Effect of the ionic strength of salts on retention and overloading behavior of ionizable compounds in reversed-phase liquid chromatography

Abstract

In a companion paper, we describe the influence of the concentration and the nature of salts dissolved in the mobile phase (methanol:water, 40:60, v/v) on the adsorption behavior of propranolol (R'-NH2+ -R, Cl-) on XTerra-C18. The same experiments were repeated on a Symmetry-C18 column to compare the adsorption mechanisms of this ionic compound on these two very different RPLC systems. Frontal analysis (FA) measurements were first carried out to determine the best isotherm model accounting for the adsorption behavior of propranolol hydrochloride on Symmetry with a mobile phase without salt (and only 25% methanol to compensate for the low retention in the absence of salt). The adsorption data were best modeled by the bi-Moreau model. Large concentration band profiles of propranolol were recorded with mobile phases having increasing KCl concentrations (0, 0.002, 0.005, 0.01, 0.05, 0.1 and 0.2 M) and the best values of the isotherm coefficients were determined by the inverse method (IM) of chromatography. The general effect of a dissociated salt in the mobile phase was the same as the one observed earlier with XTerra-C18. Increasing the salt concentration increases the two saturation capacities of the adsorbent and the adsorption constant on the low-energy sites. The adsorption constant on the high-energy sites decreases and the adsorbate-adsorbate interactions tend to vanish with increasing salt concentration of the mobile phase. The saturation capacities decrease with increasing radius of the monovalent cation (Na+, K+, Cs+, etc.). Using sulfate as a bivalent anion (Na2SO4) affects markedly the adsorption equilibrium: the saturation capacities are drastically reduced, the high-energy sites nearly disappear while the adsorption constant and the adsorbate-adsorbate interactions on the low-energy sites increase strongly. The complexity of the thermodynamics in solution might explain the different influences of these salts on the adsorption behavior.

Effect of mobile phase anionic additives on selectivity, efficiency, and sample loading capacity of cationic drugs in reversed-phase liquid chromatography

“…Based on experimental results obtained on different bonded phases, they also concluded that the loading capacity of ionized analytes increases with increasing salt concentration in the mobile phase due to a decrease in the repulsive interaction among the sorbed analytes. In addition, they used specific salt-solute electrostatic interactions to explain the differences in adsorption behavior of ionizable analytes [25,37,38]. …”

Nonmolecular Solvents in Separation Methods: Dual Nature of Room Temperature Ionic Liquids

“…Since ions have different lyotropy or hydrophoby, they may interact differently with the chromatographic stationary phases. The adsorption mechanisms of molecular as well as ionic species on RPLC stationary phases and especially Kromasil C 18 were recently extensively studied by Gritti and Guiochon. − They found that the adsorption isotherms of molecular and/or ionic compounds were greatly modified by the presence of ionic species. The nature of the added salts used was very important. − The retention time of propanolol, a cationic solute at pH 5, was very dependent on the buffer nature and concentration in agreement with the chaotropic model .…”

Hydrophilic interaction chromatography: A promising alternative to reversed‐phase liquid chromatography systems for the purification of small protonated bases

“…Additionally, the buffer strength affects the intensity of the electrostatic interactions between ions (since the Debye length is inversely proportional to the square root of the ionic strength) but it also impacts the binding constants and the saturation capacities of the weak and strong adsorption sites of silica‐C 18 solid adsorbents. This has been supported by frontal analysis adsorption data performed on a large variety of silica‐C 18 bonded phases .…”