Behavior of adsorbed and fluid phases versus retention properties of amino acids on the teicoplanin chiral selector

The relationship between adsorption equilibria of two amino acids, i.e., l,d-threonine and l,d-methionine on the teicoplanin chiral selector and their phase behavior has been analyzed. The experimental and numerical methods have been proposed to determine activity coefficients of amino acids in different solvent systems. The procedure was based on the analysis of solubility properties of the amino acids in aqueous solutions of methanol, ethanol and propanol-2-ol used as the mobile phases in chromatographic elution. The solubility measured in mixed alcohol–water solutions was correlated with the non-random-two-liquid (NRTL) model for the activity coefficients. The values of activity coefficients were incorporated into the adsorption isotherm equation, which allowed the analysis of retention properties of the amino acids versus their fluid phase behavior. For the investigation the experimental data of adsorption equilibria of amino acids as well as of the mobile phase constituents acquired in a previous work were exploited. The composition of both the mobile and the adsorbed phases was found to affect the retention properties of the amino acids. For water-rich mobile phases the activity in the adsorbed phase determined the retention mechanism, while for the alcohol-rich systems activity in the mobile phase was predominant.