Systematic approach to links between separations in capillary electrophoresis and liquid chromatography IV. Application of binding constant-retention factor relationship to the separation of 2-, 3- and 4-methylbenzoate anions using β-cyclodextrin as selector

Abstract

Quantitative links between binding constants determined in CE and retention factors in HPLC for 1:1 analyte-selector complexes, developed in a previous paper and applied to separations of cationic tioconazole enantiomers with β-cyclodextrin (β-CD), have been extended to the separation of anionic positional isomers. In this paper we apply our unified theory to the separation of anionic methylbenzoate anions (MB −, again using β-CD as selector. There is only partial resolution for all of the three isomeric methylbenzoates without β-CD present, either in the CE background electrolyte or immobilised in the HPLC experiments. Using the same buffer as in CE (ammonium phosphaae pH 7.0), and calculating the effective concenyration of selector from the amount of β-CD divided by the volume of the mobile phase in the column, HPLC is shown to give effective binding constants, K′, which are in excellent agreement with binding constants, K, determined from the variation of CE mobility with β-CD concentration. K′=11,41,87 M′ and K=13.0, 45.5, 85.3 M −1 for 2-, 3-, 4-MB − respectively. This demonstrates the quality of our unified treatment of separations based on binding equilibria, and suggests the potential of method development in CE and information transfer to HPLC or vice versa. Quantitative determination of K in CE also generates mobilities of free analyte and analyte-selector complexes, which after correction for non-ideality give radii and diffusion coefficient of the MB − and MB −: β-CD ions. This in turn allows a quantitative treatment of variance contributions from diffusion to CE peak widths. Good agreement is found between calculated and observed peak widths when these are measured with the fastest possible data acquisition parameters.