The Convergence Coordinates of the Affinity of a Component to the Stationary Phase vs. the Composition of Mobile Phase in HPLC

By plotting of the affinity of component to adsorbent (_a) vs. the logarithm of the solvent activity in the mobile phase in a reversed phase HPLC system, it is shown that when the concentration of organic solvent in the mobile phase increases, _a of a series of aromatic alcohol homologues tends to converge to a point. A theoretical model for evaluating the convergent coordinates was presented by using the linear relationship between _a and log a_D and the stoichiometric displacement theory for retention (SDT-R). The physical meaning of the coordinates of convergence point was also elucidated. Convergence occurs when the activity of the displacing solvent approaches the value when in a pure state. The experimental results fit the equations and the coordinate of the convergence point well. At the same time, a unified equation to express the quantitative relation between parameter _a describing the adsorption process in a liquid-solid system and k describing the retention process in a liquid chromatographic system is also derived.