How to read a certificate of analysis (COA) for research peptides

A certificate of analysis (COA) is a laboratory’s report on a sample from one batch of material. For research peptides it is usually the most useful document a supplier can give you. It is also the most misread one. This guide walks through a typical peptide COA field by field, explains what the two core methods measure, and lists the limits that no certificate can overcome.

The parts of a peptide COA

Purity: what HPLC measures

High-performance liquid chromatography (HPLC) separates the components of a sample as they pass through a column. A detector draws them as peaks over time. For a synthetic peptide, the main peak is the target sequence and smaller peaks are related impurities, such as truncated or modified sequences. “Purity” is normally the main peak’s share of total peak area at the detection wavelength.

Two consequences matter. First, purity is a ratio. A sample can be 99% pure and still contain less material than the label says. Second, HPLC purity only covers what the detector sees. Water, salts and counter-ions such as acetate or trifluoroacetate do not show up in that percentage. The literature on peptide impurities and counter-ions explains why the “peptide content” of a powder is always below 100%.

Identity: what mass spectrometry adds

Mass spectrometry (MS), often electrospray ionisation (ESI-MS) for peptides, measures the mass of the molecules in the sample. If the observed mass matches the expected mass of the sequence, that is evidence the main component is the right molecule. A purity figure without an identity result tells you the sample is mostly one thing, but not which thing.

Content is not purity

Many peptide certificates also report quantity, such as “10.33 mg” for a 10 mg vial. That figure answers a different question: how much material was in the tested vial. It can be above or below the label. In the partner certificates we reviewed for this guide, a 10 mg BPC-157 vial measured 9.72 mg, and two 100 mg GHK-Cu vials from different batches measured 86.10 mg and 109.58 mg. Blends are often reported only by content for each component, because one purity figure cannot describe several peptides.

What a COA cannot show

• Other batches. A certificate covers the sample tested, from that batch.
• Handling after testing. Storage, shipping and time all happen after the report was issued.
• Tests that were not run. Endotoxin, sterility and heavy-metal results appear only if the laboratory ran those tests. Some supplier libraries in our directory include them; many do not.
• Suitability for any use. A COA is an analytical report, not an approval.

A note on accreditation

ISO/IEC 17025 is the international standard for the competence of testing laboratories. Accreditation applies to specific methods at a specific laboratory. If a supplier says its laboratory is “accredited”, ask for which tests, and check with the accreditation body.

Next step: verify the report

Reading a certificate correctly is half the job. The other half is confirming that it is genuine, which our guide on verifying third-party testing claims covers.