ENANTIOMERIC SEPARATIONS IN CAPILLARY ZONE ELECTROPHORESIS USING A CHIRAL CROWN-ETHER

Chiral separations of optically active amines using 18-crown-6 tetracarboxylic acid (18C6H(4)) as pseudo-stationary phase in capillary zone electrophoresis were studied. The buffer composition and pH strongly affected the performance of the separation. Although good separations were achieved using tertiary (triethylamine) or quaternary amines (tetramethylammonium) as cationic buffer constituents, with respect to speed of analysis, separation factor and efficiency, the best results were obtained with Tris. pH was shown to be an important factor for method optimization. Although inclusion complexation of primary amines is enhanced with increasing pH, the difference in the Gibbs free energy of the complexation of D,L-tryptophan was highest at pH 2.5. A synergistic effect on the resolution of chiral amines was observed when IsC6H(4) and cyclodextrin were dissolved in the same buffer. With this system the enantiomers of noradrenaline could be baseline separated even though neither of the two chiral selectors by itself was able to reserve the enantiomers. The separation factor depends strongly on the concentrations of the two selectors. It was demonstrated that simply complexation of the cyclodextrin rather than chiral recognition was the basis for the improved separation.