Dependence of retention on the organic modifier concentration and multicomponent adsorption behavior in reversed-phase chromatography.

A three-parameter equation is derived to express the dependence of the logarithmic retention factor, kappa, on the volume fraction of the retention modulator, phi, in a binary eluent (such as the organic modifier in the hydro-organic eluents used in reversed-phase chromatography). It is based on the competitive binary adsorption isotherm of the eluite and the modulator generated by employing the ideal adsorbed solution (IAS) method. The equation is found to describe adequately the trends in the kappa-phi relationship experimentally observed in reversed-phase systems. Furthermore, the expression affords an estimation of the single-component adsorption isotherm of the eluite from the corresponding kappa versus phi plot and thus provides a simple means to gather data of importance in the design of separations by non-linear chromatography. For instance, the method can be used to determine whether a pair of eluite isotherms cross one another, a situation that could lead to difficulties in preparative separations. The inherent limitations of the IAS approach may restrict the usefulness of the expression in specific cases. Nevertheless, the approach presented here establishes an explicit, thermodynamically consistent link between the eluite-modulator multicomponent isotherm and corresponding plots and allows a rational description of the generally observed retention behavior in reversed-phase chromatography. The results of this work also illustrate the limitations of the competitive Langmuir isotherm, which is most frequently used to treat competitive adsorption, in the study of the kappa-phi relationship specifically and in investigating and modeling non-linear chromatography at large.