Effect of the ionic strength on the adsorption process of an ionic surfactant onto a C18-bonded charged surface hybrid stationary phase at low pH.

The adsorption behavior of nortriptylinium hydrochloride was investigated in detail on a new C18-bonded charged surface hybrid stationary phase (CSH-C18, Waters) at a constant W(S)pH of 2.60 (phosphate buffer dissolved in an acetonitrile/water mixture, 34/66, v/v) in order to promote strong repulsive electrostatic interactions between the surface of the packing material and the analyte molecules. This new stationary phase was recently designed to reduce the peak tailing of ionizable compounds in RPLC. Overloaded band profiles were recorded over a broad range of column loading factors (0.03-2.18%) and eluent ionic strengths (5-50mM). The experimental data were compared to those predicted by some electrostatically modified adsorption isotherms, including the Langmuir and the Moreau isotherms. For small loading factors (<0.1%), the electrostatic repulsion definitely plays a role on the adsorption behavior and the retention factors increase with increasing ionic strength due to the expected diminution of the Debye length through the bulk solution. Still this effect could not be consistently described by either of the above two electrostatic isotherm models over the whole range of ionic strengths. For higher loading factors (>0.1%), the compound was mostly adsorbed as a neutral species and the adsorption behavior was best represented by an apparent Langmuir-Moreau isotherm model.