A technique used to determine relative atomic masses and the relative abundance of isotopes and for chemical analysis, as in the identification of metabolites, drugs, and other molecules isolated from biological samples. In a mass spectrometer a sample (usually gaseous) is ionized and the positive ions produced are accelerated into a high-vacuum region containing electric and magnetic fields. These fields deflect and focus the ions onto a detector. The fields can be varied in a controlled way so that ions of different types can impinge on the detector. A mass spectrum is thus obtained consisting of a series of peaks of variable intensity to which mass/charge (m/e) values can be assigned. For organic molecules, the mass spectrum consists of a series of peaks, one corresponding to the parent ion and the others to fragment ions produced by the ionization process. Different molecules can be identified by their characteristic pattern of lines. Analysis of mixtures can be done by gas chromatography-mass spectrometry (see gas-liquid chromatography).

Mass spectrometry is commonly used to identify protein components of complex mixtures. One approach involves separating different proteins by chromatography and electrophoresis, extracting the individual protein bands, and cleaving the proteins into small peptide fragments using protease enzymes. The fragments are then analysed by mass spectrometry. Large peptides can also be analysed ‘top-down’ using tandem mass spectrometry, which involves multiple phases or rounds of spectrometry in a single machine. Such a technique can isolate a particular peptide from a mixture, subject it to fragmentation through collisions with gas molecules in a vacuum (collision-induced dissociation), then determine the m/e values of the resulting fragments. The resultant ‘signature’ of the fragments is compared with a database of known protein fragmentation patterns to establish the identity of the unknown protein. See also matrix-assisted laser desorption/ionization-time of flight; protein profiling.

http://www.astbury.leeds.ac.uk/facil/MStut/mstutorial.htm The principles and biological applications of mass spectrometry described by Alison Ashcroft, University of Leeds