Mass Spectrometry and Peptide Identity Testing
Why mass spectrometry is used to confirm peptide identity, what 'observed vs. theoretical mass' means and what it does not prove.
Identity, not purity
Mass spectrometry (MS) measures the mass-to-charge ratio of ionised molecules. For a peptide, the measured mass can be compared with the theoretical mass calculated from its amino-acid sequence and modifications. A match supports that the intended molecule is present.
MS alone does not quantify purity; that is the role of HPLC. The two techniques are complementary and are often combined (LC-MS).
Common ionisation methods
Electrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI) are both widely used for peptides. ESI often produces multiply-charged ions, so the spectrum shows several peaks that are deconvoluted to a single molecular mass.
What to look for on a COA
Look for the method used, the theoretical mass, the observed mass and whether the result conforms. A statement of 'conforms' without any figures is less informative than reported values.
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