Direct enantiomeric separation of chiral pesticides by liquid chromatography on polysaccharide-based chiral stationary phases under reversed phase conditions

Abstract

The chiral separation of the racemic pesticides, including isocarbophos, fenpropathrin, imazalil, ethofumesate, myclobutanil, paclobutrazol, fipronil and hexythiazox, was carried out using cellulose (tris-3,5-dimethyl-phenylcarbamate) (CDMPC) and amylose (tris-3,5-dimethylphenyl-carbamate) (ADMPC) coated silica as chiral stationary phases by liquid chromatography (LC) under reversed phase (RP) conditions. The influence of the chiral stationary phases (CSPs), the mobile phase composition, water content and column temperature on chiral separation was investigated and the chromatographic conditions were optimized. The results showed that the two CSPs exhibited different chiral recognition ability in RP-LC and methanol, acetonitrile and water content in the mobile phase affected the enantiomeric resolutions and retentions. Isocarbophos, fenpropathrin, imazalil, ethofumesate, myclobutanil, paclobutrazol, fipronil and hexythiazox underwent baseline or partial separation with the two CSPs. Fipronil was separated only on the CDMPC-CSP. Hexythiazox was separated only on the ADMPC-CSP. The retention factors (k) for the enantiomers of investigated pesticides decreased as the temperature increased. The thermodynamic parameters calculated based on linear van't Hoff plots showed that the chiral separation was controlled by enthalpy. Better resolutions were not always at low temperature. The elution orders of the eluting enantiomers were determined by a circular dichroism (CD) detector. In short, CDMPC- and ADMPC-CSPs provided different retentions, resolutions and elution orders of some of the enantiomeric pairs.