How do you read a CoA?

The market for research peptides has grown rapidly in recent years, and with it the number of suppliers. The quality of those suppliers varies widely. Almost every vendor now claims “third-party tested” or “≥ 99% pure”, but such claims are only worth as much as the document backing them up. This article explains what a Certificate of Analysis (CoA) actually contains, what a purity percentage does and does not tell you, and which signals point to a weak or unreliable CoA.

What does a complete CoA contain?

A proper CoA for a research peptide generally contains at least the following elements:

• A unique batch or lot number traceable to the specific product you received
• The test date and the name of the laboratory that performed the analysis
• An HPLC chromatogram (High-Performance Liquid Chromatography) with the reported purity percentage
• A mass spectrometry (MS) result confirming the identity of the molecule
• Appearance/identity checks (colour, form, solubility) and sometimes water content or residual solvents

If either of the first two items is missing — a linked batch number and the name of the testing laboratory — the document cannot be verified in practice and is therefore worth little, however professional it looks.

What an HPLC percentage does tell you

HPLC separates the components of a sample based on their chemical properties and typically reports a “peak area percentage”: the share of the total detected signal area attributed to the main peak (the target peptide). A value of ≥ 99% means impurities — incomplete sequences, degradation products, residual solvents — make up a negligible portion of the detected signal.

That is a real and useful indicator, but not an absolute guarantee of mass purity: peak area normalisation assumes every substance present responds equally strongly at the wavelength used, which is not always the case in practice. This is why a purity figure without a chromatogram — that is, without the underlying measurement itself — is less reliable than a figure accompanied by one.

Why purity alone is not enough

An HPLC report shows how pure a sample is, but not necessarily what exactly is in it. A sample can be 99% “pure” and still contain the wrong molecule, or a truncated variant of the intended sequence — if it responds as strongly as the real peptide, it will not appear as a separate peak.

Mass spectrometry (MS) measures the exact molecular weight and thereby confirms the identity of the substance. A complete CoA therefore always combines HPLC (purity) with MS (identity) — the two measurements answer different questions and do not substitute for one another.

Red flags to watch for

• A CoA without a batch or lot number, or where the number on the document does not match the label on the packaging
• No mention of the laboratory that performed the test, or a laboratory that cannot be verified
• Only a final percentage, with no chromatogram or MS spectrum as supporting evidence
• The same document (or the same test report with only the date changed) reused across multiple different batches
• In-house testing only, without any independent verification

Independent versus in-house testing

In-house testing — where the supplier uses its own equipment to test its own product — is better than no testing at all, but provides no independent confirmation. An independent external laboratory with no commercial stake in the outcome adds a layer of scrutiny that in-house testing cannot offer. When comparing suppliers, the distinction between “tested” and “independently tested” is often the single most decisive difference in documentation quality.

Questions worth asking a supplier

• Can I get the CoA for the specific batch I receive, linked to the lot number on the packaging?
• Which laboratory performed the analysis, and is that laboratory independent of the supplier?
• Does the CoA include both an HPLC chromatogram and an MS result, or only a final percentage?
• Is every batch tested, or only a sample from the production line?