Separation of positional and structural isomers by cyclodextrin-mediated capillary zone electrophoresis

Previously derived models for optimization of cyclodextrin (CD)-mediated capillary zone electrophoresis (CZE) referred only to the separations of enantiomers. These models assume that the mobility of the inclusion complexes of the two solutes are equal (i.e., mu>(ACD) = mu(BCD)). With other types of solutes, such as positional and structural isomers, this assumption is not valid (i.e., mu(ACD)not equal mu(BCD)). In this work, the effectiveness of the model of Wren and Rowe, which was developed for enantiometric separations, is evaluated for cyclodextrin-mediated CZE of other types of solutes. Experimental data is obtained for the alpha-cyclodextrin-mediated separation of positional and structural isomers, modelled by nitrophenols and phenylbutyric acids, respectively. It was found that the mobilities of the inclusion complexes of the isomers differed from one another (mu(ACD)not equal mu(BCD)) and that the complex mobility did not correlate with the solute mobility, the formation constant or the ''quality of fit''. Despite the complex mobilities for the positional and structural isomers not being equal, the Wren and Rowe model is nonetheless effective for predicting the optimum cr-cyclodextrin concentration. Only when the formation constants for two isomers are approximately equal (K(ACD)approximate to K-BCD) does the optimum alpha-cyclodextrin concentration differ from that predicted.