Journal of Chromatography, 595 (1992) 149-154 Elsevier Science Publishers B.V., Amsterdam CHROM. 23 927 Ion-exclusion chromatography using mobile phases containing /?-cyclodextrin Bronislaw K. GIod*, Paul R. Haddad* and Peter W. Alexander Department of Analytical Chemistry, University of New South Wales, P.O. Box 1, Kensington, N.S. W. 2033 (Australia) (First received August 14th, 1991; revised manuscript received November 20th, 1991) ABSTRACT Ion-exclusion chromatography of some aromatic carboxylic acids is performed using an Aminex HPX-87H organic acids column with a mobile phase comprising ethanol-water (20:80, v/v) containing camphorsulphonic acid and fi-cyclodextrin. Retention volumes of the solutes increased with increasing mobile phase concentration of camphorsulfonic acid. On the other hand, some solutes (gallic, o-nitrobenzoic, acetylsalicylic and m-nitrobenzoic acids) showed a decrease in retention volume when the mobile phase concentration of /I-cyclodextrin was increased. This behaviour was attributed to the formation of inclusion complexes in the mobile phase, leading to an equilibrium shift which decreases the amount of solute in the stationary phase and hence its retention. This provides an additional parameter for control of the solute retention. A theoretical equilibrium model is presented which considers the equilibrium reactions occurring in the chromatographic system. Retention data are then used to evaluate the equilibrium constants for these reactions. In this way, the inclusion constant for each solute may be determined. INTRODUCTION Ion-exclusion chromatography, first introduced by Wheaton and Baumann [l] in 1953, involves the use of strong anion- or cation-exchange resin for the separation of ionic solutes from weakly ionized or neutral solutes. In this mode of chromatography ionic solutes tend to be repelled (or excluded) from the resin, whilst neutral solutes are able to penetrate the resin and are hence retained. In addition to the charge on the solute, several other factors are known to play a part in the retention process; these include the size and especially the hydrophobicity of the solute [2]. The mechanism by which acidic solutes may be separated by ion-exclusion chromatography using a cation-exchange resin has been described previously [336]. We now introduce a further parameter which * Present address: Institute of Physical Chemistry of the Pol- ish Academy of Sciences, Kasprzaka 44152, 01-224 Wars- zaw, Poland. exerts an influence on retention in this form of chro- matography, namely the concentration of an inclu- sion compound in the eluent. fi-Cyclodextrin (CD) was selected as a model inclusion compound. The most characteristic property of cyclodextrins is their remarkable ability to form molecular inclusion compounds with some organic and inorganic com- pounds and also with ions. The stereoselective na- ture of CD inclusion has been exploited in high- performance liquid chromatography (HPLC) through the use of chemically bonded CD-silica stationary phases and by the application of CD as a mobile phase component. CD was first utilized as a mobile phase modifier by Uekama et al. [7] for ion- exchange chromatography and later applied [8-l l] to reversed-phase HPLC. The main purpose of the present paper is to demonstrate that CD may also be utilized as an eluent component in ion-exclusion chromatography. A simple scheme describing the separation process occurring in this system is pro- posed and equations relating the distribution coeffi- cient of a solute to the concentration of CD are derived. These equations are used to calculate in- 0021-9673/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved