How to Read a Certificate of Analysis for Research Peptides

A certificate of analysis, usually shortened to COA, is the main piece of evidence a supplier can give you about what is in a vial. It is also easy to skim past. A purity figure in large type can look reassuring while the details that give it meaning are missing.

This guide walks through a typical peptide COA from top to bottom: the header, the test table, the three results that matter most, and the lab details that let you check the document. It ends with what a COA can't tell you and a short checklist you can use on any supplier's paperwork.

What a COA is, and the parts to check first

A COA is a summary report. It lists the tests run on one production batch, the method behind each test, the limit the batch had to meet and the result it actually got. In regulated pharmaceutical manufacturing, a COA is issued for each batch, gives numerical results, and is dated and signed by someone in the quality unit. Research suppliers aren't held to that standard everywhere, but it is a sensible benchmark.

A COA is not the raw data. The chromatogram and mass spectrum behind the numbers are separate pages, and a careful supplier attaches them. The example below marks the parts worth checking first.

Certificate of analysisExample only

Product	Example peptide, 10 mg vial
Batch / lot	EX-2611-04	1
Test date	14 Oct 2026
Method	HPLC (UV 214 nm), ESI-MS	2
Purity (HPLC)	99.2%	3
Identity (MS)	Observed mass matches expected	4
Content	10.3 mg per vial
Laboratory	Independent lab name and address	5
Verification	Report number you can check with the lab	6

The header: product, batch number and dates

The top of the certificate identifies what was tested. Expect to see:

• Product name and stated amount, for example “Example peptide, 10 mg vial”.
• Sequence or molecular formula, and the theoretical molecular weight that the identity result is compared against.
• Batch or lot number, such as EX-2611-04.
• Dates: manufacture, testing, and a retest or expiry date.

The batch number is the single most useful field. It should match the label on the vial you receive, character for character. A COA with no batch number, or with one that never changes from order to order, describes a product line rather than your material.

Check that the dates make sense. Testing should come after manufacture, and a certificate dated years before the batch was first offered deserves a question.

The test table: method, specification and result

The body of most COAs is a table with four or five columns. A single row might read:

Purity | HPLC, UV 214 nm | ≥98.0% | 99.2% | Conforms

Read it left to right. The test names what was measured. The method says how. The specification is the acceptance limit the batch had to meet, set before testing. The result is what this batch actually measured, and the last column says whether it passed.

Look for numbers in the result column. A row that says only “Pass” tells you the batch met a limit, but not by how much. Also check that the specification is shown: a 99.2% result means more when you know the limit was 98.0%.

Other common rows include appearance (for example, “white lyophilized powder”), water content by Karl Fischer titration, and counter-ion content such as acetate or trifluoroacetate (TFA). Each row covers only what it says. A test that isn't listed wasn't reported.

Purity, identity and content answer different questions

Three results do most of the work on a peptide COA, and they are easy to confuse.

• Purity comes from HPLC. It is the main peak's share of the total peak area on the chromatogram, so it tells you how much of what the detector saw was the main compound.
• Identity usually comes from mass spectrometry. The line should show an observed mass next to the expected mass, within a stated tolerance, for example “expected 1,532.8 Da; observed 1,532.9 Da”.
• Content tells you how much peptide is actually present, either as a net peptide content percentage or as a measured amount, such as “10.3 mg per vial”.

None of these substitutes for another. A high purity figure with no identity result could describe a very clean sample of the wrong molecule. A correct mass with no purity result says nothing about what else is in the vial. Neither tells you quantity, because purity is a ratio, not a weight. The guides to HPLC and to purity percentages cover each of these in more depth.

The lab, signatures and report number

The bottom of the COA should tell you who stands behind it. Look for:

• The laboratory's name and address. If an outside lab did the testing, it should be named, not described only as “a third-party lab”.
• A report or certificate number unique to this document, which the lab can look up.
• A signature or approval with a name, role and date.
• Page numbering such as “page 1 of 3”, so you can tell whether attachments are missing.

When a reseller passes along a manufacturer's certificate, good practice is for the document to identify the lab that actually ran the tests and the original batch it refers to. A COA on the seller's letterhead with no lab named leaves you unable to confirm anything.

The report number is what makes checking possible. Many labs will confirm whether a report number exists and matches the sample description, though policies vary. Verifying a COA and spotting a fake are covered in a separate guide.

What a COA can't tell you

Even a complete, genuine certificate has limits.

• It describes the sample the lab received. It can't prove your vial came from the same batch, which is why the batch match matters.
• It covers only the tests listed. If residual solvents, heavy metals, endotoxin or sterility aren't on the report, the report says nothing about them.
• Purity depends on the method. A different column or solvent gradient can separate an impurity that one method hides under the main peak.
• It reflects the material on the test date. It can't account for how the vial was stored or shipped afterward.

A COA is evidence, not a guarantee. Its value depends on whether it is batch-specific, complete and verifiable.

A quick checklist for any COA

1. The batch number matches your vial label exactly.
2. The test date is recent and falls after the manufacture date.
3. Each test shows a method, a specification and a numerical result.
4. Purity comes from HPLC, with the wavelength stated and a chromatogram attached.
5. Identity comes from mass spectrometry, with observed and expected mass and a spectrum attached.
6. Content is reported, and it is clear whether it means gross powder weight, net peptide or a measured amount.
7. The lab is named, with an address and a report number you can check.
8. The document is signed or approved and dated, with no pages missing.

If several of these are missing, the purity figure at the top carries much less weight. Ask the supplier for the full report before relying on it.