Prediction for the separation efficiency of a pair of enantiomers during chiral high-performance liquid chromatography using a quartz crystal microbalance

A simple and rapid screening method of the chiral stationary phase during high-performance liquid chromatography (HPLC) utilizing a quartz crystal microbalance (QCM) has been developed for the chiral separation of a pair of enantiomers. The outline of the method is as follows: a self-assembled monolayer (SAM) is constructed on the gold electrodes of the QCM sensor chips by utilizing the interaction between thiols and gold. The chiral selectors used as chiral stationary phases in the HPLC are then immobilized, and a pseudostationary phase is constructed on the electrodes. Subsequently, the sensors are equilibrated in the solutions, the targeted chiral samples are injected, and the frequency changes are observed. Four kinds of chiral molecules and three kinds of chiral stationary phases were examined in this study. When chiral separation is possible using the chiral stationary phase immobilized on the sensors, significant differences in the frequency changes are observed because the intensities based on interactions differ among the isomers. The developed method can predict not only the possibility for chiral separation but also the elution order from the chiral stationary phase column. Furthermore, the degree of the mutual separation of a pair of enantiomers seems to be roughly predictable from the difference in the frequency change (DeltaF) and first-order association rate constant (k(obs)). The method does not require several different kinds of chiral columns that are more expensive than achiral ones such as the octadecylsilica (ODS) column. The required amounts of the chiral stationary phases are extremely small, and the sensors with immobilized chiral selectors are reusable. In addition, the method requires only a few minutes to complete the analysis. Thus, considerable reductions in both cost and time are realized. By applying the developed method to many chiral molecules and chiral stationary phases, its superiority may be corroborated; thus, it is expected that the method can be effectively used for the selection of chiral stationary phases.