Tartaric acid-based ionic liquid-type chiral selectors: Effect of cation species on their enantioseparation performance in capillary electrophoresis

Ionic liquids (ILs) have proven to be a promising alternative to traditional chiral selectors in chiral capillary electrophoresis (CE). However, the number of IL-type chiral selectors currently available is quite limited and their types of structure are generally restricted to tetraalkylammonium salts. In this work, six tartaric acid (TT)-based ILs (TTILs) were designed containing different cation species, including di-1-butyl-3-methylimidazolium-L-tartrate ([BMIM]2-L-TT), di-1-butyl-pyridinium-L-tartrate ([BPY]2-L-TT), di-tetrabutylphosphonium-L-tartrate ([TBP]2-L-TT), di-choline-L-tartrate ([CHO]2-L-TT), di-1-butyl-1-methylpiperidinium-L-tartrate ([BMPIP]2-L-TT), and di-1-butyl-1-methylpyrrolidinium-L-tartrate ([BMPYR]2-L-TT). These novel IL-type chiral selectors were successfully synthesized and applied to the enantioseparation of ten amino alcohols in CE. Under selected CE conditions (60 mM TTILs, 150 mM boric acid in 80–90% methanol, pH 4.5), [BMPIP]2-L-TT showed the best chiral separation performance, followed by [BMPYR]2-L-TT and [TBP]2-L-TT. Most amino alcohols were resolved with enantioresolutions (Rs) above 4.0. The enantioselectivity demonstrated by [CHO]2-L-TT was relatively moderate (Rs ranging from 1.50 to 4.50), but it proved that the suppression of electroosmotic flow (EOF) is not the main contribution of TTILs to chiral CE. The enantioselectivities of [BMIM]2-L-TT and [BPY]2-L-TT were also shown to be satisfactory but their interference with UV detection is a negative factor in CE application. To the best of the authors’ knowledge, this is the first time that the influence of various achiral cation species of IL-type chiral selectors on CE enantioseparations has been systematically evaluated. Since there has been controversy regarding the true role of chiral ILs in CE enantioseparations, the parallel comparison of these TTILs in this work clearly shows that their cations are able to participate in the enantiorecognition process by introducing steric hindrance effect, rather than merely influencing the EOF in the separation systems.