S-METHYL ISOMER CONTENT OF STORED MALATHION AND FENITROTHION WATER-DISPERSIBLE POWDERS AND ITS RELATIONSHIP TO TOXICITY

A study was undertaken to determine the role of S-methyl isomer formation in the toxicity of malathion and fenitrothion water-dispersible powders held in tropical storage. The S-methyl isomers of malathion and fenitrothion were synthesized, high-pressure liquid chromatography and gas-liquid chromatography methods were developed for their determination in powder formulations. Samples of malathion water-dispersible powders returned from storage in tropical countries were found to have S-methyl isomer contents as high as 3.3% and rat oral LD50 values of 500 mg/kg compared to fresh powders containing less than 0.3% of the isomer and having rat oralLD50 values of above 2500 mg/kg. Increasing S-methyl isomer content and toxicity were also observed in malathion water-dispersible powders held at elevated temperatures in the laboratory. The rate of formation of S-methyl isomer of malathion was found to depend upon the type of “inert” ingredients in the formulation. No increase in toxicity was observed in fenitrothion powders stored at elevated temperatures although the S-methyl isomer was formed in concentrations up to 1%. © 1979, American Chemical Society. All rights reserved.