Effects of the surface concentration of fixed charges in C18-bonded stationary phases on the adsorption process and on the preparative chromatography of small ionizable compounds

• The adsorption isotherm of nortryptilinium onto BEH-C18 shows two adsorption sites.
• The density of high energy sites correlates to the amount of fixed charges on the surface.
• The density of low energy sites reflects the specific surface area of the adsorbent.
• The active sites are closer to the adsorbent surface than the weak adsorption sites.
• This charge doped surface permits the production of large amount of ionizable compound.

The effects of the surface concentration of positive charges attached to the surface of research BEH-C18 hybrid particles on the overloaded band profiles and the adsorption isotherms of a neutral (caffeine) and a positively charged (nortryptilinium hydrochloride) compounds were measured and investigated. The inverse method (IM) of chromatography was used to determine the isotherm parameters. Three columns were packed with endcapped BEH-C18 particles doped with three different charge densities on their surfaces (LOW, MEDIUM and HIGH). Two other columns packed with unbonded, non-endcapped, and endcapped BEH-C18 particles served as standard reference materials. Minor disturbance method (MDM) experiments were conducted with acetonitrile/water mixtures in order to assess qualitatively the surface densities of the fixed positive charges. A more quantitative approach based on the solution of the linearized Poisson–Boltzmann equation and the decrease of the experimental Henry constant was also applied. The results show that the surface concentrations of the fixed charges in the LOW, MEDIUM and HIGH columns were 0.029, 0.050, and 0.064 μmol/m2, e.g., close to two orders of magnitude smaller than the surface density of bonded C18 chains (2.1 μmol/m2).The adsorption isotherm of the ionizable compound nortryptilinium onto the BEH-C18 columns is consistent with a two-sites adsorption model. The density of the high energy sites correlates directly to the total amount of the fixed charges and isolated silanols amidst the C18-bonded chains. The amount of low energy sites reflects the specific surface area of the adsorbent. The binding constants on the high- and low-energy adsorption sites are respectively ten and two times lower on the HIGH column than on the reference endcapped column. The active sites are closer to the adsorbent surface than the weak adsorption sites. Finally, a higher production rate of ionizable compounds can be achieved in preparative chromatography with the charge doped than with the reference RPLC-C18 columns. For the same analysis time, larger recovery yields are observed despite a slight loss in selectivity due to non-selective repulsive electrostatic interactions.