Effect of ion source pressure on ion formation of carbamates in particle-beam chemical-ionisation mass spectrometry

Abstract

In this study the mass spectra of fourteen carbamate pesticides, obtained with desorption chemical ionisation (DCI) and flow injection (FIA)-particle beam (PB)-ammonia positive chemical ionisation (PCI)-mass spectrometry (MS), are compared. The mass spectra from the FIA-PB-PCI-MS experiments exhibit higher relative abundances for fragment ions. The influence of the ion source pressure and temperature on the ion abundances under ammonia positive chemical ionisation conditions was studied. The results indicate that thermal degradation of the carbamate pesticides takes place in the FIA-PB-MS system. In addition, the [M + NH 3 + H] + and [M + NH 3 + H - CH 3NCO] + ion intensities are strongly dependent on the ion source pressure, especially for carbofuran as an extreme. Both the ion source pressure and the temperature cause irreproducibility of the ammonia PCI mass spectra of carbamates under liquid chromatography PB-PCI-MS conditions and it is therefore of utmost importance to use well defined experimental conditions for the quantitative determination of carbamates.