The utility of mass spectrometry for protein and peptide analyses lies in its ability to provide highly accurate molecular
weight information on intact molecules. The ability to generate such accurate information can be extremely useful for protein
identification and characterization. For example, a protein can often be unambiguously identified by the accurate mass
analysis of its constituent peptides produced by either chemical or enzymatic treatment of the sample. Furthermore, protein
identification can also be facilitated by analysis of the protein’s proteolytic peptide fragments in the gas phase; fragment ions
generated inside the mass spectrometer via collision-induced dissociation (CID) to yield information about the primary structure
and modifications. This overview describes how electrospray ionization (ESI) and matrix-assisted laser desorption/ionization
(MALDI) mass spectrometry is being used for peptide and protein characterization focusing on its application to molecular
weight determination.