Identification and determination of fenitrothion photolysis products in water—methanol by gas chromatography—mass spectrometry

Abstract

The photodegradation of fenitrothion was examined in distilled water containing methanol. After 7 h of UV irradiation with a high-pressure mercury lamp, a fractionation step was carried out on a semi-preparative silica gel column. Eighty-eight fractions were collected by increasing the polarity of an eluent mixture of n-hexane—ethyl acetate (95 + 5) up to 100% ethyl acetate. Three more polar fractions in eluent mixtures of n-hexane—acetone (50 + 50), pure acetone and acetone—methanol (25 + 75) were obtained. The breakdown products formed were identified by gas chromatography—mass spectrometry with electron impact (EI) ionization. An estimate of the amount of fenitrothion degraded indicated that after 7 h of irradiation, 10% of the parent compound still remained and different oxidation, isomerization and solvolysis products were produced. The P = S group was oxidized to give fenitrooxon and trimethyl phosphate in amounts of 0.1% and 3%, respectively. Carbomethoxyfenitrothion, due to oxidation followed by solvolysis, and the isomerization products O,O,S-trimethyl phosphorothioate and the S-methyl isomer of fenitrothion were obtained as photoalteration products at 10%, 10% and 3%, respectively. Other compounds corresponded to denitrofenitrothion, carbomethoxydenitrofenitrothion, parathion-methyl and 4-nitro- m-cresol at 0.2%, 0.4%, 0.4% and 0.4%, respectively. The EI mass spectra and the structures of the ions of the different fenitrothion photolysis products are given.