Page 204 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

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MS/MS experiments are capable of selecting the compounds of interest
and isolating them from the interfering components that coelute from the
GC. In aromatic compounds, the molecular ion is selected, while in alkanes,
which form fragment ions a typical fragment ion such as m/z 85 is chosen.

5.1.3.2 MS/MS Examples
Figure 5.3 illustrates the chromatogram of pyrolysis products (top) and
pyrolysis products with a number of the components in gasoline added to
form a total mixture concentration of 1,000 ppm (middle) along with a
1,000 ppm gasoline standard (bottom), using GC/MS analysis. Pyrolysis
products were formed by burning a 2" ¥ 2" section of nylon carpet and
extracting the headspace over the burned carpet using an activated charcoal
strip (ACS) measuring 8 ¥ 20 mm and eluting the ACS with 100 ml of carbon
disulfide. Figure 5.4 shows the same samples analyzed under GC/MS/MS
conditions. The GC/MS/MS chromatograms are more easily interpreted,
and the gasoline components are clearly isolated from the coeluting inter-
ferants from the burned carpet. In order to conduct such an experiment,
the expected retention times for the target compounds must be known and
the MS/MS conditions applied for each retention time. The gasoline stan-
dard was used to determine the retention times and MS/MS conditions. The
MS/MS conditions using the Varian Saturn 2000 ion trap are summarized
in Table 5.1. The trap temperature is set lower than normal operation of
the trap (~ 100˚C to 170˚C). The excitation voltages for each of the com-
pounds of interest are first determined in a preliminary experiment and the
resulting MS/MS spectra are saved in a library for comparison to spectra
generated by the samples.
In a second example of the utility of GC/MS/MS analysis, a small amount
(<1 ml) of gasoline was added to previously burned nylon carpet and the
heated headspace above the burned debris was extracted using an ACS. The
resulting chromatograms from the GC/MS analysis (Figure 5.5) and the
GC/MS/MS analysis (Figure 5.6) further illustrate the improvements in selec-
tivity of the MS/MS technique and the simplification of the interpretation
of the analytical results. A gasoline standard is shown on top of both Figure
5.5 and Figure 5.6. The gasoline components in the bottom chromatogram
of Figure 5.5 are masked when using only GC/MS analysis. The gasoline
components isolated from the interfering pyrolysis products are clearly
shown on the bottom chromatogram of Figure 5.6.
Figure 5.7 and Figure 5.8 illustrate the similarities and differences
between the MS spectrum of m-xylene (Figure 5.7) and the MS/MS spectrum
of m-xylene (Figure 5.8). Similarly, Figure 5.9 and Figure 5.10 illustrate the
differences in mass spectra for naphthalene for the MS and MS/MS modes
of operation, respectively.

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