Modelling of cation retention in ion chromatography using fixed-site and dynamically coated ion-exchange columns

A retention model is derived for inorganic cations eluted from cation-exchange columns with eluents containing a single competing cation and a complexing ligand. This model is evaluated using divalent solute cations and a low-capacity fixed-site cation-exchange column, and good agreement is obtained between theoretical and experimental results both for simple cation exchange and also when complexation effects are present. Sodium ions added to the eluent during pH adjustment were found to contribute significantly to the elution of solute cations and should be included in all calculations using the retention model. Retention characteristics were also studied for an ion-interaction chromatographic system using a C18 column, octanesulfonic acid as the ion-interaction reagent, and oxalate as the complexing ligand in the eluent. Experimental data for this system did not show close agreement with the ion-exchange retention model. Discrepancies are attributed to variations in the ion-exchange capacity of the dynamically coated ion-exchange column during the study. However, qualitative agreement with the ion-exchange retention model was obtained when the concentration of octanesulfonate in the eluent was varied.