On-line coupling of electrokinetic chromatography and mass spectrometry

The use of pseudostationary phases (PSPs) in capillary electrophoresis provides powerful separation systems of high efficiency, selectivity and flexibility. Such electrokinetic chromatographic (EKC) systems are particularly useful for chiral analysis or for the analysis of samples containing a broad range of compounds. As the availability of mass and/or structural data on (unknown) sample constituents is increasingly important, the on-line coupling of EKC and mass spectrometry (MS) has gained attention. However, commonly used PSPs, such as micelles and cyclodextrines, may strongly interfere with electrospray ionization (ESI), making on-line EKC-MS quite a challenging task. This review covers the various approaches that have been proposed and developed to combine EKC and MS. A distinction is made between methodologies that prevent the PSP from entering the MS system, and methodologies that allow introduction of PSPs into the ion source. Various approaches such as partial filling of the separation capillary with PSP, use of reverse-migrating PSPs, employment of volatile PSPs, and alternative ionization modes, are outlined. Specific applications are described and overview tables are provided. It is concluded that there is no general solution for EKC-MS available yet, but new ionization techniques like atmospheric pressure photoionization may offer attractive perspectives for achieving full compatibility.