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What does 99% purity actually mean on a peptide test?
The short answer
It almost always means chromatographic purity: the main peak accounted for 99% of the integrated peak area at the stated wavelength. It is a ratio among things the detector sees. Water, salts and counter-ions are largely invisible to it, so a vial can be 99% pure by chromatography and still contain considerably less peptide by mass.
In detail
Two different questions hide behind the word purity. First, of the peptide-related material present, how much is the target molecule — that is the chromatographic figure. Second, of the total mass in the vial, how much is peptide at all — that is net peptide content, and it is a different measurement.
This is why purity and quantity are reported separately on a certificate. High purity with a low quantity result is a perfectly coherent outcome, and a common one.
When comparing two vendors' numbers, check that both are quoting the same kind of purity at the same wavelength before concluding anything.
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