Chloride attachment negative chemical ionization mass spectra of organophosphate pesticides

This paper reports the dichloromethane-mediated negative chemical ionization mass spectra of 17 organophosphorus pesticides having the general structure (RO)2P(S)S(CH2)nR (R = Me, Et or iso-Pr; n = 0, 1, or 2; R′ contains one or more heteroatoms) and demeton-O (O, O, S-triethylphosphorothioate). In all cases the spectra are simple and characteristic of the compound in question. In general, the [(RO)2PS2]− anion was the base peak, and a chloride attachment ion was invariably present; this ion allows molecularly specific confirmation of the compound. Molecules having stable potential anions in the R′ moiety, e.g. carbophenthion and phencapton, selectively form the stable anion. Dissociative capture or nucleophilic displacement on the R group of methyl and ethyl esters increased in prominence at higher temperatures; [M – R]− was the base peak in the negative chemical ionization mass spectrum of dimethoate (R = Me, n = 1, R′ = C(O) NHMe) above 200°C.