Simultaneous Derivatization and Trapping of Volatile Products from Aqueous Photolysis of Thiamethoxam Insecticide

Abstract

An aqueous photolysis study was conducted with radiolabeled thiamethoxam, 4H-1,3,5-oxadiazin-2-imine, 3-[(2-chloro-5-thiazolyl)methyl]tetrahydro-5-methyl-N-nitro, to establish the relevance of aqueous photolysis as a transformation process for (14)C-[thiazolyl]-thiamethoxam. (14)C-[thiazolyl]-thiamethoxam was applied to sterile sodium acetate pH 5 buffer solution at a dose rate of approximately 10 ppm. The resulting samples were incubated for up to 30 days at 25 degrees C under irradiated and nonirradiated conditions. The irradiated samples were exposed to a 12-hour-on and 12-hour-off light cycle. Volatile fractions accounted for up to an average of 56.76% of the total dose for the irradiated incubations and <0.08% for the nonirradiated incubations. These fractions were proposed to be a mixture of carbonyl sulfide (COS) and isocyanic acid (CONH). Verification of these components was accomplished by trapping with cyclohexylamine and formation of the thiocarbamate and the isocyanic acid derivatives. A similar method of trapping thiocarbamate metabolites was reported (Chen and Casida, 1978) where filter paper saturated with isobutylamine in methanol was arranged to trap (14)COS and (14)CO(2) under a positive flow of O(2) at 25 degrees C. Mass spectroscopy of the derivatized components confirmed the presence of carbonyl sulfide as the cyclohexylamine thiocarbamate and of isocyanic acid as its cyclohexylamine derivative. Evidence from this study indicates that thiamethoxam degrades significantly under photolytic conditions.