An evaluation of atmospheric pressure ionization techniques for the analysis of N-methyl carbamate pesticides by liquid chromatography mass spectrometry

Abstract

Atmospheric pressure ionization (API) techniques are evaluated for the mass spectral analysis of N-methyl carbomate pesticides. Atmospheric pressure chemical ionization (APCI) using a heated nebulizer interface provided both protonated molecules and abundant, characteristic fragment ions. With ion spray (ISP; pneumatically assisted electrospray ionization), which utilizes a milder “ion evaporation” process, primarily protonated molecules were obtained, although fragment ions similar to those observed in APCI could be induced by variation of the API orifice voltage. Product ion spectra of ISP-derived protonated molecules, generated by tandem mass spectrometry using collision-induced dissociation, are also presented. The APCI and ISP spectra of the carbamates are compared to those obtained with a thermospray interface and also to their electron ionization and methane CI spectra obtained with a particle beam interface. For all four interfaces, combined liquid chromatography mass spectrometry methods using conventional (4.6 mm i.d.) columns are described for the separation and detection of pesticide mixtures. These methods are applied to the confirmatory analysis of three representative carbamate pesticides, spiked at the 0.1-ppm level in green peppers. For those carbamates amenable to gas chromatography mass spectrometry, comparative results are presented.