How to Read a Peptide Certificate of Analysis

July 31, 2026 ·4 min read · Last reviewed August 28, 2026

A Certificate of Analysis is the only objective evidence that a vial contains what its label claims. It is also a document most buyers glance at and file away. Knowing how to actually read one is the difference between verified material and a marketing claim.

What a COA is — and what it is not

A COA is a laboratory report for one specific batch. It is not a general statement about a supplier, and it does not transfer between lots. Two vials of the same compound from the same vendor can come from different syntheses with different impurity profiles. This is why a COA is only meaningful when its batch number matches the batch number on the vial you hold.

A COA also is not a safety assessment or an approval of any kind. It reports what the analysis measured, nothing more.

The header: batch, date, and who did the testing

Start at the top. You are looking for three things:

• Batch or lot number — must match the vial. If it does not, the document tells you nothing about your material.
• Analysis date — a report from three years ago describes the batch as it was then, not necessarily as it is now.
• Testing laboratory — is the analysis from an independent third party, or was it produced in-house by the seller? “Tested” with no named laboratory is a claim, not evidence.

Every batch we stock is analysed by Janoshik Analytical, an independent laboratory, and the report is issued in their name rather than ours.

The HPLC chromatogram: reading purity

High-performance liquid chromatography separates the components of a sample by how strongly they interact with a column. The output is a chromatogram — a trace with peaks, where each peak is a compound and the area under each peak is proportional to how much of it is present.

Purity is normally quoted as the main peak’s area as a percentage of total peak area. A figure of 99.2% means the target peptide accounts for 99.2% of the detected material.

What to look at beyond the headline number:

• Baseline quality. A clean, flat baseline suggests a well-run analysis. A noisy or drifting baseline makes small peaks unreliable.
• Peak shape. A sharp, symmetrical main peak is what you want. Broad or tailing peaks can indicate a poorly resolved mixture.
• Impurity peaks. A single 0.8% impurity is a different situation from eight peaks of 0.1% each, even though both give “99.2% purity”. In peptide synthesis, larger secondary peaks often correspond to deletion sequences — chains missing a residue — which are structurally similar and hard to separate.
• Where the trace was integrated. A percentage calculated over a narrow retention window can exclude impurities that elute outside it.

Mass spectrometry: confirming identity

HPLC tells you how much of the sample is one dominant compound. It does not tell you which compound. That is the job of mass spectrometry, which measures molecular mass.

On the report, compare the observed mass against the theoretical mass for the sequence. They should agree closely. A discrepancy of roughly 18 Da can indicate a hydration or dehydration difference; around 1 Da may point to a deamidation; a mismatch matching a residue mass suggests a deletion or substitution.

A COA with a purity figure but no mass confirmation is only half a document. High purity of the wrong molecule is still the wrong molecule.

Additional tests worth having

Depending on the intended application, a fuller report may include:

• Endotoxin testing — bacterial lipopolysaccharide contamination, relevant for any cell-based work, since endotoxin can confound results at very low concentrations.
• Heavy-metal screening — residues from synthesis reagents or catalysts.
• Water and acetate content — lyophilised peptides retain residual water and counter-ions, so the actual peptide mass in a vial can be lower than the gross weight. Peptide-content analysis quantifies this.

That last point is worth dwelling on. A vial labelled 5 mg refers to gross lyophilisate mass unless stated otherwise. Net peptide content is typically somewhat lower.

Warning signs

Treat these as reasons to ask questions:

• No batch number, or a batch number that does not match the vial.
• An image of a chromatogram with no axis labels, no retention times and no integration table.
• “In-house tested” with no laboratory named.
• The same COA reused across visibly different lots.
• Purity quoted to implausible precision, such as 100.0%.

Requesting a COA

A supplier should be able to produce the report for your specific batch on request. For any order placed with us, contact us with your order number and we will send the batch-matched Certificate of Analysis covering purity, identity, endotoxins and heavy metals. Further detail is in our FAQ, and the full catalog is here.

Our guide to judging the best research peptide supplier in Europe puts all of this into a single nine-point checklist.

All products and information referenced are for in-vitro research and laboratory use only. Nothing here is medical advice, and no therapeutic claim is made or implied.