Purity tells you how much of a sample is the target peptide. It does not confirm that the target is the correct molecule. That confirmation comes from mass spectrometry. This explains what the method does and why it belongs on every Certificate of Analysis alongside the purity result.
What mass spectrometry measures
Mass spectrometry, or MS, measures the mass-to-charge ratio of a molecule. For a peptide, the useful output is the measured molecular weight. Every peptide sequence has a specific theoretical molecular weight calculated from its amino acids. MS compares the measured weight of the sample against that expected value. A match confirms the molecule is the intended sequence.
Why purity is not enough on its own
An HPLC purity result describes the composition of a sample: how much is one main component versus impurities. But HPLC alone does not prove the main component is the right peptide. A sample could be highly pure and still be the wrong molecule, or the right molecule carrying an unintended modification. MS closes that gap by confirming identity.
How the two methods work together
HPLC and MS answer different questions, and a complete analytical picture needs both:
- HPLC answers how much: the purity percentage.
- MS answers what: confirmation of the molecular weight and identity.
The two are often run in sequence, and a batch that passes one but not the other has not been fully characterized. This is why a credible Certificate of Analysis reports both a purity figure and an identity confirmation, not just one.
What to look for on a COA
A COA should state the identity method, commonly listed as MS or LC-MS, and show that the measured mass matches the expected mass for the sequence. If a certificate reports purity with no identity confirmation, the material has been half-characterized. Identity is not an optional extra. It is half of what analytical testing exists to establish.
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