How to read a peptide certificate of analysis (COA)

A certificate of analysis, or COA, is a laboratory's report on a sample it tested. For research peptides it usually covers two things, identity and purity, and sometimes a third, content. Reports vary a great deal in format and quality, from a one-line summary to several pages with spectra and chromatograms attached. This guide walks through a typical report section by section.

One thing before we start. However good the report, it describes the sample the laboratory received. Whether that sample has anything to do with a particular vial is a separate question, and we cover it in why an unmarked peptide vial means a certificate can describe anything.

1. Who did the testing

The report should name the laboratory, give an address, and be signed or authorized by a named person. Three things are worth checking.

Does the laboratory exist independently of the seller? It should have its own website, address and contact details that you can find without going through the seller. A laboratory that only seems to exist on the seller's documents is a reason for caution.

Is it accredited, and for what? Testing laboratories can be accredited to ISO/IEC 17025 by an accreditation body; in the United States, A2LA and ANAB are two of the main ones. Accreditation always has a scope, meaning a list of the specific tests it covers, and a laboratory accredited for one test is not thereby accredited for everything it runs. The scope documents are public. We go into this in what a laboratory's ISO 17025 accreditation actually covers.

Can the report be confirmed with the laboratory? Some laboratories offer a lookup by report number, or will confirm a report by email. When it's available, that is one of the strongest checks there is.

Not being accredited doesn't make a result wrong. A lot of good analytical work is done by unaccredited laboratories. What matters is that the status is stated accurately and you know what it covers.

2. What was tested, and when

Look for a sample description, a sample or batch identifier, the date received and the date tested.

The identifier. A batch or lot number on the report only helps if you can connect it to your product. If your vial carries no such number, the identifier can't be matched to anything you hold.

The date. A report from long before the product was sold may well describe a different batch.

The claimed amount. Many reports repeat the label claim, "10 mg" say. That is what the sample was said to be, not what was measured. It is surprisingly easy to read the one as the other.

3. Identity: is it the right compound?

Identity asks whether the compound is what it is supposed to be. For peptides this is usually done by mass spectrometry (MS), often coupled to liquid chromatography (LC-MS).

A useful identity result shows the theoretical mass of the declared compound, the observed mass the instrument found, and ideally the spectrum itself rather than just the word "conforms."

The observed and theoretical masses should agree closely. Peptides often appear at several charge states, so the spectrum may show peaks at a half or a third of the full mass. That is normal and not a cause for concern.

What identity by mass doesn't tell you: a matching mass confirms that the molecule weighs what it should. On its own it doesn't confirm the amino acid sequence, and it can't rule out a different molecule with the same mass. Techniques such as tandem mass spectrometry (MS/MS) go further, and a report that includes them is saying more.

4. Purity: how clean is it?

Purity is usually measured by HPLC (high-performance liquid chromatography) with a UV detector. The sample is separated into its components, which appear as peaks on a chart called a chromatogram. The reported purity is normally the area of the main peak as a percentage of the total area of all the peaks.

Look for three things.

The chromatogram. A purity figure with no chromatogram is hard to assess. With one, you can see for yourself whether there are other peaks and how large they are.

The method. Column, mobile phase, gradient and detection wavelength (often around 214 nm for peptides). Different methods can give different numbers for the same material, so a purity figure without a method is a number without a context.

How precise the number claims to be. A figure like 99.874% implies a precision the method rarely supports between laboratories. Treat the decimals with some skepticism.

Purity is a ratio, and it leaves out a great deal. Our piece on what a purity number does and doesn't tell you goes through what.

5. Content: how much is in the vial?

Content, sometimes called quantity or assay, asks how much of the compound is actually in the vial. It is reported as a mass, 9.6 mg for example. It is harder to measure than purity, because it needs either a characterized reference standard for that compound, which most research peptides don't have, or a technique such as amino acid analysis that doesn't need one. That is a large part of why it's reported less often. For a buyer it can matter more than purity does.

When a report includes content, check whether the figure was measured or just repeats the label; the method used; and the basis, meaning whether the number refers to the peptide alone or to the whole powder, which also contains water and salts.

6. What is not on the report

Absence matters as much as presence. A typical identity-and-purity report says nothing about sterility, endotoxin, heavy metals or residual solvents. If those tests weren't run, the report is silent on them, and a high purity figure is not evidence either way.

Watch, too, for marketing language that goes beyond the document. A product page that says a compound is "tested for sterility" when the certificates only show identity and purity is making a claim the paperwork doesn't support.

7. Pass, fail and "conforms"

Many reports end with a verdict: "pass," "conforms," "meets specification." That verdict is measured against a specification that someone chose, and the someone is often the seller. Look for what the specification actually was. A pass against "purity at least 95%" tells you the number was above 95%, which the number itself already told you.

For research compounds there is usually no established standard behind such a specification. The better practice, in our view, is to publish the measurement and its method and let the reader judge the number, rather than attach a verdict to it.

A short checklist

1. Can I find the laboratory without going through the seller?
2. Is its accreditation status stated, and does the scope cover this test?
3. Can the report be confirmed with the laboratory directly?
4. Is the test date recent relative to the product I have?
5. Does identity show observed and theoretical mass, ideally with a spectrum?
6. Does purity come with a chromatogram and a stated method?
7. Is content measured, and on what basis?
8. What was not tested?
9. Can I connect this sample to my vial at all?

The last question is the one most certificates can't answer. It is also the one that matters most.