Direct Enantiomeric Separation of Chiral Pesticides by LC on Amylose Tris(3,5-dimethylphenylcarbamate) Stationary Phase under Reversed Phase Conditions

Amylose tris(3,5-dimethylphenylcarbamate) chiral stationary phase was used by liquid chromatography under reversed-phase conditions for the chiral separation of 20 pesticides, of which ten samples were separated directly under suitable conditions. The influence of mobile phase composition and column temperature from 0 to 40 A degrees C on the separation was investigated. The mobile phases were methanol/water or acetonitrile/water at a flow rate of 0.5 mL min(-1) with UV detection at 230 nm. The two enantiomers of fenamiphos, terallethrin, fenoxaprop-ethyl, benalaxyl and lactofen could obtain base separation under optimized conditions, while the enantiomers of quizalofop-ethyl, metalaxyl, napropamide, fluroxypyr-meptyl and 2,4-D-ethylhexyl got partial separation. The retention factors (k) and selectivity factor (alpha) for the enantiomers of most investigated pesticides decreased with increasing the temperature. The ln alpha-1/T plots for enantiomers of chiral pesticides were linear at the range of 0-40 A degrees C except for that of metalaxyl, fenoxaprop-ethyl and 2,4-D-ethylhexyl enantiomers in methanol/water. The thermodynamic parameters calculated based on linear Van't Hoff plots showed the chiral separation was controlled by enthalpy. Better separation was not always at low temperature. The chiral recognition mechanisms were discussed. The elution orders of the eluting enantiomers were determined by a circular dichroism detector.