Enhanced enantioselectivity of tartaric acid in capillary electrophoresis: From tartaric acid to tartaric acid-based ionic liquid

Abstract

This work presents, for the first time, a systematic and step-by-step methodology on how to improve the enantioseparation performance of tartaric acid (TT) in capillary electrophoresis (CE) using ionic liquid (IL) technology. A series of chiral and achiral ILs were applied in different separation modes to evaluate their influence on TT's enantioselectivity. Unlike previous ILs-related chiral CE studies, which typically report positive synergistic effect, in this work, negative synergistic effect was observed between TT and most chiral ILs (including several amino acids ILs and a saccharide-based IL). Instead, achiral ILs (the precursors of chiral ILs, mainly tetraalkylammonium hydroxides) exhibited significantly better performance in system modifications when used in combination with TT. The influence of the configuration of TT and chiral ILs on enantioseparation was investigated to explore the synergistic mechanism. The result showed that the existence of the chiral part of the ILs may cause interference rather than facilitation in the selector-selectand interactions. On the basis of the above results, we further designed a novel TT-based IL-type chiral selector, di-tetramethylammonium-D-tartrate ((TMA)2-D-TT). It is able to achieve excellent enantioseparation of all model analytes without adding any other chiral or achiral additives. This study demonstrates that TTs, which have almost been abandoned in chiral CE, are still able to provide satisfactory enantioselectivities with the help of ILs. In some application modes, e.g., under the achiral ILs modification or using TT-based ILs, their chiral separation performance is comparable or even superior to the well-established and widely used TT ester-type chiral selectors