How does mass spectrometry allow for protein detection?

- peptide mass fingerprinting: the measured proteolytic peptide masses are matched to the theorectical proteolytic peptide masses of the proteins. this can be done because you know where trypsin is supposed to cut. if you find peptides that are very rare, proteins are easily identified. but if you find peptides that are very common, many different peptides are needed to identify the protein. - labelling: the sample is labelled with stable isotopes, which are visualised after analysis.