A peptide is only as useful as it is verified. Two analytical techniques do most of that verification work, and they answer two different questions.
High-Performance Liquid Chromatography (HPLC) answers "how pure is it?" The sample is pushed through a densely packed column under high pressure. Different molecules travel through that column at different speeds depending on how strongly they interact with the packing material, so the target peptide separates from truncated sequences, deletion products, and other impurities. A detector records each component as a peak, and the area under the main peak relative to the others gives a purity percentage. A clean chromatogram with one dominant, sharp peak is the signature of a well-made peptide.
Mass Spectrometry (MS) answers "is it the right molecule?" The instrument ionizes the sample and measures the mass-to-charge ratio of the resulting ions with high precision. Because every peptide sequence has a specific, calculable molecular weight, the measured mass either matches the expected value or it does not. A match confirms identity; a mismatch flags a synthesis error, an unexpected modification, or contamination.
The two techniques are complementary, and this is the key point. HPLC could show a single clean peak while the peptide is still the wrong compound. MS could confirm the correct mass while significant impurities go unnoticed. Used together — often as LC-MS, where the two are coupled in sequence — they establish both identity and purity in a single analytical workflow.
This is why a credible certificate of analysis references both methods rather than one. Purity without identity, or identity without purity, is only half the picture.
