How to Read a Peptide COA (Certificate of Analysis)

Every vendor worth buying from will hand you a COA. Most people look at the purity number, see 99%, and close the file. That number is close to the least useful thing on the page.

Here is what the document contains, what each test can and cannot establish, and what to check before you trust it.

What is a COA?

A Certificate of Analysis is a lab report on one batch of one compound. It says: we tested this material, here is how pure it was, here is evidence it is the molecule it claims to be.

The words doing the work there are "one batch." A COA does not certify the vendor, the product line, or the compound in general. It covers a single production lot. Most of the checks below follow from that.

What does the purity number mean?

Purity comes from HPLC, high performance liquid chromatography. The sample gets pushed through a column that separates it into its components, and a detector records what comes out and when. The output is a chromatogram, a line with peaks on it, each peak a different substance in the sample.

The percentage is the area of the main peak divided by the area of all peaks combined. A result of 99.1% means the main component accounted for 99.1% of what the detector saw.

So HPLC tells you the sample is mostly one thing. It cannot tell you what that thing is. A vial can test at 99% and be the wrong peptide, purely and consistently. Identity is a separate question, answered by a separate test.

What purity is good enough?

Most vendors claim 98% or higher and that is what to expect. Above 99% is common for short, simple peptides. Long or structurally complex ones can come in at 95% without anything being wrong.

Below 95%, ask what the impurities are. Below 90%, walk away.

The decimal places matter less than people think. A purity figure depends on the column, the gradient, and the detection wavelength, and two competent labs testing identical material can report results a couple of percentage points apart. This is why a decent COA states the method, something like "98.6% by HPLC at 220 nm," instead of a bare number.

A 99.6% claim on a document you cannot verify is worth less than a 98.1% result from a named lab on a lot number that matches your vial.

Does the lot number match your vial?

Find the batch or lot number on the COA. Look at the label on your actual vial. If they do not match, the report describes material you do not have.

If your vial has no lot number at all, nothing can be tied to it, and the COA is decoration.

Check the test date while you are there. A report dated two years ago, attached to something you just bought, means the vendor is reusing it across new batches.

What does mass spectrometry prove?

Mass spectrometry measures molecular weight. Every peptide has a fixed weight set by its amino acid sequence, so a measured mass that matches the expected mass is strong evidence you have the right molecule.

On a COA this shows up as two numbers, theoretical or expected mass and observed or found mass. They should sit within roughly one unit of each other for most peptides. A wider gap means something else is in the vial.

Two limits worth knowing. Peptides built from the same amino acids in a different order weigh exactly the same, so a matching mass supports identity without proving it. Confirming the actual sequence takes MS/MS or peptide sequencing, which consumer-facing COAs almost never include. Mass spec also says nothing about how much of the vial is that molecule, which is what the purity test is for.

Between them the two tests cover the two questions. Mass spec, very probably the right compound. HPLC, most of what is in the vial is that compound.

Who ran the test?

A vendor testing its own product and an independent lab testing it are different things.

An in-house COA still shows the vendor has quality control at all, which many do not. The incentive problem is obvious enough that it should never be the only document you have.

A third-party COA should name the lab clearly, with contact details or a website, and usually a report number. Some labs let you enter that number on their own site and pull up the original. Do it when the option exists. Most COA fraud is not fabricated from nothing, it is a real report from a real lab with the fields edited, and checking against the source kills it.

An unnamed lab, a generic letterhead, or a free email address as the only contact means you are holding a vendor claim rather than independent evidence.

How can you tell if a COA has been edited?

Open the file properties. Right-click, then Properties or Get Info, then the document details.

You are looking at two things: what software created the file, and when. Lab reports come out of lab or reporting software. If the creating application is an image editor or a word processor, someone built that document by hand. If the creation date falls weeks after the stated test date, ask about it.

Try to highlight the text as well. A COA that is one flat image with nothing selectable has usually been screenshotted and re-saved, which is what you would do to hide an edit.

How do you read the chromatogram?

A stated percentage with no chromatogram is a claim. Ask for the version with the trace on it.

Look for one dominant peak with a flat baseline around it. That is a clean sample.

Small peaks clustered right beside the main one are usually related impurities, sequences that came out of synthesis slightly wrong, missing a residue or carrying an extra fragment. A few tiny ones are normal. A crowd of them, or one that is a real fraction of the main peak, points to sloppy synthesis or poor purification.

A large peak at the very start of the run is the solvent front. Normal, and labs generally exclude it from the calculation.

Somewhere on the page there should be a detection wavelength, usually 214 or 220 nanometres. Its absence tells you how much care went into the report.

What is net peptide content?

Purity is a percentage of the peptide-related material the detector saw. It is not a percentage of the powder in your vial.

Synthesised peptides carry counterions from purification, usually trifluoroacetate or acetate, plus some residual water. That is real weight in the vial that is not peptide. Net peptide content is the figure that accounts for it, measured separately by amino acid analysis or nitrogen determination.

Most COAs in this space do not report it. Without it you cannot tell whether the milligram number on the label is gross weight or actual peptide, which is one of the quieter reasons two vials of the same nominal size can behave differently. Ask a vendor about it directly. Whether they understand the question is informative on its own.

What a COA does not cover

Sterility and bacterial endotoxin are separate analyses that standard peptide COAs leave out. Labs do offer them, so a vendor paying for endotoxin testing is telling you something real about how seriously they take this.

Fill accuracy is separate too. The batch can be exactly what the report says and your particular vial still be short.

Handling is outside it entirely. Material that tested at 99% can be degraded by heat in transit or by months in a warm warehouse.

Then there is the join between the report and the vial in your hand, which rests on the vendor's word and the lot number matching.

What are the red flags on a fake COA?

Any one of these is reason to stop and ask questions.

• No lot number, or a lot number that does not appear on your vial.
• A purity figure with no chromatogram behind it.
• The lab is not named, or cannot be found.
• A stated purity of exactly 100.0%. Real analytical results essentially never land there.
• No method or wavelength given alongside the purity figure.
• The same chromatogram appearing under different compound names on one vendor's site.
• Mismatched fonts, misaligned text, or fields pasted into an otherwise clean document.
• A PDF created in an image editor or word processor, or a flat image with no selectable text.
• A test date that predates the batch, or no date at all.
• A compound name that does not exactly match what you ordered.

The first one on that list does most of the work. A vendor who will not give you a report tied to your lot number has already told you what you needed to know.

What your own vial should look like

Since the COA stops at the lab door, your eyes handle the rest.

A properly freeze-dried peptide is a white to off-white cake or powder. It might be a solid disc at the bottom, it might be loose powder, it might have broken up in transit. All normal.

Yellow or brown tint is not. Neither is an oily or wet look, a cake collapsed into a sticky smear, a loose stopper, or a damaged seal. Those point to moisture getting in or the vial getting warm, and no certificate covers what happened in shipping. If a mixed solution comes out cloudy or has anything floating in it, do not use it.

Can you test it yourself?

You can, and it is the answer if you finish this and conclude you cannot fully trust a document handed to you by someone selling you something.

Independent labs will test a sample you send. You buy the vial, ship a portion, and get a report requested in your name rather than the vendor's. Pricing varies enormously, from labs that specialise in serving this community to pharmaceutical analytical services charging several times more for the same core tests, so get a current quote before assuming either. Group testing, where several people who bought the same batch split the cost, is common and brings the per-person figure down.

Nobody does this for every vial. Doing it once, on the first order from a new vendor, tells you more about the rest of their catalogue than any COA they send you.

Working through one

Ask for the COA matching your lot number rather than the one on the product page. Check the lot, the date, the lab, and the file properties. Compare observed mass against expected. Look at the chromatogram if there is one.

A COA describes what a sample of a batch looked like on one day in one lab. The rest of the picture is the vendor, which is the next thing to work through.

Common questions