Study of the retention behavior in zwitterionic hydrophilic interaction chromatography of isomeric hydroxy- and aminobenzoic acids

The retention behavior of fifteen isomeric hydroxy- and aminobenzoic acids in zwitterionic hydrophilic interaction chromatography was studied using a sulfobetaine phase (ZIC-HILIC). By an inspection of their molecular structures, the retention was related to the number, the position and hydrogen bond properties of the functional groups. The effect of the chromatographic conditions was analyzed in order to investigate the retention mechanism of the stationary phase. The increased retention observed for negative charged compounds when the mobile phase pH decreased was ascribed to a diminishing of the electrostatic repulsion with the underivatized silanol groups. Also the salt buffer concentration in the mobile was proved to have a great influence in the modulation of the electrostatic interactions. However, the retention behavior of the benzoic acids was not described by conventional ion-exchange models. Subsequently, a systematical analysis of partition, adsorption, and hydrophilic chromatographic models was presented. The results from the fittings indicated that partition processes govern mainly the ZIC-HILIC separation, but also adsorption processes via hydrogen bonds occurred for hydrogen donor analytes. Finally, the influence of the chromatographic conditions on the water enriched layer in which partition takes place has been evaluated by the elution behavior of toluene.