Mass spectrometry.

A technique that measures molecular mass to confirm identity.

Mass spectrometry ionizes molecules and measures their mass-to-charge ratio, producing a precise molecular weight. For peptides it is the standard identity check, because a peptide's mass follows directly from its amino-acid sequence, so a measured mass matching the calculated one is strong evidence that the intended molecule is present. It is the natural complement to HPLC: chromatography shows how many components are present and in what proportion, mass spectrometry shows what they are, and the two together are the usual basis of a Certificate of Analysis. More advanced approaches fragment the peptide inside the instrument and read the pieces, which can confirm the actual sequence rather than only the total mass. The limitation worth understanding is that mass alone is not a complete identity proof. Two peptides built from the same amino acids in a different order have exactly the same mass, and so do certain stereoisomers, meaning a molecule with the same atoms arranged in a mirror-image configuration at one or more positions. Those differences can matter biologically while being invisible to a simple mass measurement. Confirming a mass is a strong check on identity, not an absolute one, and it says nothing about purity, sterility, or safety.