A feasibility study on the use of hydrophobic eutectic solvents as pseudo-stationary phases in capillary electrophoresis for chiral separations

Abstract

A critical challenge in using deep eutectic solvents (DESs) in capillary electrophoresis (CE) is to develop separation systems in which a DES can really work as a single entity. To achieve this, the authors recently demonstrated a novel strategy that takes advantage of the aqueous dispersibility of hydrophobic DESs (or more accurately hydrophobic eutectic solvents (HESs)). However, the previous work was limited only to the separation of achiral analytes, e.g., analogues, homologues, and isomers. The present study was designed as a follow-up study in order to explore the feasibility of employing HES-type pseudo-stationary phases (PSPs) in CE for chiral separations. By using carboxymethyl-β-cyclodextrin (CM-β-CD) as a model chiral selector, we provide the first evidence that there is a potential synergistic effect between HESs and traditional chiral selectors. Specifically, the combined use of HES (−)-menthol:octanoic acid and CM-β-CD allowed excellent enantioseparations of several basic drugs which were not able to be resolved in the single CM-β-CD system. The enantioresolutions were significantly improved while the migration times of the enantiomers were also shortened due to the hydrophobic mechanism of the HES-type PSP. Critical factors influencing the novel chiral CE system were systematically investigated. Since HESs are considered as “designer” solvents with highly tunable properties, this study demonstrates the potential of employing HESs (or HDES)-type PSPs in CE for chiral separations.