High-efficiency enantioseparations of N-derivatized amino acids by packed capillary electrochromatography using ODS silica and a quinine-derived chiral selector as ion-pair agent

The enantiomers of N-derivatized amino acids, e.g., 3,5-dinitrobenzoyl, 3,5 -dinitrobenzyloxycarbonyl, 2,4-dinitrophenyl and 9-fluorenylmethoxycarbonyl amino acids, have been separated by enantioselective ion-pair formation an d packed capillary electrochromatography (CEC) using RP18 silica particles. Thus, a CEC-Hypersil ODS-3 mu m packed capillary column, 335 mm (effective length 250 mm)x0.1 mm I.D., was used in combination with a quinine carbama te type chiral ion-pair agent (selector, SO) which was added to aqueous and non-aqueous buffered mobile phases, respectively. The negatively charged a nalyte enantiomers interact with the positively charged chiral SO by multip le intermolecular interactions to form a pair of transient diastereomeric i on-pairs which may differentially adsorb to the ODS-stationary phase. Enant ioseparation is achieved due to different observed mobilities of the analyt e enantiomers originating from different ion-pair formation rates of the en antiomers and/or differential adsorption of the diastereomeric ion-pairs to the ODS-stationary phase. Countercurrent-like electrophoretic migration of oppositely charged ion-pair agent and solute enantiomers may give rise to enhanced enantioselectivity. At high electrolyte concentrations (>10 mM), e .g. if the chiral ion-pair agent is added to the mobile phase that contains , compared to typical RP-CEC conditions (<10 mM), relatively high amounts o f background electrolyte, electrophoretic transport of the analytes is domi nating (negative polarity mode); this method is characterized by high effic iency with theoretical plate numbers up to 170 000 per meter, e.g. for DNP- Val, but with moderate enantioselectivity. In contrast, at low electrolyte concentrations (<10 mM), e.g. if the chiral ion-pair agent itself is used a s electrolyte without any other background electrolyte, the analytes cue dr iven through the capillary column with the electroosmotic flow (positive po larity mode); this operation mode is characterized by relatively poor effic iency, but high enantioselectivity. The influence of several mobile phase p arameters (aqueous versus non-aqueous, selector concentration, type of back ground electrolyte) on observed mobility, enantioselectivity and efficiency was evaluated. (C) 1999 Elsevier Science B.V. All rights reserved.