Photochemical analysis of 14C-fenhexamid in aqueous solution and structural elucidation of a new metabolite

The photodegradation kinetics and the break down pathway of fenhexamid were studied in aqueous systems using [phenyl-UL- 14C]- and [carbonyl- 14C]-labelled compounds. The photolysis of fenhexamid followed first-order kinetics. The degradation rate of fenhexamid was significantly influenced by the solution pH with rate constants ( k) of 2.11 × 10 −2, 4.47 × 10 −2, 6.11 × 10 −1 and 1.69 h −1 at pH 5.0, 6.6, 7.3 and 9.0, respectively. Fenhexamid exhibited no significant change in degradation rate in the presence of acetone and hydrogen peroxide, while humic and fulvic acids retarded the degradation rate, because they shielded the active molecules from light. However, in phosphate medium, the photolysis rate was significantly enhanced as a function of concentration. About 3–8% and 10–25% photo mineralization were observed, using [carbonyl- 14C]- and [phenyl-UL- 14C]-labelled fenhexamid in aqueous solutions at different pH, respectively. In addition to four known metabolites, one major and five minor photoproducts out of which one is reported for the first time, were identified using high resolution LC–MS/MS and NMR. The toxicity of the new metabolite was tested against the fish Oncorhynchus mykiss with no lethal effect at 100 mg L −1.