Stacking or On-line Sample Concentration in CE-MS for Metabolomics
ESI-MS is a powerful/sensitive detector for CE in metabolomics. For trace analysis of certain metabolites, off-line sample preparation will be required prior to CE-ESI-MS. An alternative and complementary sample preparation approach is stacking, which was initially developed for CE with UV detection to improve detection sensitivity. Stacking also offers the possibility of sample clean-up during analyte focusing, providing the opportunity to improve the tedious and long analysis commonly associated with bioanalysis. This chapter introduces the different stacking techniques developed in CE-UV, and their evolution into CE-ESI-MS with special emphasis on applications for metabolomics. The nature of metabolites (typically charged small molecules) makes it easy to apply the different stacking techniques currently available in the literature. However, it seems like stacking in CE-ESI-MS for metabolomics is still in its development/testing stage, and hopefully younger scientists will pursue research in this relevant area of study.