Chiral separation by capillary electrokinetic chromatography with hydrophobic deep eutectic solvents as pseudo-stationary phases

This study demonstrated for the first time that hydrophobic deep eutectic solvents (HDESs) can be used in capillary electrophoresis (CE) for chiral separations. We found that the an HDES methyltrioctylammonium chloride:octanoic acid (N8881Cl:OctA) can exist in the form of nano-sized microdroplets in CE background electrolyte solutions, and show hydrophobic effects as a new type of pseudo-stationary phase (PSP) during CE separation. When used in combination with various cyclodextrin (CD)-type chiral selectors, the presence of N8881Cl:OctA significantly improved the enantioresolutions of several model drugs. Moreover, the migration time of the enantiomers can also be reduced when an anionic CD (e.g., carboxymethyl-β-cyclodextrin (CM-β-CD)) was used. Critical factors influencing the chiral separations were systematically investigated including the HDES concentration, hydrogen-bond acceptor (HBA)/hydrogen-bond donor (HBD) ratio, CD concentration, buffer pH, and applied voltage, etc. An insight into chiral recognition mechanism with HDES is provided for reference. A comparison of the chiral CE performance of HDESs with traditional surfactants was also performed to demonstrate their superiority as a new type of PSP.