THERMOSPRAY MASS-SPECTROMETRY OF PHOSPHOROTHIONATE PESTICIDES AND THEIR OXYGEN ANALOGS

Positive and negative ion modes thermospray mass spectrometry were used for the characterization of the phosphorothionate pesticides parathion-ethyl, fenchlorphos, bromophos-ethyl and fenitrothion and their oxygen analogues. A convenient and easy procedure using dimethyldioxirane has been developed for the preparation of the triester phosphates from the corresponding phosphorothionates. The spectra reported from paraoxon-ethyl and fenitrooxon are compared with those of standards available. For the first time, the characterization of the oxygen analogues of fenchlorphos and bromofos-ethyl by any mass spectrometric technique is reported. By employing the positive ion mode, the base peak was [M + NH4]+ for all the studied pesticides with a second abundant ion corresponding to [M + H]+ whereas in the negative ion mode typical negative chemical ionization mechanisms involving electron capture, dissociative electron capture and anion attachment have been observed. The corresponding specific fragments were: [M - R]-, R being a methyl or ethyl; [FG]-, FG being the functional group fragment which contains the phosphorus moiety; M.-, [M - Cl]-, phenolate and thiophenolate anions and adducts with the ionizing additive. Special emphasis is given to the changes in the spectra of the phosphorothionate pesticides as compared to their oxygen analogues. With the positive ion mode, sensitivity was about one and a half orders of magnitude better for the oxygen analogues than for the parent pesticides, whereas in the negative ion mode identical sensitivities were observed.