High sensitivity capillary zone electrophoresis-electrospray ionization-tandem mass spectrometry for the rapid analysis of complex proteomes

The vast majority of bottom-up proteomic studies employ reversed-phase separation of tryptic digests coupled with electrospray ionization tandem mass spectrometry. These studies are remarkably successful for the analysis of samples containing micrograms of protein. However, liquid chromatography tends to perform poorly for samples containing nanogram amounts of protein, presumably due to loss of trace-level peptides within the chromatographic system. Capillary zone electrophoresis provides a much simpler flow system and would appear to be an attractive alternative to liquid chromatography for separation of small peptide samples before electrospray ionization and mass spectrometry detection. However, capillary zone electrophoresis has received very little attention as a tool for analysis of complex proteomes. In 2012, we reported the use of capillary zone electrophoresis for the analysis of the secretome of Mycobacterium marinum, a model system for tuberculosis. Roughly 400 peptides and over 100 proteins were identified from this medium-complexity proteome; this identification required analysis of a set of 11 fractions and occupied three hours of mass spectrometer time. We have recently employed an improved capillary zone electrophoresis system for the analysis of 100 ng of the Escherichia coli proteome and observed over 1300 peptides and nearly 350 proteins in a single separation. More interestingly, analysis of 1 ng of the E. coli proteome yielded over 600 peptide and 140 protein groups. This sample size approaches that of a large eukaryotic cell, suggesting that capillary zone electrophoresis may ultimately be a useful tool for chemical cytometry.