Chemistry · Glossary
Peptide purity
Also written: HPLC purity · chromatographic purity · peak-area purity · COA purity
Definition
Purity is the share of detected material that is the target peptide—a peak-area percentage, not a measure of how much peptide is in the vial.
Peptide purity is the share of detected material assigned to the target peptide in a chromatographic test, usually reported as a peak-area percentage. A 99% result means the target peak produced 99% of the integrated signal under that method—not that the vial contains 99% peptide by weight or the labeled fill.
What does 99% purity mean?
What does 99% purity mean? In the usual HPLC purity calculation, the laboratory separates a sample, measures the area under each detected peak, and divides the target peak area by the total integrated peak area. The result describes what the chosen method detected and separated; the denominator is not the powder’s total weight.
Suppose a BPC-157 sample produces 1,000 integrated units and the assigned target peak accounts for 990. The peptide purity is 99%. This is hypothetical, not a real batch result. A peer-reviewed USP reference-standard paper likewise reports HPLC impurities as a percentage of total detected area while measuring counterions and other material separately by weight.
What does HPLC purity leave out?
HPLC purity leaves out anything the method does not detect or resolve, and the headline percentage does not establish sterility, endotoxin level, net peptide content, or correct fill. Closely overlapping compounds can also share one apparent peak. “99%” is useful evidence about a chromatogram, not a quality score.
| Question | Does a purity percentage answer it? |
|---|---|
| Is the sample sterile? | No |
| Did it pass an endotoxin test? | No |
| How much peptide is in the vial? | No |
| Does the vial contain the correct fill? | No |
| How much detected peak area belongs to the target? | Yes, under the stated method |
The FDA’s Q6A guidance treats dosage-unit uniformity, sterility, endotoxins, assay, identity, and impurities as distinct tests. One tidy number cannot moonlight as all six.
Is peptide purity the same as potency?
Peptide purity vs potency is a category error. Purity describes the relative detector signal assigned to the target; potency describes strength or biological activity under a suitable assay. A clean chromatogram does not show how strongly a peptide activates a receptor, and a potency result does not reveal every impurity.
How should you read a COA purity result?
Read a COA purity result as one batch-specific measurement: find the method, chromatogram, target-peak assignment, detection conditions, and lot identifier. Then stop the conclusion where the test stops. A percentage without those details is difficult to audit, however many decimal places decorate it.
Use net peptide content for the separate mass question and the guide to reading a peptide COA for the other test categories. Neither answer is hiding inside the COA purity percentage.