Micellar electrokinetic capillary chromatography (MECC) is suitable for the separation of mixtures of uncharged and charged solutes. In this paper, the migration behavior of acidic compounds in MECC is quantitatively described in terms of different models. These equations describe the relationships between the two migration parameters in MECC (retention factor and mobility) and the two important experimental parameters (pH and micelle concentration) that have a great influence on the migration behavior and selectivity. Interestingly, the mobility and retention factor of a given solute could behave differently with the variations in pH. This would raise a question of which parameter actually represents the migration behavior of a solute in MECC: retention factor (a chromatographic parameter) or mobility (an electrophoretic parameter). The consequences of micellar-mediated shifts of ionization constants on selectivity and optimization strategies in MECC are discussed. The mathematical models would allow the prediction of migration behavior of solutes based on a limited number of initial experiments. This would greatly facilitate the method development and optimization of separations of ionizable compounds by MECC and, in addition, important physical and chemical characteristics of solutes such as their apparent ionization constants in micellar media and their partition coefficients into micelles (over a wide range pH values) can be determined. The models were verified, as good agreements were observed between the predicted and the experimentally observed migration behavior. Based on the preliminary results, the pH and micelle concentration are likely to be interactive parameters in many situations. As a result, simultaneous optimization of these two parameters would be the most effective strategy to enhance the MECC separation of acidic solutes.

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The migration behavior and optimum separation of a set of substituted phenols is presented. A mathematical model that describes mobility in CZE in terms of fundamental constants of each solute (acid dissociation constant Ka, mobility of the dissociated acid microA-) and the pH of the buffer was used to predict the mobility of each solute as a function of pH. The resolution between each peak pair was calculated, and a window diagram of minimum resolution as a function of pH was employed in determining the optimum pH. A comparison of the predicted and actual electropherograms shows the usefulness of this technique.