Fragmentation of protonated O,O-diethyl O-aryl phosphorothionates in tandem mass spectral analysis.

The gas-phase ion chemistry of protonated O,O-diethyl O-aryl phosphorothionates was studied with tandem mass spectrometric and ab initio theoretical methods. Collision-activated dissociation (CAD) experiments were performed for the [M+H](+) ions on a triple quadrupole mass spectrometer. Various amounts of internal energy were deposited into the ions upon CAD by variation of the collision energy and collision gas pressure. In addition to isobutane, deuterated isobutane C4D10 also was used as reagent gas in chemical ionization. The daughter ions [M+H-C2H4](+) and [M+H-2C2H4](+) dominate the CAD spectra. These fragments arise via various pathways, each of which involves γ-proton migration. Formation of the terminal ions [M+H-2C2H4-H2O](+), [M+H-2C2H4-H2S](+), [ZPhOH2](+), [ZPhSH2](+), and [ZPhS](+) [Z = substituent(s) on the benzene ring] suggests that (1) the fragmenting [M+H](+) ions of O,O-diethyl O-aryl phosphorothionates have protons attached on the oxygen of an ethoxy group and on the oxygen of the phenoxy group; (2) thiono-thiolo rearrangement by aryl migration to sulfur occurs; (3) the fragmenting rear-ranged [M+H](+) ions have protons attached on the oxygen of an ethoxy group and on the sulfur of the thiophenoxy group. To get additional support for our interpretation of the mass spectrometric results, some characteristics of three protomers of O,O-diethyl O-phenyl phosphorothionate were investigated by carrying out ab initio molecular orbital calculations at the RHF/3-21G* level of theory.

Fragmentation of protonated O,O-dimethyl O-aryl phosphorothionates in tandem mass spectral analysis.

A study was carried out on the fragmentation of 12 protonated O,O-dimethyl 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 the [M + H](+) ions to investigate the dissociation behavior of the ions with various amounts of internal energy. On collisionally activated dissociation, the [M + H](+) ions decompose to yield the [M + H - CH3OH](+), (CH3O)2PS(+) (m/z 125), and (CH3O)2PO(+) (m/z 109) ions as major fragments. The ions [M + H - CH3OH](+) and (CH3O)2PS(+) probably arise from the [M + H](+) ions of the O,O-dimethyl O-aryl phosphorothionates 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 many cases, surprisingly, the phenyl group and not the reagent gas. This was confirmed by using deuterated isobutane, C4D10, as reagent gas in Cl. The fragment ions (CH3O)2PO(+) and [ZPhS](+) are the results of thiono-thiolo rearrangement reaction. The precursor ion for the ion (CH3O)2PO(+) 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 sites carry the extra proton and that these sites get the proton usually upon ionization. The stability order and some characteristics of three protomers of O,O-dimethyl O-phenyl phosphorothionate were investigated by ab initio calculations at the RHF/3-21G* level of theory.

Multiple-stage mass spectrometry in structural characterization of organophosphorus compounds.

Multiple-stage mass spectrometry involving consecutive collision-activated dissociation reactions was used to examine the structures of fragment ions commonly formed on electron ionization of organophosphorus esters. The compounds studied include several aryl thiophosphates, some of which are analogs of common pesticides. Energy-resolved collisionactivated dissociation experiments allow the dissociation of the molecular ions of these compounds in such a manner that only a few fragment ions dominate the spectrum. An abundant fragment ion of m/z 109, formed from all of the compounds studied, can have at least four different stable structures: (CH3O)2PO(+), CH3CH2OP(O)OH(+), CH2 =CHOP(H)(OH)2 (+), and (CH2O)2P(H)OH(+). The structure of the fragment ion of m/z 109 was found to reflect the phosphorus-containing part of the compounds studied. Another abundant fragment ion obtained for all the aryl esters studied is structurally characteristic of the aromatic moiety of the molecule. This fragment ion is the result of a complex rearrangement involving transfer of an alkylene group to the aromatic ring from the phosphoruscontaining part of the molecular ion. The utility of these fragment ions in the structural characterization of unknown organophosphorus esters is discussed.