Retention modelling of electrostatic and adsorption effects of aliphatic and aromatic carboxylic acids in ion-exclusion chromatography

The retention mechanism of aliphatic and aromatic carboxylic acids in ion-e xclusion chromatography has been investigated with consideration of simulta neous electrostatic repulsion effects and hydrophobic adsorption effects. A mathematical relationship between the retention factor of the analyte and the mobile-phase composition (sulfuric acid concentration and percentage of methanol), the type of analyte (pK(a) and hydrophobicity) and some physica l characteristics of the stationary phase has been derived. Thirteen carbox ylic acids (comprising mono- and divalent, aliphatic and aromatic acids) we re chosen and used to acquire retention data on three different cation-exch ange stationary phases (in which the sulfonate functional groups are bound to polystyrene-divinylbenzene, polymethacrylate or silica) using 14 mobile- phase compositions of varying pH and percent methanol. These retention data were used to derive the parameters necessary to solve the retention model, using non-linear regression, In this way, a quantitative measure of the ef fects of adsorption phenomena on analyte retention were obtained. The model was then used to optimise the separation of nine carboxylic acids. (C) 199 9 Published by Elsevier Science B.V. All rights reserved.