Chiral stability of synthetic pyrethroid insecticides

Synthetic pyrethroids are chiral compounds consisting of multiple stereoisomers. Evaluation of enantioselectivity in environmental fate and ecotoxicity requires analytical methods that preserve stereoisomer integrity during analysis. In this study, we characterized the stability of stereoisomers from four commonly used pyrethroids, cis-bifenthrin (cis-BF), permethrin (PM), cypermethrin (CP), and cyfluthrin (CF), during gas chromatography (GC) analysis and sample preparation. Stereoisomers of cis-BF and PM were found to be stable, but those of CP and CF were unstable, under heat or in water. Isomer conversion occurred only at the aC in CP or CF, causing the analyte stereoisomer to convert to an epimer. At a GC inlet temperature of 260°C, about 9% conversion occurred for CP and CF. In organic solvents and sterile water, stereoisomers of cis-BF and PM were stable, but slow isomer conversion was observed for CP and CF in water at ambient temperature. However, isomer conversion for CP and CF was relatively insignificant (2-3%) when the GC inlet temperature was kept at ≤ 180°C or when on-column injection was used. Isomer conversion at the αC in water suggests that abiotic processes may also contribute to enantioselectivity observed in the environment for pyrethroids with the asymmetric αC.