Tetraalkylammonium-l-tartrate ionic liquids as sole chiral selectors in capillary electrophoresis

Abstract

In the field of separation science, the use of tartaric acid-based ionic liquids (TTILs) for chiral separation has not yet been reported. In this work, six novel mono- and di-tetraalkylammonium l-tartrate ILs (TAA-l-TTs) with different alkane chain length were evaluated for their potential as sole chiral selectors in capillary electrophoresis (CE). In comparison with conventional l-tartaric acid (l-TT) chiral selector, these IL-type chiral selectors exhibit remarkably improved enantioseparation performance, which is even better than most previously reported TT ester-type chiral selectors. Major factors influencing CE separation were investigated, including the background electrolyte (BGE) composition, ILs concentration, type and proportion of organic modifier, buffer pH, etc. The results show that the l-TT-based IL-type chiral selectors prefer a weak acid buffer (150 mM boric acid at pH 4.5) with a high proportion of methanol (80%-90%). The best resolutions (Rs) were obtained when the IL concentration was set at 60 mM. We further carried out a series of comparative experiments and a molecular simulation study to explore the enantiorecognition mechanism of TTILs. The result indicates that the TAA-l-TTs cannot be considered as a simple combination of TAA salt and l-TT. The dissociation degree of TTILs may be an important factor influencing the enantiorecognition process.