Mass spectral peaks, pesticides

MS detection does not necessarily require as highly resolved GC separations as in the case of selective detectors because the likelihood of an overlapping mass spectral peak among pesticides with the same retention time is less than the likelihood of an overlapping peak from the same element. Unfortunately, this advantage cannot always be optimized because SIM and current gas chromatography/tandem mass spectrometry (GC/MS/MS) methods, it is difficult to devise sequential SIM or MS/MS retention time windows to achieve fast GC separations for approximately > 50 analytes in a single method. 

Some difficulties were encountered in the absolute confirmation of configurational and structural isomers where mass spectral peak intensities and mass values of characteristic fragment ions were too similar under the conditions of the analysis. In those instances, however, identification was readily made on the basis of gas chromatographic retention time data. The estimated lower limit of detectability of organochlorine pesticide residues by this analytical scheme and instrument configuration was determined to be approximately 0.05 to 0.1 ppm. 

Table I. Gas Chromatographic Peak Identities and Characteristic Mass Spectral Fragments and Intensities of Some Organochlorine Pesticide Residues Isolated From Human Adipose Tissue...

Low resolution mass spectrometry (MS), especially in tandem with gas chromatography, and nuclear magnetic resonance (NMR) spectroscopy have been reviewed with respect to their application to pesticide residue analysis. Sample preparation, direct probe MS analysis, GC-MS interface problems, spectrometer sensitivity, and some recent advances in MS have been studied. MS analyses of pesticide residues in environmental samples (malathion, dieldrin, dia-zinon, phenyl mercuric chloride, DBF, and polychlorinated biphenyls) have been illustrated. Fragmentation patterns, molecular ions, isotope peaks, and spectral matching were important in the identification of these pesticides. The sensitivity limitations of NMR and recent improvements in sensitivity are discussed along with examples of pesticide analyses by NMR and the application of NMR shift reagents to pesticide structure determinations. 

GC xGC-ToF MS linearity was excellent in solvent and only slightly affected by the matrix, and the LODs were in general below 20 gg/kg. Furthermore, the acquisition of full-range mass spectra provided powerful confirmation of the pesticides in the sample, even for nontarget analytes [45,48,49]. This potential is in principle reinforced by the automated peak find and spectral deconvolution software capabilities, although in practice intensive manual data revision is often needed [45,48,50]. 

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