Aqueous photodegradation of sethoxydim herbicide: Qtof elucidation of its by-products, mechanism and degradation pathway

The photochemical fate of sethoxydim herbicide in water was studied under simulated sunlight radiation (Suntest apparatus). This compound exhibited a rapid degradation rate in water with a half-life of approximately 1 h, which is in accordance with the high calculated value of the quantum yield (Phi = 0.26). Consequently, the photolysis of sethoxydim should be considered as an efficient route of sethoxydim removal in aqueous media. During the photodegradation studies, ten by-products were detected, and their identification was accomplished using high-performance liquid chromatography (HPLC) coupled with quadrupole time-of-flight mass spectrometry (Qtof) and electrospray ionization in positive mode (ESI +). A detailed and exhaustive study of the mass spectra of the precursor ions and their MS/MS fragmentation patterns allowed for their identification. The photodegradation products resulted from the cleavage and isomerization of the N-O bond of the oxime, oxidation of the sulfur atom, oxidative C-S bond cleavage and Beckmann rearrangement followed by intramolecular cyclization. Additionally, the reactive species involved and a plausible mechanism for the photodegradation of sethoxydim in water are discussed. Finally, based on the identified products, a rational pathway for the photodegradation of sethoxydim in water is proposed. (C) 2013 Elsevier B.V. All rights reserved.