How to Read a Peptide Certificate of Analysis (COA)

What a COA is — and what it is not

A Certificate of Analysis (COA) is a lab report. It documents what testing found in a specific batch of a specific product: how pure it was, whether its identity was confirmed, and — depending on the testing performed — what else was in the vial. For research peptides, where no regulatory body verifies label claims, the COA is the closest thing to a window into what you are actually holding.

It is worth being clear-eyed about the limits, though. A COA describes a sample from a batch at the time it was tested. It does not guarantee the vial in your hand matches that sample, it does not certify the product as safe for human use, and — because the vendor usually commissions it — it is only as trustworthy as the lab that produced it and the vendor's honesty in passing it along. A COA cannot make a grey-market product safe. What it can do is tell you whether a vendor is even measuring what they sell, and how carefully. That is genuinely useful information, if you know how to read it.

The header: the lot number is the whole point

Before reading a single test result, check that the COA is tied to a lot or batch number — and that the number matches the one printed on your vial.

This is the most common way COAs are misused. A vendor runs proper testing on one batch, gets a clean COA, and then displays that same COA on the product page indefinitely — long after that batch has sold out and been replaced by untested ones. The COA is real; it just has nothing to do with the vial you received. A COA with no lot number, or a lot number that does not match your vial, tells you about some batch, not yours. The header should also carry the product name, the test date, and the name of the testing lab.

HPLC purity: reading the chromatogram

High-performance liquid chromatography (HPLC) is the standard purity test. The sample is pushed through a column that separates its components by how strongly each interacts with the column material; a detector records what comes off and when. The output is a chromatogram — a plot with peaks.

The intended peptide produces one main peak. Impurities — truncated sequences, degradation products, leftover synthesis byproducts — produce additional peaks. Purity is reported as that main peak's area expressed as a percentage of the total area of all peaks. A research-peptide COA will typically claim something like 98-99% purity.

If the chromatogram itself is shown — and it should be — look at the shape, not just the number. A single, tall, symmetrical, sharply-resolved peak is what high purity looks like. Be skeptical of broad peaks, peaks with shoulders, or a cluster of small peaks near the main one. One detail many people miss: purity percentages depend on the detection wavelength (commonly 214 nm or 220 nm, which detect the peptide bond itself). A purity figure with no chromatogram and no stated wavelength is a number you cannot check.

Mass spectrometry: confirming it is the right peptide

HPLC tells you how pure the sample is. It does not, on its own, tell you the main peak is the peptide you ordered. That is what mass spectrometry (MS) is for.

MS measures molecular weight very precisely. Every peptide has a theoretical molecular weight set by its amino acid sequence. A COA should state the expected (theoretical) mass and the observed mass, and they should match within a small tolerance. If they match, the COA has confirmed identity — the sample really is a molecule of the right weight. If a COA reports a purity number but shows no MS data at all, it has told you the sample is pure something, without confirming what. For an unregulated product, that is a meaningful gap.

Net peptide content: the number nobody mentions

This is the single most under-discussed line on a COA, and the one most likely to surprise people.

When a vial is labeled "10 mg," that is usually the total mass of dry solid in the vial — and that solid is not 100% peptide. Synthetic peptides come out of purification carrying counterions (typically trifluoroacetate or acetate, left over from synthesis and purification), residual water absorbed from the air, and sometimes residual salts. Net peptide content — sometimes called peptide content or assay — is the percentage of that total mass that is actually peptide.

Net peptide content commonly runs somewhere around 70-90%. So a vial labeled "10 mg" might genuinely contain only 8 mg of peptide, with the remaining 2 mg being counterions and water. This is not necessarily dishonest — it is the normal chemistry of synthetic peptides — but it matters, because two vials both labeled "10 mg" can hold meaningfully different amounts of actual peptide. A COA that reports net peptide content is being more honest and more complete than one that reports only HPLC purity. The two are different measurements: purity asks "of the peptide present, what fraction is the right peptide," and net content asks "of the total solid, what fraction is peptide at all."

Counterions, water, and residual solvents

A thorough COA accounts for the non-peptide mass directly. Counterion content (trifluoroacetate or acetate) may be listed as its own line. Water content is measured by a method called Karl Fischer titration. Residual solvents from synthesis may be reported. None of these needs to be zero — they are expected. Their value is that, together with net peptide content, they make the COA add up: a complete COA accounts for essentially all the mass in the vial. A COA that reports 99% HPLC purity and nothing else has left most of that accounting invisible.

Endotoxin and sterility

Purity and identity testing say nothing about biological contamination. A peptide can be 99% pure by HPLC and still carry endotoxin — fragments of bacterial cell wall that survive even after the bacteria themselves are gone, and that the body reacts strongly to.

Endotoxin is measured by a test called the LAL (Limulus amebocyte lysate) assay; sterility testing checks for viable microorganisms. These tests are standard for anything manufactured for injection, and they are routinely absent from research-peptide COAs — which is one of the clearest signals of the gap between a research-use-only product and a pharmaceutical-grade one. Their absence is information in itself.

Red flags: a quick checklist

Pulling it together, treat these as warning signs on a COA:

• No lot number, or one that does not match your vial — the COA may describe a different batch entirely.
• No mass-spec data — purity reported without confirmed identity.
• A purity figure with no chromatogram — an unverifiable number.
• No net peptide content — the vial's true peptide mass is left unstated.
• No testing-lab name, or testing done only by the vendor — independent third-party testing is more credible than self-reported results.
• A COA that is years old, or visibly reused across multiple products.
• No endotoxin or sterility data — expected for research-grade material, but worth recognizing as a real limitation.

The honest read

A COA is a useful tool and a limited one. Used well, it lets you tell the difference between a vendor who tests carefully and documents it — lot-matched, third-party, with MS identity and net peptide content — and one who posts a single purity number with nothing behind it. That distinction is real and worth knowing.

What a COA cannot do is convert a research-use-only product into something verified safe for human use. It does not test the specific vial you received, and it does not address the regulatory and quality gaps that define the grey market. Read the COA for what it is: evidence about how seriously a vendor takes characterization — not a safety guarantee.