THERMOSPRAY MASS-SPECTRAL STUDIES OF PESTICIDES - TEMPERATURE AND SALT CONCENTRATION EFFECTS ON THE ION ABUNDANCES IN THERMOSPRAY MASS-SPECTRA

The dependence of the ion abundances in the thermospray (TSP) mass spectra of several pesticides, including anilides, carbamates, N-heterocyclic and organophosphorus compounds and phenylureas on the vaporizer and the gas-phase temperatures and under collision-activated dissociation conditions was investigated. The results clearly demonstrate that fragmentation in the TSP mass spectra of the investigated pesticides was mainly caused by gas- or liquid-phase ''chemical dissociation'' reactions of neutral analyte molecules or, in some instances, of the quasi-molecular ions in the ion source or the vaporizer probe, probably induced by solvent or buffer ions which can be described by well defined mechanisms. A linear relationship was observed for most of those pesticides which showed combinations of the two quasi-molecular ions [M + H]+ and [M + NH4]+, when the logarithm of the ion abundance ratio for the [M + H]+ ion relative to the [M + NH4]+ ion was plotted against the reciprocal of the absolute temperature of the gas phase. It is. shown that this dependence can be used to generate TSP mass spectra with mainly one quasi-molecular ion. This may be of value for selected-ion monitoring experiments, because the total ion current (i.e., the sum of the [M + H]+ and [M + NH4]+ ions) is less dependent on the gas-phase temperature than the ion currents of the individual quasi-molecular ions. It was found that additional adduct ions beside these quasi-molecular ions could be observed in the spectra of several pesticides for which the formation was limited to low gas-phase temperatures. In addition, the results for the investigated quaternary ammonium compounds clearly show that the addition of a volatile buffer salt to the mobile phase induces chemical reactions in the gas phase which have a strong influence on the ion abundances. However, addition of buffer salt was necessary to obtain intense signals although the compounds are already completely dissociated in the aqueous solvent and fragmentation was enhanced as the buffer concentration was raised.