Effect of the pH and the ionic strength on overloaded band profiles of weak bases onto neutral and charged surface hybrid stationary phases in reversed-phase liquid chromatography

This work reports on the effects of the solution pH and its ionic strength on the overloaded band profiles and the parameters of the adsorption isotherms of nortriptylinium hydrochloride on the bridge ethylene hybrid (BEH) and the charged surface hybrid (CSH) C-18-bonded columns. The mobile phases used were mixtures of acetonitrile and water buffered with hydrogenophosphate, formate, acetate, and dihydrogenophosphate buffers. The results show that the adsorption behavior of this protonated base onto the BEH-C-18 column depends barely on the mobile phase pH and is slightly affected by its ionic strength. From both physical and statistical viewpoints, the Linear-Langmuir model is the most relevant adsorption isotherm. According to the inverse method of chromatography, this model is consistent with weak dispersive interactions taking place onto the C-18-bonded chains and some strong ion dipole interactions with residual silanols. In contrast, adsorption on the CSH-C-18 column depends on the applied (s)(w)pH e.g., on the degree of ionization of the amine groups tethered to the CSH surface. For (s)(w)pH < 3, the electrostatically modified Langmuir model (EML) is acceptable because analyte molecules cannot access and interact with any active sites due to the electrostatic repulsion by the positively charged adsorbent surface. At (w)(s)pH > 7.0, the Linear-bi-Langmuir model describes best the weak adsorption of the protonated base molecules onto the C-18 chains and their strong adsorption onto the residual silanols and neutral amine groups. (C) 2013 Elsevier B.V. All rights reserved.