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 cyclodext rin (CD) concentration dependence of the electrophoretic mobilities as well as the separation selectivities observed during the capillary el ectrophoretic (CE) separation of the enantiomers of weak acids, is ext ended 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 val ues to obtain the ionic mobilities and base dissociation constant of t he weak base enantiomers, (ii) one with constant, low pH background el ectrolytes with varying concentrations of beta-cyclodextrin to obtain the mobilities and formation constants for the protonated base enantio mer-beta-cyclodextrin complexes, and (iii) one with constant, high pH background electrolytes with varying concentrations of beta-cyclodextr in to obtain the formation constants for the nonprotonated base enanti omer-beta-cyclodextrin complexes. Homatropine was used as a model subs tance to test the validity of the model and an excellent agreement has been found between the calculated and the measured mobility and selec tivity values. Baseline separation of the enantiomers of homatropine c ould be achieved in less than 15 min using a pH 6.25 phosphate buffer, which contained 15 mM beta-cyclodextrin.