Journal of Chromatography A, 654 (1993) 197-205 Elsevier Science Publishers B.V., Amsterdam CHROMSYMP. 2923 Model for the mixed ion-exclusion-adsorption retention mechanism in ion-exclusion chromatography* Bronislaw K. GEd* and Janusz Stafiej Polish Academy of Sciences, institute of Physical Chemistry, Kasprzaka 44152, 01- 224 W arsaw (Poland) (Received September 2Oth, 1992) ABSTRACT The model elaborated in a previous paper for the retention mechanism in ion-exclusion chromatography was generalized to include adsorption of the solute. The computer modelling of the column performance by the Craig method was used in the case of an unbuffered mobile phase. The retention process in the case of a sufficiently buffered mobile phase turned out to be governed by a linear partition isotherm and can be described globally by simple equations. The adsorption constants of several compounds were calculated from the data available in the literature. INTRODUCTION Ion-exclusion chromatography (IEC) is an efficient method for the separation of partially ionized species [l-12]. The ion-exclusion mecha- nism of solute retention is based on the phenom- enon that neutral molecules penetrate the ion- exchange resin while the counter ions with re- spect to the exchanged ion are repulsed or, in other words, excluded from it [l]. Therefore, by this mechanism acidic compounds can be sepa- rated on cation-exchange resins and basic com- pounds on anion-exchange resins. This is the opposite situation to ion-exchange chromatog- raphy, where an anion-exchange resin is used to separate anions and a cation-exchange resin to separate cations. Tanaka et al. [13] have shown for a cation- exchange resin that the dependence of the dis- tribution coefficient, K,, on the pK, values of * Corresponding author. * Presented at the International Zon Chromatography Sy mposium 1992, Linz, September 21- 24, 1992. The ma- jority of the papers presented at this symposium were published in J. Chromatogr., Vol. 640 (1993). various acidic compounds is analogous to the dependence of K,, on the logarithm of the molecular mass in size-exclusion chromatog- raphy. They interpreted this as evidence for an ion-exclusion mechanism of acidic solutes sepa- rated on a cation-exchange resin. A more quantitative description of these find- ings was attempted in a previous paper [14], where the following equation was derived: K d = zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM 1 + 2clK, - qm 2clK, - 2 (1) This equation expresses the distribution coeffi- cient as a function of the solute acidic dissocia- tion constant and the solute concentration at the peak maximum, c. Eqn. 1 is inconvenient from the analytical point of view because the solute concentration at the peak maxium is not easy to determine and one would prefer the analytical solute concen- tration instead [14]. Also, the simplifications involved in eqn. 1 are not generally justifiable WI. The improved approach in ref. 15 removes the inconvenience and some of the simplifications. The price to pay for the improvements is that the OO21-9673/93/$06.00 @ 1993 Elsevier Science Publishers B.V. All rights reserved