Purity Percentages on COAs: What 98% Actually Means

"Purity: 98.7%" is the most quoted number on a peptide COA — and one of the most misunderstood. The percentage is an area-under-the-curve estimate from HPLC, not a guarantee that 98.7% of the vial's mass is active peptide.

What the number measures

HPLC purity typically reports the main peak area divided by the sum of all detected peak areas at a specific UV wavelength. That means:

• Only compounds that absorb at that wavelength and elute from the column are counted.
• Solvents, counter-ions (TFA, acetate), and water content are not in the percentage.
• Peptides that co-elute with the main peak may be invisible as separate impurities.

So "98% pure" means "98% of what HPLC sees looks like one peak" — contingent on method, column, and detection settings.

What it does not tell you

• Identity. A pure peak of the wrong molecule still gets a high purity number. Pair with MS identity confirmation.
• Potency. Purity and biological activity are different metrics.
• Future stability. Purity was measured on the test date for that batch — not a shelf-life promise.

How to read it critically

Always request the chromatogram and integration table behind the number. Check whether impurity peaks are labeled and whether the method is described. A lone percentage without supporting data is marketing, not evidence.

For the visual side, see reading HPLC chromatograms. For vendor comparison, use our vetting scorecards.