How to Read a Peptide Certificate of Analysis (COA)

A certificate of analysis (COA) is the lab report a manufacturer issues for a specific batch of a substance, stating what tests were run and what the results were. For peptides it is the closest thing a buyer has to proof of what is actually in the vial — the identity of the molecule and the purity of the sample. It exists because you cannot tell a correct peptide from a wrong, degraded, or under-filled one by looking at the powder. Reading a COA well is a core sourcing skill, and it sits alongside the larger question of where to get peptides safely in the first place. This guide walks the document line by line, then lists the red flags that should make you put your card away.

One framing matters before anything else. A COA characterizes a sample. It does not certify that a product is safe, sterile, or legal for human use, and a clean COA on a “research use only” vial does not change what that vial legally is. Regulatory specifications for drug substances treat identity, assay, and impurity limits as the backbone of a release test — but those are quality attributes of the material, not a clearance to inject it.^[1]

HPLC purity: what the percentage means

The headline number on most peptide COAs is a purity figure from high-performance liquid chromatography (HPLC), usually reverse-phase. HPLC separates the components of a sample as they pass through a column, and a detector records each as a peak. Purity is reported as area percent: the area under the main peptide peak divided by the total area of all peaks, times 100. A claim like “≥98%” means the target peptide accounts for at least 98% of the detected material, with the remaining ~2% being related impurities — truncated or deleted sequences, oxidation products, and other synthesis byproducts that specifications for drug substances are designed to bound.^[1]

Two caveats keep that number honest. First, area-% reflects only what the detector sees at the chosen wavelength under one method; an impurity that absorbs poorly can be undercounted. Second, purity is not the same as quantity — a sample can be 99% pure peptide and still contain far less active peptide than the label weight, which is what the net-content line is for. A credible COA shows the actual chromatogram, not just a bare percentage, so you can see the main peak and whatever else eluted.

Mass spectrometry: confirming identity

HPLC tells you how clean the sample is; it does not, on its own, prove the peak is the peptide you ordered. That is the job of mass spectrometry (MS). MS measures the molecular weight of the compound, and a correct result — an observed mass matching the calculated mass of the intended sequence — is strong evidence the molecule is what the label claims. Pairing a purity method with an orthogonal identity method is exactly how degradation products and mis-synthesized peptides get caught, and the same HPLC–MS pairing is what published analyses use to expose mislabeled gray-market product.^[2] A COA with a purity number but no MS identity result has skipped the question that matters most: is this even the right peptide?

Net peptide content versus gross weight

This is the line most buyers miss, and it is the one most likely to mean you are getting less than you paid for. Synthetic peptides are almost never shipped as pure, free peptide. They come as salts — most often trifluoroacetate (TFA) or acetate from the purification process — and they carry bound water and residual counterions. So a vial labeled “5 mg” describes the gross weight of the powder, of which the salt, water, and counterion are a real fraction. The honest figure is net peptide content (also called peptide content), the percentage of that gross weight that is actual peptide, typically established by amino acid analysis. Net content of 70–90% is common; a “5 mg” vial at 80% net content holds roughly 4 mg of peptide. A COA that reports gross weight but omits net peptide content is leaving out the number that determines what you actually receive.

Counterion, residual solvents, and water content

The counterion line names the salt form, and it is not a trivia field. TFA is a common residue from reverse-phase purification, and the counterion a peptide is paired with can change its pharmacokinetics and tolerability, which is why some manufacturers deliberately exchange TFA for a more benign salt such as acetate.^[3] A thorough COA also reports residual solvents (traces of the organic solvents used in synthesis and purification) and water content (often by Karl Fischer titration). High water content inflates the gross weight and shortens shelf life — one more reason that, once you have the powder, how you treat it matters as much as what the COA says; see our guide on how to reconstitute peptides.

Sterility and endotoxin: the gap for anything injected

Here is the testing most people assume is covered and usually is not. Identity and purity testing say nothing about whether a product is sterile or free of bacterial endotoxin — the two attributes that matter most for anything introduced by injection. Sterility and endotoxin (pyrogen) testing are separate, specialized assays, and the overwhelming majority of “research” peptide vials are not tested for either. A COA showing 99% HPLC purity and a confirmed mass tells you the chemistry is right; it does not tell you the contents are safe to put in your body. The published record on unregulated injectables is consistent: products bought outside the pharmacy system are repeatedly found mislabeled, under-dosed, or contaminated.^[4][2] A purity figure cannot fix that.

Red flags on a COA

• No batch or lot number. A COA describes one batch. Without a lot number that matches your vial, the document cannot be tied to what you received — it is decoration.
• No named third-party lab. An in-house number from the seller is a self-graded exam. Prefer a COA from a named, independent analytical lab; third-party testing is worth far more than “tested in-house.”
• A “COA” that is just a number. A purity percentage with no chromatogram and no mass-spec result is an assertion, not an analysis. Real COAs show the data.
• Identical COAs across batches. Genuine batches vary slightly. The same purity, masses, and trace values reproduced batch after batch suggests a template, not a measurement.
• Net peptide content missing. Gross weight without net content hides how much active peptide you are actually buying.
• No sterility/endotoxin data on something marketed for injection. Expected for most research vials — which is exactly why “research use only” injectables are a known risk, not a verified product.

The honest bottom line

A good COA is batch-specific, comes from a named third-party lab, and shows real data: an HPLC chromatogram with an area-% purity figure, a mass-spectrometry result confirming the molecular weight, a net peptide content number, and the counterion, residual-solvent, and water values. Those line items tell you the sample is the right molecule, how pure it is, and how much active peptide you are actually getting. What no COA can tell you is that a product is sterile, safe, or legal to inject — a COA proves identity and purity of a sample, nothing more. The most reliable way to know what you are putting in your body is to obtain a peptide through a route with real oversight; our editors’ roundup of vetted peptide providers ranks the prescription-based options, and our guide to where to get peptides safely explains why oversight, not a PDF, is the thing that actually protects you. This is general sourcing information, not medical or dosing advice.