What HPLC purity is actually measuring
When a peptide is synthesized, the reaction doesn't produce a perfectly clean batch of the target molecule. It produces a mixture — the intended peptide, plus a range of byproducts: truncated sequences that stopped forming early, deletion sequences missing an amino acid, and other synthesis-related impurities.
HPLC separates this mixture by running it through a column that causes different molecules to travel at different speeds based on their chemical properties. The output is a chromatogram — a series of peaks, each representing a distinct compound in the sample. The lab calculates purity by comparing the area under the peak that corresponds to the target peptide against the total area under all peaks combined. If that peak accounts for 99% of the total signal, the batch is reported as 99% pure.
What purity does not tell you
It doesn't confirm identity on its own
HPLC purity tells you how clean a peak is — not definitively what that peak is. Two different molecules can, in some cases, elute at similar retention times. That's why a rigorous COA pairs HPLC purity with mass spectrometry, which measures the molecular weight of the compound and confirms it actually matches the expected peptide. A purity number without a mass spec identity check is a weaker claim than it looks.
It says nothing about sterility or safety
Purity is a chemistry measurement, not a safety one. Endotoxin testing (measuring bacterial contamination, typically reported in EU/mg) and checks for residual solvents or heavy metals from synthesis are separate analyses entirely. A vial can be 99% pure by HPLC and still carry these risks if those tests weren't run.
It doesn't tell you the actual quantity in the vial
Purity is a percentage, not a mass. A vial can be 99% pure and still contain less peptide than the label claims, if the total fill weight is off. That's a separate measurement — net content or mass by weight — that a thorough lab report includes alongside purity, not instead of it.
Why two "99% pure" vials can still differ
Because purity is one measurement among several, two vendors can both advertise "99% purity" while providing very different levels of actual assurance:
- Who ran the test. A vendor's in-house lab grading its own product is a different standard of evidence than an independent third-party lab with no financial stake in the result.
- What was actually tested. Purity alone is a narrower claim than purity + mass spec identity + endotoxin + net content, even if both vendors print "99%" on the label.
- Whether the report is lot-specific. A purity number from a past batch, reused across every listing, tells you nothing about the vial you're actually receiving.
- Whether you can see it at all. A COA that exists only "on request," and one published publicly and searchable by lot number, are not the same level of transparency in practice.
How to actually read a COA
Here's a real example from our own COA library — Retatrutide 10mg, batch RETA10-0626-101, tested by Janoshik Labs:
Reading this properly: the sample was assayed twice, and the two readings differ slightly (99.571% and 99.640%) rather than showing one clean figure. That small variance is normal and, if anything, a sign the report is real: a single suspiciously round number repeated across every product and batch is often a red flag, not a good one.
When you're evaluating any vendor's COA, check for: an independent testing lab named explicitly, a lot number that matches what's printed on your actual vial, and — ideally — more than just a purity percentage alone.
The bottom line
A purity percentage is a real, useful measurement — but it's one data point, not a complete quality picture. The number matters less than the questions behind it: who tested it, what else they tested, and whether you can actually see the report for the specific lot in front of you.