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 (N 8881 Cl: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 N 8881 Cl: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.