Instrument that produces and measures a mass spectrum. It separates ion according to the ratio of their mass to charge, allowing scientists to determine the abundance of each isotope. Mass spectrometry of proteins digested into smaller peptides is used to identify peptide sequences and phosphorylation states of specific amino acids. Two primary methods are applied, i.e. electrospray ionization (ESI) for ionization of whole proteins and matrix-assisted laser desorption/ionization (MALDI).