Comparison between positive, negative and chloride-enhanced negative chemical ionization of organophosphorus pesticides in on-line liquid chromatography-mass spectrometry.

Positive and negative chemical ionization (PCI and NCI, respectively) have been used for the characterization of ten organophosphorus pesticides in on-line liquid chromatography-mass spectrometry (LC-MS). LC analyses were performed on a 20 cm X 0.7 mm I.D. C8-bonded phase using acetonitrile-water (70:30) or acetonitrile-water-chloroacetonitrile (69:30:1) as eluent. With PCI, molecular weight information was obtained with both eluents. For NCI considerable differences in the spectra were found using the two eluents. Without chloroacetonitrile the spectra were dominated by the functional group fragment and with chloroacetonitrile the base peak was [M - R]- with R being methyl or ethyl, while the spectra further contained the functional group ions. Special emphasis was devoted to the occurrence of chloride attachment at different source temperatures. With several compounds the [M + Cl]- ion was formed and its relative intensity strongly increased when the source temperature decreased. With NCI the sensitivity was about one order of magnitude better than with PCI. This advantage was partly lost when 1% of chloroacetonitrile was used in the eluent; on the other hand, complementary structural information was obtained. As an application, the determination of three organophosphorus pesticides in sediment is reported.