Enantioseparation of malathion, cruformate, and fensulfothion organophosphorus pesticides by mixed-mode electrokinetic capillary chromatography

Mixed-mode electrokinetic capillary chromatography (mixed-mode ECC) has been used for the enantioseparation of organophosphorus pesticides. In mixed-ECC, a combination of three pseudostationary phases including surfactants, neutral, and charged cyclodextrins, are used to resolve very challenging enantioseparation problems. The conditions mimic a mixture of micellar electrokinetic capillary chromatography (MECC) and dual-cyclodextrin electrokinetic capillary chromatography (dual-CECC) conditions. In this work SDS, carboxymethyl-o-CD, hydroxypropyl-BCD, and organic modifiers were mixed at various concentrations in order to achieve enantioseparation of three organophosphorus pesticides - cruformate (ruelene), malathion, and fensulfothion. The best condition for separation of ruelene enantiomers was by using a mixture of 70 mM SDS/15 mM carboxymethyl-BCD/45 mM hydroxypropyl-BCD/20% (v/v) acetonitrile in 20 mM borate buffer at pH 8.6, with applied voltage of 25 kV at 25degreesC. Malathion enantiomers were successfully resolved using either 10 mM SDS/50 mM CM-BCD/40 mM hydroxypropyl-BCD or 50mM CM-BCD/50 mM hydroxypropyl-BCD/20% (v/v) methanol in 20 mM borate buffer. Fensulfothion enantiomers were successfully resolved using a mixture of 75 mM SDS/12.5 mM carboxymethyl-BCD/45 mM hydroxypropyl-BCD in the same 20 mM borate buffer. The results demonstrate the versatility of the mixed-mode ECC technique in handling very difficult separations such as the organophosphoramidate enantiomers. It offers options for selectivity control by combining three or more pseudostationary phases in the background electrolyte (BGE). The approach to optimization in mixed-mode ECC is generally more straightforward than the use of a solid stationary phase(s) in HPLC. In theory, the separation selectivity of such mixed-mode ECC system can be modified to the extremes of MECC and the dual-CECC of the chiral selectors.