Chiral electrokinetic chromatography using dual pseudo-stationary phases: SBE-β-CD with hydrophobic (deep) eutectic solvents versus micelles and ionic liquids

Two hydrophobic (deep) eutectic solvents (H(D)ESs), menthol:octanoic acid (Men:OctA) and menthol:trioctylphosphine oxide (Men:TOPO), were prepared, characterized, and then employed for the first time in combination with sulfobutylether-β-cyclodextrin (SBE-β-CD) for chiral separations in capillary electrokinetic chromatography (EKC). The “H(D)ES + SBE-β-CD” dual pseudo-stationary phase (PSP) systems yielded not only enhanced enantioselectivities, but also accelerated migrations of primaquine and chlorpheniramine, two drugs that cannot be resolved using the single SBE-β-CD system. The enhancement originated from the combined effects of stereoselective complexation and hydrophobic partitioning involving the H(D)ES phase, which moved faster than the anionic CD phase under the separation conditions. The best results in terms of enantioresolution and migration time were obtained using a 20 mM phosphate buffer (pH 7.0) containing 2.0 % SBE-β-CD and 0.2 % Men:OctA at an applied voltage of 15 kV. We further compared the chiral EKC performance of the H(D)ESs with a series of conventional PSPs, including three micelles (Tween-20, Brij-35, SDS) and three ionic liquids (ILs). The results showed that none of these modifiers could achieve comparable EKC performance, suggesting that in certain cases, H(D)ESs could be a superior alternative to traditional modifiers for chiral EKC separations.