Distribution equilibria of solvent components in reversed-phase liquid chromatographic columns and relationship with the mobile phase volume

Abstract

Various experimental methods for measuring the adsorption of solvent components from a mixed mobile phase in a liquid-solid chromatographic column are discussed in terms of the surface excess description. The adsorption from aqueous mixtures of acetonitrile and methanol on to reversed-phase packings was measured. As the derivation of individual isotherms for both solvent components from these data is impossible, two models for the adsorption process, viz., exclusive adsorption of organic modifier and adsorption of a mixture of constant composition, are discussed. The former model is discarded, partly because it cannot explain the occurrence of an azeotropic point. The second model can explain the data over a limited range of mobile phase compositions. The discussion shows that simple experimental estimates of the mobile phase (zero) volume, such as the retention volumes of a mobile phase disturbance or of isotopically substituted solvent components, are in general unreliable. Their validity under certain conditions can be argued only from the full adsorption isotherm, accepting specific assumptions about the adsorption process