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 Melanotan II certificates
Melanotan II has an average molecular weight of 1024.18 g/mol. Its main ions are expected near m/z 1025.2 (one charge, [M+H]+) and 513.1 (two charges, [M+2H]2+).
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.