Page 245 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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is analyzed by acquiring a precursor ion with no collision energy added. In
a chemical-class specific analysis, an ion at m/z 71 or 85 is chosen for mon-
itoring alkanes and is subsequently reconstructed for data review and assess-
ment as in GC/MS. This ion is acquired in a very similar fashion to that in
selected ion monitoring (SIM) in GC/MS. As evidence of branched alkanes
is required for distillate or gasoline identification, this class represents the
limiting factor in sensitivity for MS/MS. Beyond zero energy addition, other
instrumental parameters can be modified to enhance alkane class sensitivity.
Some researchers have worked on obtaining a response for the molecular ion
to use as a precursor ion in order to obtain an associated daughter ion in
16
MS/MS. These parameters have been recently reported by researchers who
have also recognized that the alkane class is a limitation to the sensitivity of
a target-class specific GC/MS/MS analysis.
Using this approach, the analysis of a stair tread had some of these
criteria, but there were insufficiently clear characteristics to define a positive
result to present in court (see Figure 5.22). In review of the TIC in the initial
analysis, this sample would have been reported as negative by GC/MS,
whereas the TIC from the GC/MS/MS analysis shows that the sample has
many additional compounds not seen in the GC/MS analysis.
A part of the examination of the stair tread sample, Figure 5.23 illustrates
the improved comparability of this sample to a gasoline reference for these

MCounts file5690.ms RIC all
1.25 GC/MS of Sample
Result Would be Negative!
1.00
0.75
TIC is dominated by strong pyrolysates
0.50 Some Ignitable Liquid components were detected
0.25

0.00
kCounts f5704ms2.ms RIC all
GC/MS/M S of Sample
30
TIC shows the focus on Ignitable Liquid Components,
pyrolysates effectively removed
20

10

0
2.5 5.0 7.5 10.0 minutes
Figure 5.22 Top profile: Sample of a stair tread by GC/MS; bottom profile:
same sample by GC/MS/MS illustrating increased detection of ignitable liquid
components and reduced effect by pyrolysates.

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