The Effect of Mobile Phase Composition and Temperature on the Adsorption Behavior of Tryptophan

Single-component adsorption isotherm data were acquired by frontal analysis for tryptophan on a C₁₈-Kromasil packed column, using acetonitrile-water solutions of various compositions (2.5, 5, and 7.5% ACN + 1% ACOH). These isotherms have a complex behavior, exhibiting at least one clear inflection point at an intermediate concentration and, possibly, another such point, close to the origin. The isotherm for 2.5 % ACN has the strongest curvature. At high concentrations, all these isotherms tend toward a limit, showing that the adsorbent has a finite saturation capacity. At low concentrations, these isotherms seem to exhibit langmuirian behavior. The isotherm model accounting best for these data is the bi-Moreau model, showing that two types of adsorption sites coexist on the surface and that strong adsorbate–adsorbate interactions take place. Large concentration band profiles of tryptophan were obtained for the three mobile phase compositions, at five different temperatures and the best values of the isotherm coefficients were determined by the inverse method (IM) of chromatography. The retention time of tryptophan decreases rapidly with increasing acetonitrile concentration in the mobile phase and so do the saturation capacities of the two types of adsorption sites, with the highest values of the two saturation capacities being found for the lowest ACN content and the lowest temperature. The adsorption constant on the low energy sites decreases with increasing acetonitrile content and temperature. In contrast, the adsorption constant on the high energy sites increases with increasing ACN content of the mobile phase but decreases with increasing temperature. The solute–solute interaction parameters for the low and the high energy adsorption sites increase rapidly with increasing ACN concentration in the mobile phase and with increasing temperature.