Molecularly bonded chitosan prepared as chiral stationary phases in open-tubular capillary electrochromatography: Comparison with chitosan nanoparticles bonded to the polyacrylamide phase

The chiral selector, chitosan (CS), was attached to the silanized capillary via a silane coupling agent, (3-glycidyloxypropyl)trimethoxysilane (GTS), to form the GTS–CS capillary, and results for this capillary were compared with those of a previous study on the copolymerization of CS with methacrylamide (MAA) (forming the MAA–CS capillary). The GTS–CS capillary did not exhibit enantioselectivity for d/l-tryptophan, whereas the GTS–BSA capillary, which was prepared by replacement of CS with bovine serum albumin (BSA), succeeded in the chiral separation with an Rs = 2.4 in Tris buffer (50 mM, pH 8.5). To increase CS attachment, the CS units were crosslinked by succinic acid, and the resulting GTS–CS–s capillary phase improved the resolution to 1.9. Alternatively, the SiH–CS–s capillary was constructed by CS attachment on the silicon hydride phase via stepwise silanization and hydrosilation reactions and crosslinking by succinic acid, but this approach could only achieve a resolution of 1.4 in Tris buffer (50 mM, pH 9.5). Although the GTS–CS–s and SiH–CS–s capillaries were still inferior to the MAA–CS capillary (Rs = 3.8), the enantioselectivities of the three capillaries were all in the range of 1.4–1.6. For the (±)-catechin sample, the plate heights of the GTS–CS–s and SiH–CS–s capillaries conditioned in pH 8.5 Tris buffer with 60% MeOH modifier were 0.9 cm ((−)-catechin) and 6.0 cm ((+)-catechin)) and 2.9 cm (−) and 3.2 cm (+), respectively, and these heights were comparable to the MAA–CS capillary (2.5 cm (−), 6.0 cm (+)) in pH 6.6 phosphate buffer with 80% MeOH. Finally, a racemate of ibuprofen, a weakly acidic anti-inflammatory drug, was successfully baseline resolved by the GTS–CS–s and SiH–CS–s capillaries in the borate buffers, which were 30 mM at pH 7.5 and 10 mM at pH 8.0, respectively.