Mass spectrometry is an analytical technique which, as its name implies, “weighs” molecules, and thereby can define molecular identity, interactions and reactions. With this technique, molecules from a source are ionized (i.e. made to carry a formal charge, either negative or positive) and subsequently enter the mass analyzer where ions are separated according to their mass/charge ratio (m/z), and then a spectrum is recorded as ions reach the detector. Initially applications were limited to small molecules but nowadays even large macromolecular complexes are amenable for analysis.