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What mass spectrometry shows

In brief

Mass spectrometry measures the mass-to-charge ratio of ionised molecules. For peptides it is the primary identity method: the measured molecular mass is compared with the mass calculated for the declared sequence. A matching mass is strong evidence of identity; its absence from documentation leaves the question "what molecule is this" open.

Illustration for the research article

How it works

Sample molecules are ionised (for peptides, usually by electrospray or MALDI), and the instrument measures the mass-to-charge ratio of the resulting ions. The compound's molecular mass is derived from the spectrum — precisely enough to distinguish molecules differing by a single amino-acid residue or less.

Why it is the identity method

Every peptide sequence has a calculable molecular mass. If the measured mass matches the calculated one, the material is consistent with the declared structure; if not, the vial contains a different molecule or a significant modification. This is why, in the "HPLC + MS" pair, chromatography covers purity while mass spectrometry covers identity.

The method's limits

A mass match is necessary but not absolute: molecules with identical mass (for example, with rearranged residues) are indistinguishable in a basic spectrum; fragmentation methods (MS/MS) exist for that. A mass spectrum also says nothing about quantity or sterility. Each property is confirmed by its own method.

References

  1. Electrospray Mass Spectrometry and Tandem Mass Spectrometry of Synthetic Multicomponent Peptide Mixtures: Determination of Composition and Purity — Analytical Biochemistry, 219(2), 261–277 (PMID 8080083; DOI 10.1006/abio.1994.1266), 1994.