Effect of organic modifier concentration on retention in reversed-phase ion-pair liquid chromatography

Abstract

The concentration of organic modifier in the eluent is one of the most important factors that affect the retention of ionized solutes in reversed-phase ion-pair liquid chromatography (RP-IPC). Linear regression analysis of ln ▪ vs. methanol concentration ( C b) according to the equation ln ▪ was carried out; ln ▪ and c ip are constants for a given solute with a given column system, where ln ▪ is determined mainly by the electrostatic and non-electrostatic free-energy change of retention at C b  0, and c ip is mainly determined by the interaction behaviour between the ion-pair reagent, the ionized solute and the mobile phase. This equation has been confirmed experimentally. The absolute values of ln ▪ and c ip in RP-IPC are much larger than those in RP high-performance liquid chromatography (HPLC), which means that there is a much stronger effect of methanol concentration on retention in RP-IPC than in RP-HPLC. On the other hand, ionized compounds with the same kind and number of charges show almost the same value of electrostatic free-energy change, and ln ▪ and ln ▪ in RP-IPC can be well correlated with ln ▪ and ln ▪ in RP-HPLC.