Pesticides, mass spectra

Figure 3.14 Background mass spectrum obtained from the LC-MS analysis of a pesticide mixture. From applications literature published by Micromass UK Ltd, Manchester, UK, and reproduced with permission.

Interpretation/report The GC retention time of a naphthalene standard and the mass spectrum of this peak confirm its presence. Because of the complexity of the chromatograms of the petroleum products and the pesticide sample, you find it impossible to examine the chromatogram of each. However, a comparison of the GC fingerprints (i.e., the matching of chromatographic peaks and comparison of peak ratios) clearly shows that the sample consists of naphthalene dissolved in kerosene. 

Fig. 21.14. Temperature-programmed capillary GC-MS total ion chromatograms for kerosene (upper trace) and Moth-Knox pesticide (lower trace). Note the similarity in the pattern of peaks, with the exception of the large peak in the pesticide sample (at a retention time of about 12.5 min). The mass spectrum and the retention time of this peak both corresponded to a standard of naphthalene.

This fact can be attributed to its lower number of substituents and consequently lower proton affinity in comparison with the other two triazines. In contrast, it has been reported that when filament-off or thermospray ionization is employed [M + H]+ is the base peak for different chloroatrazines (21,6) similarly as when DLI LC-MS was used (13). Such a difference in the relative abundance of the different adducts in the mass spectrum between filament-on and filament-off has been previously observed for other groups of pesticides (22,23). For ohlorotriazines a [M + 60]+-ion was the base peak using an eluent of methanol-water and ammonium... 

Biros interfaced a small time-averaging computer (Varian C-1024) with a GC-MS to scan and average repetitively a small portion of the mass spectrum (such as the molecular ion region), subtract background peaks, normalize, and plot the partial spectrum. This was done for several chlorinated hydrocarbon pesticide standards (25) and for heptachlor epoxide in human liver tissue (26). 

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