Proteomics is the study of the complete protein component of a living organism, tissue or cell and yields unbiased data without a priori knowledge. The workhorse of modern proteomics is the mass spectrometer and although it is not a new technology per se, it was for a long time confined to areas outside the biological sciences. However, it was the advent of matrix-assisted laser desorption ionization (MALDI)53 and in particular of electrospray ionization (ESI)54—which enables liquid chromatography (LC) systems to be interfaced directly to mass spectrometers—that MS branched from analytical chemistry into biology.