Reading a Peptide COA: What HPLC Purity and Mass Spec Tell You

Two things decide whether a research peptide is usable: how clean it is, and whether it's really the molecule named on the label. People tend to treat those as one question, but they're separate. A certificate of analysis (COA) answers each with independent lab data, and getting comfortable reading one is the most useful habit you can pick up as a buyer.

Purity and identity are different measurements

Purity asks how much of what the method detects is the target peptide. Identity asks whether the main component is the correct molecule in the first place. A sample can be extremely pure and still be the wrong compound, which is why the two tests below are used together.

On an HPLC trace, the target shows up as one large peak, and impurities appear as small bumps around it.

What a ≥99% HPLC result actually means

High-performance liquid chromatography splits a sample into its components and records each one as a peak. Purity is reported as the target peak's share of the total detected peak area. A result of ≥99% means the target peak accounts for at least 99% of the detected peak area, with all other peaks together under 1%. On the chromatogram that looks like one dominant peak and a few very small ones.

• Target peptide — about 99% of detected peak area (the big peak)
• Impurities — about 1% of detected peak area (everything else)

That number is not the share of the vial's mass that is peptide. Water and counterions don't show up as peaks the same way, so a vial can read 99% pure by HPLC and still contain noticeably less than 99% peptide by weight. Net peptide content is a separate assay and is reported separately when it's tested.

Mass spec matters just as much

HPLC tells you how clean the sample is. Mass spectrometry tells you what it is, by measuring the molecule's mass and comparing it with the expected value. Purity without identity is only half the picture. A COA you can rely on reports both.

How to work through a COA

• Batch or lot number: it should match the vial you're holding.
• HPLC purity against the stated specification, for example ≥99%.
• Mass spec result against the expected molecular weight.
• Testing lab and test date: independent and recent is better.
• A clear pass or fail for each analyte.

Warning signs

• A purity figure with no chromatogram or mass spec data to support it.
• No batch number, so there's no way to link the report to your vial.
• A COA that doesn't name the lab that ran the test, or the date.

FAQ

What does ≥99% HPLC purity mean?

It means the target peptide's peak makes up at least 99% of the total peak area detected by high-performance liquid chromatography, with other peaks under 1%. It is not a percentage of the vial's mass; net peptide content is measured by a separate assay.

Why is mass spectrometry on a COA?

HPLC measures purity. Mass spec confirms identity by checking that the measured mass of the molecule matches its expected molecular weight. You need both.

How can I tell whether a COA belongs to my vial?

Compare the batch or lot number on the certificate with the one printed on the vial. If they don't match, the report is about something other than what you have.

Related guides

• What research peptides are