What 99% purity on a peptide COA does and does not tell you

99% pure is the most repeated number in the peptide market and one of the least understood. On a certificate of analysis, that figure usually comes from a single test, and it answers a narrower question than most readers assume.

What HPLC purity measures

Peptide purity on a COA is typically determined by high-performance liquid chromatography (HPLC). The result is the percentage of the detected signal that falls under a single peak. It tells you that most of what the detector saw was one compound. It does not tell you what that compound is.

That gap matters. A sample can read 99% on HPLC and still be the wrong sequence, a truncated version of the right sequence, or the correct molecule present in the wrong amount. HPLC purity is necessary, but on its own it is not sufficient.

Three results a COA should show

A certificate of analysis worth relying on reports three separate results, each answering a different question:

• Purity (HPLC): how much of the sample is one compound.
• Identity (mass spectrometry): whether that compound has the molecular weight it should.
• Quantity: whether a 10 mg vial actually holds about 10 mg.

Purity without identity leaves open the possibility of a clean sample of the wrong molecule. Identity without quantity leaves open the possibility of the right molecule at a fraction of the labeled amount. Only the three together describe what is in the vial.

The details that show testing is real

Beyond the three results, a few administrative details separate a real COA from a decorative one:

• A named third-party laboratory that can be looked up independently.
• A lot number on the COA that matches the lot number on the vial.
• Results that vary from batch to batch.

The last point is the easiest to overlook. Real analytical results move slightly between lots. A COA that shows identical numbers on every batch is not reporting measurements at all.