Using mass spectrometry to highlight structures of degradation compounds obtained by photolysis of chloroacetamides: Case of acetochlor

• Photooxidation of acetochlor – the characterization of photodegradation products is achieved using LC–MS and GC–MS.
• Elucidation of structures using complementary experiments (MS/MS, high resolution experiments, deuterated compound).
• Quantitative structure–activity relationship (QSAR) approach – potential toxicity effect of acetochlor and its by-products.

The photooxidation of acetochlor (a pesticide belonging to the acetamides group) using a polychromatic UV irradiation in ultrapure water was studied. This study reports the efficiency of mass spectrometry for the characterization of photodegradation products of acetochlor. Decompositions of protonated ions MH+ are proposed in electrospray (ESI) mode for LC–MS, while electron ionization (EI) and chemical ionization modes (CI) are used for GC–MS. The knowledge of fragmentation and the use of a combination of experiments (MS/MS, high resolution) allow the characterization of photoproducts. Structural elucidation is assisted by the use of photolysed deuterated compounds. Fifteen major degradation products have been characterized, five by LC-QTOF, six photoproducts by GC-ITMS, and four are observed by both techniques. In vitro bioassays based on the quantification of receptor-mediated activity demonstrated that acetochlor photolysis engenders a moderate but significant estrogenic activity. Moreover, a quantitative structure–activity relationship (QSAR) approach was used to assess the potential toxicity effect of acetochlor and its by-products. The predictions were analyzed showing a variety of toxicity profiles of acetochlor photoproducts depending on the toxicological investigated endpoint.