Peptide Purity Standards Explained: What Does 99% Really Mean?

A "99% purity" claim is one of the most heavily advertised specifications in the research peptide industry — and one of the most commonly misunderstood. Purity percentage is a specific, calculable figure, not a marketing term, and knowing what it actually measures helps in evaluating whether a Certificate of Analysis is meaningful.

What Purity Percentage Actually Measures

HPLC purity is calculated as the area of the target peptide's peak divided by the total area of all peaks detected in the chromatogram, expressed as a percentage. A 99% purity result means that, of everything detected by the column, 99% corresponds to the target compound — the remaining 1% represents related impurities, truncated sequences, or degradation products.

Why Two "99%" Results Can Still Differ

Purity alone doesn't capture everything. Two vials can both report 99% HPLC purity while differing in which impurities make up that remaining 1%, and HPLC purity doesn't independently confirm identity — that's what mass spectrometry is for, which is why credible suppliers report both. See our guide to HPLC and mass spec testing for how the two methods work together.

What to Check on a CoA

A meaningful Certificate of Analysis should show the actual chromatogram (not just a stated percentage), a batch or lot number matching the vial, the testing date, and ideally corroborating mass spec data confirming the correct molecular weight. Our guide on reading a Certificate of Analysis covers this in more detail.