Identity testing answers a narrower question than purity testing. It doesn't ask "how much of the target compound is in this sample." It asks "is the molecule in this sample the one it's supposed to be." The standard method is called mass spectrometry, often paired with a separation step and written as LC-MS.
What the method does
A mass spectrometer weighs molecules. For a peptide, the expected weight can be worked out ahead of time from its amino acid sequence: each amino acid adds a known, fixed amount, so the total has one predictable number. The lab compares the weight it measures in the sample against that expected number.
A match within a very small, accepted margin supports the conclusion that the molecule in the sample has the same makeup as the one that was expected. When the method also separates the sample first (LC-MS), it can further show that the target molecule is the main thing present, not a minor part of a mixture.
What a weight match confirms
A weight match confirms that the molecule the lab detected weighs what it should. For a peptide, that's strong evidence the right amino acids are present in the right number, since even one wrong amino acid almost always changes the total weight enough to notice.
What a weight match does not confirm
It does not confirm how much is in the vial. Identity testing establishes what the molecule is, not how much of it there is. That's purity testing's job. See what HPLC purity does and doesn't tell you.
It does not rule out every similar molecule. In rare cases, two different molecules can weigh almost exactly the same. A basic weight check alone can't always tell them apart. A lab addressing this uses a more thorough version of the test (called MS/MS) that breaks the molecule into pieces and weighs each one, rebuilding the sequence piece by piece.
It does not screen for unrelated substances outside what it's looking for. Identity testing is built to confirm that one expected molecule is present and dominant. It is not a general check for every possible substance a sample could contain.
Why identity and purity are reported separately
These are different questions answered by different methods. A certificate that reports one without the other isn't necessarily incomplete: it may simply mean that test wasn't run for that lot. Check lab results and the lot's own certificate to see exactly which tests were run. See reading a certificate of analysis for how.
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