Forensic Applications of Gas Chromatography - Forensic Applications of Gas Chromatography - page 122

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Time --> 1.00 2.003.004.005.006.007.008.009.0010.00 11.00

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12.0013.00 14.0015.00 16.0017.00 18.0019.00

Figure 8.33 Total ion chromatogram for control fire debris.

By comparing a chromatogram of fire debris, the sample is pyrolysed
(burnt). This means that comparing the output from the debris to a pure
liquid standard of ignitable liquids will be a little more difficult. Most of the
lighter (by weight) components found at the beginning of a chromatogram
will disappear from the chromatogram and decomposition products of these
lighter components may not be found. Pyrolysis products from the material
that has been consumed during the fire will also have to be considered. For
this reason, a control sample should always be analysed alongside the fire
debris. This control sample should be of the same material as the 'suspect'
sample; however, the control should be collected as far away as possible from
the sampling site of the 'suspect' sample. A control sample of the same mate-
rial was analysed and the chromatogram is shown in Figure 8.33.
Although the peaks look large, what happens with the software on our
GC-MS, and many other manufacturers' instruments, is that the chromato-
gram scales the chromatogram to have all peaks proportional to the largest
peak. If you look at the top left-hand side of the y-axis of the chromatogram
shown in Figure 8.33, you will note that, for the control sample, the abun-
dance scale reaches approximately 6500. However, if you look at the same
point on the chromatogram in Figure 8.30 for the 'suspect' sample, the abun-
dance scale reaches approximately 600,000.
By examining the extracted ion chromatograms for each of the classes
of compound (Table 8.5) in each of the pure ignitable liquid standards to
that of the unknown sample (fire debris), some of the hydrocarbon fuels can
be eliminated. Figure 8.34 shows the extracted ion chromatogram for petrol
compared to the unknown sample.
What you should be able to see are the similarities in the peaks between
the extracted ions (91, 105 and 119 Da for both the 'suspect' and petrol