What is Mass Spectrometry? (Peptide Glossary)

Mass spectrometry (MS) measures the mass-to-charge ratio of ionized molecules, producing an extremely precise molecular weight reading. For peptides, this answers the identity question: is the white powder in the vial actually the peptide on the label? Every peptide has a known theoretical molecular weight - BPC-157 is 1419.5 g/mol, semaglutide is 4113.6 g/mol - and a mass spec reading that matches confirms identity.

HPLC and MS answer different questions, which is why a proper COA includes both. HPLC tells you how pure the sample is; MS tells you what the sample actually is. A vial could be 99% pure by HPLC and still be the wrong peptide entirely - only mass spectrometry catches that. This is also how counterfeit substitutions (a cheap peptide sold as an expensive one) are detected.

On a COA, look for the measured molecular weight matching the theoretical weight within a small tolerance. Some labs report the full spectrum; others report a summary value. Either is acceptable if it comes from a verifiable third-party lab with the batch number recorded.