FRAGMENTATION OF PROTONATED O,O-DIMETHYL O-ARYL PHOSPHOROTHIONATES INTANDEM MASS-SPECTRAL ANALYSIS

A study was carried out on the fragmentation of 12 protonated O,O-dime thyl O-aryl phosphorothionates by tandem quadrupole mass spectrometry. Some of the studied compounds are used in agriculture as pesticides. Energy-resolved and pressure-resolved experiments were performed on th e [M + H](+) ions to investigate the dissociation behavior of the ions with various amounts of internal energy. On collisionally activated d issociation, the [M + H](+) ions decompose to yield the [M + H - CH3OH ](+), (CH3O)(2)PS+ (m/z 125), and (CH3O)(2)PO+ (m/z 109) ions as major fragments. The ions [M + H - CH3OH](+) and (CH3O)(2)PS+ probably aris e from the [M + H](+) ions of the O,O-dimethyl O-aryl phosphorothionat es with the proton on the sulfur or on the oxygen of the phenoxy group . The origin of the hydroxy proton of the methanol fragment was in man y cases, surprisingly, the phenyl group and not the reagent gas. This was confirmed by using deuterated isobutane, C4D10, as reagent gas in C1. The fragment ions (CH3O)(2)PO+ and [ZPhS](+) are the results of th iono-thiolo rearrangement reaction. The precursor ion for the ion (CH3 O)(2)PO+ arises from most compounds upon chemical ionization, whereas the precursor ion for the ion [ZPhS]+ arises only from a few compounds upon chemical ionization. The observed fragments imply that several s ites carry the extra proton and that these sites get the proton usuall y upon ionization. The stability order and some characteristics of thr ee protomers of O,O-dimethyl O-phenyl phosphorothionate were investiga ted by ab initio calculations at the RHF/3-21G level of theory.