Enantioselective silica-based monoliths modified with a novel aminosulfonic acid-derived strong cation exchanger for electrically driven and pressure-driven capillary chromatography.
Silica monoliths modified with trans-(1S,2S)-2-(N-4-allyloxy-3,5-dichlorobenzoyl)amino cyclohexanesulfonic acid were tested for enantioselective separations of various chiral bases by aqueous and nonaqueous CEC as well as nano-HPLC. The optimization of the immobilization procedure showed that an intermediate selector (SO) coverage, as does result from a single static immobilization cycle in the capillary at 60 degrees C with an 8% (m/v) SO solution in methanol, affords maximal EOF and optimal enantioselectivity values, while a second immobilization cycle does not lead to any improvements. Furthermore, the mobile phase composition was examined regarding the effectiveness of aqueous phases (ACN/water and methanol/water) compared to nonaqueous eluents (ACN/methanol) in terms of separation selectivity and efficiency. Additionally, different acids of varying strengths were tested as co-ions in the ion-exchange process, including formic acid, acetic acid, methanesulfonic acid, and TFA (pK(a) from 4.75 to 0.5). It turned out that the effects regarding EOF and enantioselectivity were largely negligible. The chromatographic efficiencies of the new capillary columns were compelling and remarkable for bases. H-u curves established for mefloquine revealed a C-term contribution (resistance to mass transfer) by a factor of about six lower in CEC than in nano-HPLC and an A-term (flow maldistribution) about three times lower in the CEC mode. Theoretical plate heights as low as around 3-5 mum could be obtained in CEC over a wide flow range (0.5-1.5 mm/s). Run-to-run repeatabilities like in HPLC and excellent system stability promise the practical usefulness of the novel monolithic capillary column for enantiomeric composition analysis of pharmaceuticals by CEC.