We established a method for quantitative analysis of fenthion (MPP) and its related compounds in water samples, using solid-phase extraction and liquid chromatography/mass spectrometry. With this method, the values of the limit of quantification ranged from 0.2 to 100 ng l−1. Using this method, we examined the fate of MPP in water and the products produced by light irradiation and chlorination. MPP decreased gradually and reached 50% of the initial concentration after 48 hours in water. In particular, MPP-sulfoxide was formed. With light irradiation, MPP decomposed immediately into MPP-sulfoxide, O,O-Dimethyl S-[3-methyl-4-(methylthio)phenyl]phosphorothioate and other compounds. With chlorination, MPP decomposed into MPP-sulfoxide, MPP-sulfone, and their oxons. The concentration of oxons increased in a time-dependent manner. In their effects on organisms, MPP, MPP-sulfoxide and MPP-sulfone showed weak inhibitory activity to cholinesterase, whereas their oxons showed strong activity. It is feared that MPP and its products exist in environmental water and are produced by the disinfection treatment process. Comprehensive evaluation of the toxicity of MPP and its related compounds is important in order to understand the effects of MPP on ecosystems and human health.

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