Non target approach for the identification of degradation products of herbicides used in rice cultivation

Nowadays pesticides of recent formulation are designed to offer higher selectivity and lower persistence in the environment than the formulations used in the past. However a lower persistence in the environment does not necessarily correspond to a lower toxicity since the pesticides undergo natural degradation reactions in the environment and new species potentially more toxic can be formed [1-4]. In this study some of the most important herbicides used in Italy in rice cultivation (imazamox, oxadiazon and profoxydim) were undergone to degradation studies simulating natural environmental conditions. To this aim: a) aqueous solutions of the species were exposed to simulated sun light irradiation in a solarbox; b) hydrolysis processes were studied preserving the solutions at dark. At prefixed times, the solutions were analyzed by using a micro liquid chromatography system interfaced to a quadrupole-time of flight (QTOF) high-resolution mass spectrometer. LC-MS/MS analyses of the samples undergone to sunlight irradiation showed a decreased intensity of the herbicide signals. Instead, no hydrolysis effect was evidenced in samples stored at the dark for three months. Moreover, solutions prepared in ultrapure water showed different degradation rates with respect to those prepared in paddy water. The LC-MS/MS analyses were carried out by using a non-target approach without any a priori knowledge of the possible species formed during the degradation process of the herbicides. All the MS/MS spectra were recorded without prefixed criteria with the help of the SWATHTM (Sequential Window Acquisition of all THeoretical Mass Spectra) acquisition method, and were used to elucidate the chemical structures of the predominant degradation products. This research has been supported by Fondazione Cariplo, grant n. 2014-1260