What does mass spectrometry confirm that HPLC purity cannot?

HPLC and mass spectrometry answer two different questions. HPLC tells you how pure a sample is: it separates the contents and reports what percent is one main component. It does not tell you what that component is. A sample can read 99% pure and still be the wrong molecule. Mass spec confirms identity. It measures molecular weight (mass-to-charge ratio), and every peptide has a specific expected mass set by its amino acid sequence. If the measured mass matches the theoretical value, that confirms the compound is what the label claims. HPLC alone can't do that, partly because a different compound can co-elute at the same retention time and hide inside the main peak. This is why a useful COA pairs both: HPLC for purity, MS for identity. Note these are research-use-only materials, not approved for human consumption.