Effects of stationary phase cross-linking and ion-exchange capacity on the retention of carboxylic acids in ion-exclusion chromatography using sulfonated resins.

Ion-exclusion chromatography (IEC) has been widely used in the separation of carboxylic acids, finding applications in the analysis of food and beverages, pharmaceuticals, biological samples, and acid rain samples.' The retention mechanism of carboxylic acids in ion-exclusion chromatography has also been investigated in many studies, and the factors which affect retention can be divided into three categories: analyte effects, mobile phase effects, and stationary phase effects. In IEC, the chromatographic system can be considered to consist of three phases: the mobile phase which comprises the eluent passing through the interstitial volume of the column, the stationary phase which comprises the occluded liquid trapped within the pores of the resin and the resin phase which is the solid resin network and functionalized groups.' In this mode of chromatography, the analytes being separated have the same charge as the functional group on the resin; that is, anions are separated on cation-exchange columns and cations are separated on &x~n-exchange c o ~ m s . Modern IEC columns are usually packed with fully functionalized resins of polystyrene-divinylbenzene or polymethacrylate substrates. Most commercial ion-exclusion resins have approximately 8% cross-linking, 5 to 10 p particle size and 5 meqlg ion-exchange capacity. The functional groups for cation-exchange columns are usually sulfonate or carboxylate functional groups or a mixture of both. Previous studies investigating the effect of the ion-exchange capacity have covered only a small range of capacities (0 to 2 meq/g) on highly cross-linked (50%) polystyrene divinylbenzene resin^.^,^ The retention of the carboxylic acids examined increased with increasing the ion-exchange capacity. This was opposite to the expected trend, as a higher concentration of functional groups on the resin should increase its electrostatic repulsion with the analyte, and hence decrease retention. Harlow and Morman have demonstrated that the degree of cross-linking of the resin has a profound effect on the retention of carboxylic acids.4 Weakly ionized species show more penetration into the occluded liquid phase on resins of low cross-linking than those of high cross-linking. They found that resins of lower crosslinking provided better resolution of highly ionized acids, such as HC1, from organic acids.