Impurity Problems in Chromatographic Method Studies for Fenthion Formulations

Abstract

Several gas chromatographic (GC) methods for analyzing fenthion were studied, and flame ionization detection, a glass column packed with SE-52 on Chromosorb GHP, and on-column injection were selected as parameters. No suitable internal standard was found because of coeluting, bias-producing impurities in fenthion. Several liquid chromatographic (LC) methods were also studied, and UV detection, a Zorbax ODS column, and a methanol-water-H3P04 mobile phase were finally selected as parameters. Dipentyl phthalate was selected as internal standard. After testing, this LC method was submitted for collaborative study. During the collaborative study, the manufacturer found an impurity, 3-methyl-4-(methylthio)anisole, co-eluting with fenthion in the collaborative method. The manufacturer's method was then evaluated as an alternative for collaborative study, but a significant unidentified impurity from the formulation blank, used for formulating emulsifiable concentrates, was also found co-eluting with fenthion. The anisole impurity was verified by LC, and the presence of the S-isomer and bis-methylthio-fenthion impurities was found by gas chromatographic- mass spectrometric (GC-MS) analyses. An anisole material obtained from Mobay was also verified by GC-MS analysis. Several other impurities were identified from literature references and from private communication with the manufacturer.