Strong versus weak chiral cation exchangers: Comparative evaluation for enantiomer separation of chiral bases by non-aqueous CEC

Abstract

Novel enantioselective silica-supported strong and weak cation exchange (SCX and WCX) materials (3.5 μm particles) based on enantiomerically pure N-(4-allyloxy-3,5-dichlorobenzoyl)-2-amino-3,3-dimethylbutanesulfonic acid and corresponding phosphonic acid as well as carboxylic acid structural analogs as chiral selectors have been evaluated for enantiomer separation of chiral bases by non-aqueous capillary electrochromatography (CEC). Capillary columns packed with these chiral stationary phases (CSPs) showed enantioselectivity in non-aqueous CEC towards a variety of chiral bases including amino alcohols such as β-sympathomimetics and β-blockers. Chromatographic and electrokinetic properties of the strong and weak chiral cation exchangers were evaluated comparatively in terms of their pH* profile, i.e. in terms of their dependence on the base-to-acid ratio of the background electrolyte. It turned out that the SCX type CSPs, and in particular the one based on the β-amino sulfonic acid show a broader window of applicable and suitable experimental conditions for CEC. For example, a strong and constant EOF was obtained on the sulfonic acid based CSP over the entire pH* range studied, while the EOF velocity of the carboxylic acid based CSP was slow under acidic conditions. In the separation of chiral bases, the ion-exchange retention mechanism dominated over electrophoretic migration under most conditions, especially on the SCX type CSPs. The SCX phases exhibited reasonable enantioselectivity over a wider pH* range, while the weak chiral cation exchanger (WCX type CSP) showed enantiomer separation capabilities for primary, secondary, and tertiary chiral amines only in the alkaline pH* range. Sulfonic and phosphonic acid based CSPs possess broad spectrum of applicability. For example, clenbuterol enantiomers were well baseline resolved both on suifonic acid based CSP (a = 1.33, R s = 14.2) as well as phosphonic acid based CSP (a = 1.13, R s = 4.9). In contrast, under the same conditions the corresponding carboxylic acid CSP exhibited enantioselectivity a of 1.08 and resolution R s of 1.3 only.