CORRELATION OF RETENTION AND SELECTIVITY OF SEPARATION IN REVERSED-PHASE HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY WITH INTERACTION INDEXES AND WITH LIPOPHILIC AND POLAR STRUCTURAL INDEXES

A simple semi-empirical model of interaction indices proposed to describe the retention in reversed-phase systems, assuming a predominant role of the interactions in the mobile phase to control the retention, and the concept of lipophilic and polar indices derived on its basis are reviewed. Specific contributions of the stationary phase to the retention are neglected to a first approximation. The model suggests that the polarities and the concentrations of the mobile phase components and the volume and polarity of the solute, characterized by its molar volume and interaction index, respectively, are the major factors controlling the retention. Based on this model, the retention and selectivity in reversed-phase systems can be understood to be comprised of lipophilic and polar contributions, which are characterized quantitatively by constants related to the structure of the solutes, i.e., by the lipophilic and polar indices n(ce) and q(i), respectively. These indices can be used to characterize and predict the retention and selectivity in reversed-phase chromatography using a retention scale calibrated with a suitable reference homologous series, the first member of which can serve as the selectivity reference standard. The two-indices approach allows the use of a single calibration equation over a wide range of compositions of a binary mobile phase with better precision of prediction of retention than single-index calibration approaches. It is also possible to characterize and predict the retention and separation selectivity in ternary mobile phases composed of two different organic solvents in water and in gradient elution chromatography. It has been verified for several types of compounds that the indices can be calculated from the additive increments of structural substituents. The present method allows the prediction of possible changes in the order of elution with changing composition of the mobile phase and optimization of the separation conditions. It was used for a quantitative description of retention in homologous and oligomeric series, where it was possible to explain and predict conditions under which some polar oligomeric series are eluted in order of decreasing molecular mass.