ELECTROSTATIC RETENTION MODEL OF REVERSED-PHASE ION-PAIR CHROMATOGRAPHY

The theoretical foundation of the electrostatic theory of ion-pair chromatography derives from colloid and surface chemistry. In the first part of this paper, the basic concepts of the theory are discussed with emphasis on the physical principles. The theory can predict retention changes of a charged solute when varying experimental parameters in ion-pair chromatographic systems. However, because of the interplay between the different parameters, such a prediction is only feasible when using iterative numerical procedures. Therefore, a simplified theory is developed in the second part where a relationship is derived which separates the contributions of various parameters, such as type and concentration of ion-pairing reagent, ionic strength, concentration of organic modifier and eluent pH. At high surface concentrations of the ion-pairing reagent, competition between the solute and ion-pairing reagent for the limited area of the stationary phase available may occur. It is shown in the third part of the paper that this results in a maximum in the relationship between capacity factor and concentration of ion-pairing reagent in the eluent. In the final section, and extended version of the electrostatic theory is developed. It accounts for the effect of accumulation of solute ions in the electrical double layer on the capacity factor. The extended form of the electrostatic theory provides the most complete treatment of the retention of charged solutes. However, this is achieved at the cost of developing a complex mathematical formulation.