Photolytic degradation of the insecticide thiamethoxam in aqueous medium monitored by direct infusion electrospray ionization mass spectrometry

Photodegradation of the insecticide thiamethoxam (1), 3‐[(2‐chloro‐5‐thiazolyl)methyl]tetrahydro‐5‐methyl‐ N ‐nitro‐4H‐1,3,5‐oxadiazin‐4‐imine, in an aqueous medium was monitored by electrospray ionization mass spectrometry in the positive ion mode, ESI(+)‐MS. An aqueous solution of (1) was incessantly exposed to a UV radiation source and aliquots were taken after reaction times of 1, 2, 3, and 4 h. Analysis by GC/NCI‐MS revealed that (1) was continuously degraded under these experimental conditions. However, the total organic carbon (TOC) content remained practically constant during the exposition period, thereby indicating that 1 was not mineralized but continuously converted into other compounds. ESI(+)‐MS monitoring revealed that whereas the intensity of the ions of m / z 292/294 ([1 + H] + ) constantly decreased, there was the emergence of other ions of m / z 247/249, 197, 168, and 116 whose intensities simultaneously increased. Their structures were proposed on the basis of: (1) the data of their ESI(+)‐MS/MS; (2) their high resolution m / z values; and (3) a plausible reactivity of the thiamethoxam molecule exposed to UV radiation in aqueous solution. Finally, these data allowed us to suggest a reaction route for the photodegradation of 1 in an aqueous medium. Copyright © 2007 John Wiley & Sons, Ltd.