What is LC-MS (liquid chromatography–mass spectrometry)?
LC-MS couples liquid chromatography with a mass spectrometer so that each separated component can be identified by its mass; it is the standard method for confirming a peptide's identity.
How LC-MS works
The sample is first separated by liquid chromatography. As components leave the column, they are ionized, most often by electrospray ionization, and passed into a mass spectrometer, which measures their mass-to-charge ratio (m/z).
Electrospray gives a large peptide several charges at once, so one molecule appears as a series of ions rather than a single peak at its full molecular weight. Identity is confirmed when the observed ions match the values calculated from the expected molecular weight.
LC-MS on GLP-3 RT certificates
GLP-3 RT has an average molecular weight of 4731.33 g/mol. Its main ions are expected near m/z 1578.1 (three charges, [M+3H]3+) and 1183.8 (four charges, [M+4H]4+). Current certificates show 1578.0–1578.3 and 1183.6, within about 0.4 m/z of prediction.
Limits
LC-MS confirms molecular weight. Two peptides with the same amino acids in a different order have the same mass, so sequence order needs fragmentation (tandem MS) or other methods. Signal intensity also depends on how easily each species ionizes, so LC-MS is not used for the purity figure.