Chirally-functionalized monolithic materials for stereoselective capillary electrochromatography

Despite the demonstrated high performance of stereoselective capillary electrochromatography (CEC) [1], the broader acceptance of this appealing separation technique has been strongly impeded by the weakness of the non-dedicated, conventional packed column technology normally employed, which is based on microparticulate materials (3–5 lm) and depends on sintered retaining frits. To alleviate the need for frits and related problems (such as bubble formation, barely reproducible manufacture, fragility) and in particular poor column longevity, researchers’ attempts to improve on the CEC column technology and thereby simplify the practical work have recently focused on monolithic type columns where the continuous porous solid is anchored to the capillary wall. Along this line, three distinct types of enantioselective monolithic CEC columns have evolved [2, 3]. (i) Enantioselective monolithic capillaries that make use of particulate CSPs have been prepared by either embedding the particles (25–30%) in a monolithic bed (such as a sol–gel matrix [4] or a polyacrylamide polymer [5]) or by stabilizing the densely-packed particulate bed via a sol–gel matrix [6]. (ii) Silica monolithbased enantioselective columns are prepared in situ in a capillary using sol–gel technology via steps of gelation and phase separation (macropore formation) and subsequent hydrothermal treatment or aging (mesopore formation), which allows independent control of mesopores and macropores and yields an inorganic material with a bimodal pore distribution and high active surface area [7, 8]. In a subsequent step, the chiral selector is immobilized through either covalent linkage [9] or via adsorptive coating of a polymeric selector [10, 11]. (iii) In contrast to the above approaches, enantioselective organic polymer monoliths may be prepared in a single step from a chiral monomer (a chiral selector with a polymerizable functionality), an ionizable co-monomer for EOF generation, another co-monomer that allows the surface polarity to be adjusted, a crosslinker, and porogens that induce phase separation during polymerization. This approach is very flexible and allows tuning of the polymer in terms of porosity, macropore diameter and surface chemistry, in order to meet the specific requirements of CEC and enantiomer separation. Chirally-functionalized polyacrylamide [12–15], brush-type chiral polymethacrylate [16], and molecularly-imprinted chiral polymer monoliths [17] have been successfully developed for enantioselective CEC by incorporating various different chiral recognition principles.