Retention modelling of electrostatic and adsorption effects of aliphatic and aromatic carboxylic acids in ion-exclusion chromatography II. Calculations of adsorption coefficients in unbuffered eluents

Previous models for the retention behaviour of carboxylic acids in ion-excl usion chromatography are applicable only when the degree of ionisation of t he analyte is constant over the entire chromatographic peak. When solutions of sulfuric acid are used as eluents, this condition applies only when the eluent concentration is considerably higher than that of the analyte. Sinc e it is common for dilute solutions of sulfuric acid to be used as eluents, a retention model which accounts for unbuffered eluents has been developed . This model also considers the effects on retention of hydrophobic adsorpt ion of the undissociated and dissociated forms of the analyte onto the stat ionary phase substrate, as well as the effects of organic solvents added to the eluent. The derivation of this model is presented and it has been eval uated using a comprehensive set of retention data obtained using three diff erent sulfonated stationary phases over a range of eluent conditions. The a dsorption coefficients calculated from the model are in accordance with exp ected trends and showed that both the undissociated and dissociated forms o f the analyte acids were retained by hydrophobic adsorption effects, althou gh this adsorption was much stronger for the undissociated analytes. (C) 20 01 Elsevier Science B.V. All rights reserved.