Artifact formation in the GC/MS analysis of Zolone EC (phosalone) insecticide formulation

In an investigation of potential causes for three grape-harvester pesticide poisoning incidents, acetone extracts of Zolone EC [active ingredient, phosalone [O,O-diethyl S-(6-chloro-2-oxobenzoxazolin-3-yl)-methyl phosphorodithioate]] were analyzed by GC/MS for the presence of possible impurities. Initial findings suggested the presence of isophosalone [O,S-diethyl S-(6-chloro-2-oxobenzoxazolin-3-yl)methyl phosphorodithioate], a phosalone isomer containing S-ethyl and oxon functionalities, as a formulation impurity. High-resolution and tandem mass spectrometry experiments confirmed the structure of isophosalone. An additional solid probe tandem mass spectrometry study and investigations of the effects of injector temperature, column length, and on-column injection verified that isophosalone was produced as an artifact of gas chromatography and was not initially present in the Zolone EC formulation. The results demonstrate the utility of high-resolution and tandem mass spectrometry experiments for structural confirmation and illustrate that the potential exists for formation of isomer artifacts of phosphorodithioate insecticides during the gas chromatographic process