Page 209 - Advances in Forensic Applications of Mass Spectrometry - Jehuda Yinon

1522_book.fm Page 190 Thursday, November 13, 2003 9:58 AM

Table 5.1 The GC, MS, and MS/MS Conditions
Using the Varian Saturn 2000 Ion Trap
GC Conditions
He flow rate (ml/min) 1
Injection mode Split and splitless
Split ratio in split mode 18:1
Injector temperature (ºC) 280
Run time (min) 23
MS Conditions
Scan time (m sec) 2
Target TIC (counts) 5000
Maximum ionization time (msec) 25000
Ion trap temperature (˚C) 180
Prescan ionization time (m sec) 1500
Background mass (amu) 38
RF dump mass (amu) 650
Scan range (amu) 40–400
MS/MS Conditions
Excitation voltage (V) 40–60 (aromatics)
0 (aliphatics)

One advantage of the MS/MS mode is the substantial reduction in back-
ground ions for a particular selected ion. Figure 5.11 is an example of the
increase in signal relative to the noise when m -xylene is detected in MS (top)
and MS/MS (bottom) modes. Table 5.2 summarizes the improvements in
sensitivity, in most cases, when MS/MS is used for the analysis of target
compounds used in fire debris analysis.

5.1.4 Conclusions
GC/MS/MS provides improved selectivity for the identification of target
compounds when coeluting compounds mask the compounds of interest.
The detection limit (S/N>3) for a single component target compound in an
ignitable liquid residue is ~ 10 times lower when using GC/MS/MS in the
splitless mode over GC/MS in the splitless mode. The combination of
improved selectivity and improved sensitivity provides GC/MS/MS with the
potential for improving both the detection and the identification of target
compounds in residues extracted from fire debris, especially in cases where
the sample concentration is very low and/or when the sample contains inter-
fering species. It is expected that future work will include the generation of
compound specific MS/MS spectra under standardized conditions for target
compounds of interest in fire debris analysis.

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