CAPILLARY ELECTROPHORETIC CHIRAL SEPARATIONS USING BETA-CYCLODEXTRIN AS RESOLVING AGENT .2. BASES - CHIRAL SELECTIVITY AS A FUNCTION OF PH AND THE CONCENTRATION OF BETA-CYCLODEXTRIN

An equilibrium model, first developed to describe the pH and cyclodextrin (CD) concentration dependence of the electrophoretic mobilities as well as the separation selectivities observed during the capillary electrophoretic (CE) separation of the enantiomers of weak acids, is extended to cover the separation of the enantiomers of weak bases as well. Model parameters are derived from three series of CE experiments: (i) one with cyclodextrin-free background electrolytes of varying pH values to obtain the ionic mobilities and base dissociation constant of the weak base enantiomers, (ii) one with constant, low pH background electrolytes with varying concentrations of beta-cyclodextrin to obtain the mobilities and formation constants for the protonated base enantiomer-beta-cyclodextrin complexes, and (iii) one with constant, high pH background electrolytes with varying concentrations of beta-cyclodextrin to obtain the formation constants for the nonprotonated base enantiomer-beta-cyclodextrin complexes. Homatropine was used as a model substance to test the validity of the model and an excellent agreement has been found between the calculated and the measured mobility and selectivity values. Baseline separation of the enantiomers of homatropine could be achieved in less than 15 min using a pH 6.25 phosphate buffer, which contained 15 mM beta-cyclodextrin.