Critical comparison of retention models for the optimisation of the separation of anions in ion chromatography II. Suppressed anion chromatography using carbonate eluents

Seven theoretical retention models, namely the linear solvent strength mode l (using the dominant equilibrium approach and competing ion effective char ge approach), the dual eluent species model, the Kuwamoto model, the extend ed dual eluent species model, the multiple species eluent/analyte model and the empirical end-points model, were used to describe the retention behavi our of anions in suppressed ion chromatography (IC). An extensive set of ex perimental retention data was gathered for 24 anions (fluoride, formate, br omate, chloride, hexanesulfonate, bromide, chlorate, nitrate, iodide, thioc yanate, perchlorate, sulfite, succinate, sulfate, tartrate, selenate, oxala te, tungstate, phthalate, molybdate, chromate, thiosulfate and phosphate) o n a Dionex AS4A-SC column using carbonate eluents of varying concentration and HCO3-:CO32- ratios. Statistical comparison of the predicted and experim entally obtained retention factors showed that the performance of the theor etical models improved with the complexity of the model. However the empiri cal model (in which a linear relationship is assumed between the logarithm of retention factor and the logarithm of eluent strength, but the slope is determined empirically) gave the most consistent performance across the wid est range of anions. The empirical end-points model was also shown to be th e most satisfactory model due to its low knowledge requirements and easy so lution. Compared with non-suppressed IC (see Part I), the retention behavio ur in suppressed IC was found to be easier to model by all retention models . (C) 1999 Elsevier Science B.V. All rights reserved.