How to Read a Peptide COA (Certificate of Analysis)

How to read a peptide COA: the short version

Knowing how to read a peptide COA means checking five things: the HPLC purity percentage, mass spectrometry identity, a matching batch or lot number, the name of an independent testing lab with a date, and endotoxin or sterility results. Everything else on the document is secondary.

A certificate of analysis is the closest thing you get to proof. But a COA is only as useful as your ability to read it, and plenty are decorative: a logo, a number, and nothing you can actually verify. This guide walks every line in plain English.

Quick disclosure: a couple of links below, including to Real Peptides, are affiliate links that support this work. They do not change what I tell you to check.

What a certificate of analysis actually is

A certificate of analysis is a batch-specific lab report stating what a peptide is and how pure it is. For research peptides it should report HPLC purity and mass spectrometry identity at minimum, tied to a lot number, and ideally produced by an independent laboratory rather than the seller's own bench.

Regulators specify the same analytical backbone for therapeutic peptides, built around chromatography and mass spectrometry (regulatory guidelines for peptide analysis). A COA applies those methods to one specific batch. The key word is batch: a COA describes the lot it was run on, not the brand in general, which is why the lot number matters so much.

Line one: HPLC purity

HPLC purity is the headline number. High-performance liquid chromatography separates the sample into its parts and reports the target peptide as a percentage of the whole. For most research peptides you want 98% or higher, shown with the actual chromatogram, not just a figure typed into a box.

Why purity is not a formality: in a 2024 analysis, online semaglutide measured between 7.7% and 14.37% pure against a claimed 99% on the label (multifactor quality analysis, 2024). The chromatogram is the evidence. A clean single dominant peak looks very different from a forest of smaller impurity peaks, and that picture is exactly what a bare percentage can hide.

Line two: mass spectrometry identity

Mass spectrometry confirms identity. It measures the molecular weight of what is in the vial and checks it against the known weight of the peptide you ordered. Purity tells you how much; mass spec tells you what. A high purity figure means nothing if the molecule itself is the wrong one.

The observed mass should match the expected mass of your peptide within a small tolerance. For difficult sequences, orthogonal methods such as nuclear magnetic resonance add a further layer of identity confirmation (NMR quality control of therapeutic peptides). For everyday verification, HPLC paired with mass spec is the standard.

Line three: the batch or lot number

The lot number ties the document to your vial. Because a COA only describes the batch it was run on, the lot number on the certificate must match the lot printed on your vial. If they do not match, the COA is for a different batch and tells you nothing about what you are actually holding.

This is the most common gap of all: a genuine-looking COA for one lot, attached to a vial from another. Always cross-check the two. Some suppliers print a lookup code so you can pull the report straight from the testing lab rather than taking the seller's copy on faith.

Line four: the testing lab and date

The lab name and test date tell you who stands behind the result and how fresh it is. A credible COA names the laboratory that ran it, ideally an independent third party, and carries a recent date. An undated report, or one with no lab named at all, is not verifiable and should be read as marketing.

An old date is not automatically disqualifying, but it should prompt a question about storage and stability since the test was run. No date and no lab name together are a hard stop.

Line five: endotoxin and sterility

Endotoxin and sterility results matter for anything that will go into solution. A peptide can be chemically pure and still carry bacterial endotoxin from contaminated production, which can cause fever and serious reactions. A thorough COA includes an endotoxin result, because contamination is a real risk, not a theoretical one.

Bacterial endotoxin can survive even when a product is otherwise sterile, which is why it is tested separately (FDA, bacterial endotoxins and pyrogens), and microbial contamination of injectable products is well documented (microbiological contamination of medicinal products). Purity and safety are two different questions.

Third-party versus in-house: who ran the test

Who ran the test changes how much the COA is worth. An in-house COA from the seller can be accurate, but the seller has an obvious incentive to look good. An independent third-party lab has none, which is why a third-party COA, or a test you commission yourself, is the gold standard for trusting a result.

If the only COA on offer is the seller's own, treat the purity number as a claim rather than a fact. The way to settle it for good is independent testing, which is its own short guide.

Red flags on a fake or useless COA

A few COA red flags are decisive on their own. No chromatogram image, no named lab, a lot number that does not match your vial, a purity figure with no method shown, or one generic COA reused for every product all mean the document cannot be trusted, however official it looks.

• No chromatogram, just a typed purity percentage.
• No named testing lab and no date.
• The lot number on the COA does not match the vial.
• One identical COA reused across different products or batches.
• An image that looks edited or pixelated around the numbers.
• No endotoxin or sterility line for an injectable-format product.

For my own research I use Real Peptides, because they publish a batch-specific COA from an independent lab rather than a generic template. Reading the COA is one link in a longer chain; see the full peptide verification checklist and the wider context in the grey market problem.

References

1. Regulatory guidelines for the analysis of therapeutic peptides. PMC11806371
2. Multifactor quality and safety analysis of semaglutide products sold online, 2024. PMC11582493
3. Quality control of therapeutic peptides by NMR. PMC6452441
4. Microbiological contamination of medicinal products. PMC12300887
5. FDA: bacterial endotoxins and pyrogens, inspection technical guide. FDA

This content is for educational purposes only. These compounds are intended for research use. Nothing here is medical advice.