Cyclodextrins as mobile phase additives in open-tubular admicellar electrochromatography for achiral and chiral separations

Native cyclodextrins (alpha-, beta- and gamma-CD) which are traditional chiral selectors in capillary electrophoresis were studied in open-tubular admicellar electrochromatography (OT-AMEC) using cetyltrimethylammonium bromide (CTAB) as dynamic stationary pseudophase. The initial mobile phase was 0.2 mM CTAB in 25 mM sodium tetraborate (pH 9.2), which forms admicelles (bilayer) at the solid surface/liquid interface inside the capillary. Different molar ratios of [CD]:[CTAB], and 0:1 to 2:1 for [alpha-CD]:[CTAB] and [beta-CD]:[CTAB] and 0:1 to 20:1 for [gamma-CD]:[CTAB] in the mobile phase were investigated. An increase in the CD concentration decreased the magnitude of the electrosmotic flow (EOF). This suggests that CDs increase the critical surface aggregation concentration of CTAB, reducing the amount of free CTAB available to aggregate at the solid surface/liquid interface. At a molar ratio of 1.5:1 for alpha-CD, 2:1 for beta-CD and 20:1 for gamma-CD, the EOF changed from anodic to cathodic in direction. This suggests that no admicelles were formed under these conditions. Consequently, the retention factors (k) of the tested analytes (dichlorprop, fenoprop, ibuprofen, ketoprofen, propranolol, verapamil, hexanophenone, verapamil and valerophenone) decreased until k = 0 (no admicelles or stationary pseudophase). We observed a change in the migration orders of the analytes because of the resulting change in the effective electrophoretic mobility. Finally, we achieved chiral separation of mecoprop and dichlorprop in OT-AMEC with alpha- or beta-CD in the mobile phase, which was not possible in electrokinetic chromatography using the same alpha- or beta-CD concentration in the background solution. The chiral separation was aided by the retention caused by the dynamic stationary pseudophase.