Effect of the ionic strength of the solution and the nature of its ions on the adsorption mechanism of ionic species in RPLC -: III.: Equilibrium isotherms and overloaded band profiles on Kromasil-C18

In two companion papers, we have described the influence of the concentration and the nature of completely dissociated salts dissolved in the mobile phase (methanol:water, 40:60, v/v) on the adsorption behavior of propranolol (R'-NH2+-R, C1) on XTerra-C-18 and on Symmetry-C-18. The same experiments were repeated on a Kromasil-C-18 column to compare the adsorption behavior of this ionic compound on these three different RPLC systems. The adsorption data of propranolol hydrochloride were first measured by frontal analysis (FA) using a mobile phase without salt. These data fit best to the Bi-Moreau model. Large concentration band profiles of propranolol were recorded with mobile phases containing increasing KC1 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 using the numerical solution of the inverse problem of chromatography. The general effect of a dissociated salt in the mobile phase was the same as the one observed earlier with XTerra-C-18 and Symmetry-C-18. However, obvious differences were observed for the shape of the band profiles recorded at low column loading (1.5 g/L, 250 muL injected). A long shoulder is visible at all salt concentrations and the band broadening is maximum at low salt concentrations. A slow mass transfer kinetics on the high-energy sites of the bi-Moreau model might explain this original shape. Five other salts (NaCl, CsCl, KNO3, CaC1(2) and Na2SO4) were also used at the same ionic strength (J = 0.2 M). As many different band profiles were observed, suggesting that specific solute-salt interactions take place in the adsorbed phase. (C) 2004 Elsevier B.V. All rights reserved.