whether the analyst
can display selected compounds that have been deconvoluted in a selected
ion profile manner to allow easy pattern recognition and comparison of
compound class characteristics.
Analysis by GC/HRMS significantly improves the analysis by improving
the mass resolution; e.g., the mass of an ion can be measured to 0.001 amu at
a resolution of 10,000, which often completely removes all pyrolysates that have
a similar mass. However, this half-million dollar instrument is usually reserved
for environmental samples that are tested for the presence of polychlorinated
dibenzo-p-dioxins and -furans at ultra trace parts-per-quadrillion levels.
GC/IRMS may show promise, but it is of high cost and is also in exper-
imental use and unproven for trace evidence work.
With the advent of the benchtop ion trap mass spectrometer, at a cost
similar to quadrupole units, the price to obtain MS/MS capability and its
specificity is not an obstacle and is now discussed further.

5.2.3 Analysis by GC/MS/MS

As references will show, the use of GC/MS/MS is well documented in other
fields of analysis including forensics. This laboratory has conducted research
and development using GC/MS/MS since April 1997. GC/MS/MS is the

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