Chiral capillary electrophoresis-mass spectrometry: modes and applications

A review is presented to highlight several approaches for coupling capillary electrophoresis (CE) and electrospray ionization-mass spectrometry (ESI-MS) for analysis of chiral compounds. A short discussion of commercially available CE-MS instruments and interface design is followed by a detail review on various modes of chiral CE-MS. In general, for each CE-MS mode, the capabilities, applications and limitations for chiral analysis have been pointed out. The first mode, chiral capillary zone electrophoresis-mass spectrometry (CZE-MS) in which neutral derivatized cyclodextrins (CDs) are used is possible using either column coupling with voltage switching or a partial-filling technique (PFT). However, some applications of direct coupling of CZE-MS mode are also reported. The second mode is a chiral electrokinetic chromatography-mass spectrometry (EKC-MS) in which a charged chiral selector such as a sulfated beta-CD or a vancomycin could be conveniently employed. This is because these chiral selectors have a significantly higher countercurrent electrophoretic mobility which prevents the entrance of these selectors into the mass spectrometer. The combination of counter-migration and PFT demonstrates that this synergism could be successfully applied to chiral analysis of a broader range of compounds. It is well-known that the on-line coupling of micellar electrokinetic chromatography to mass spectrometry (MEKC-MS) is problematic because the high surface activity and nonvolatile nature of conventional surfactant molecules lower the electrospray ionization efficiency. However, a recent report demonstrates that this hyphenation is now possible with the use of molecular micelles. Various MEKC-ESI-MS parameters that can be used to optimize both chiral resolution and ESI response are discussed. Finally, two recent examples that demonstrate the feasibility of using either open-tubular or packed chiral CEC with MS are reviewed. This survey will attempt to cover the state-of-the-art on various modes of CE-MS from 1998 up to 2002.