A certificate of analysis (COA) is the document that turns a label into evidence. Without it, a vial is a claim. With it, a researcher can verify identity, purity, and lot traceability before a single experiment begins. This article explains the structure of a credible COA and the checks that separate real verification from cosmetic paperwork.

What is a certificate of analysis?

A certificate of analysis is a formal report issued by an analytical laboratory that describes the composition and purity of a specific lot of material. For research peptides, the COA documents that a specific batch matches its label across a defined set of tests, most commonly identity (mass spectrometry), purity (HPLC), and where relevant peptide content, water content, and residual solvents.

A COA is lot-specific. It is not a marketing document. Two different lots of the same peptide require two different COAs, and a COA without an explicit lot number tied to the vial in hand has limited evidentiary value.

Reading the HPLC report

The HPLC section presents a chromatogram and a purity percentage. The chromatogram should clearly show a dominant main peak, a labeled retention time, and any minor peaks integrated and reported. The purity value is calculated as the area of the main peak divided by the total integrated area, typically detected at 214 nm or 220 nm where the peptide bond absorbs strongly.

Look for the method conditions: column chemistry, gradient, flow rate, detection wavelength, and injection volume. A COA that prints a number without the conditions that produced it has skipped the step that makes the number reproducible.

Reading the mass spectrometry report

The MS section confirms identity. It should report the observed molecular ion, the theoretical mass calculated from the amino acid sequence, and the mass error in parts-per-million or daltons. For most research peptides, the observed mass should match theoretical within a small fraction of a dalton.

If the report shows the right purity but the wrong mass, the sample is pure — of the wrong molecule. Identity and purity are independent properties and both must pass.

Interpreting purity percentages

A purity percentage describes the fraction of the integrated chromatogram attributable to the target peptide. A value of 99% means the remaining 1% consists of related substances, truncations, deletions, or oxidation products — not contaminants in the toxicological sense, but anything that is not the target sequence.

Higher is not always meaningfully better. A well-characterized 98% peptide with a clean impurity profile is often more useful than a marketing claim of 99.9% with no chromatogram attached.

Identity verification

Identity verification answers the question: is this the molecule on the label? Mass spectrometry is the primary tool. For complex sequences, peptide mapping or amino acid analysis may supplement the basic mass check.

When a COA presents only a purity number without an identity confirmation, the researcher has no way to distinguish the target from a near-isobaric impostor. Identity always comes first.

Batch verification

A COA must reference a specific lot number that appears on the vial itself. Cross-check the lot on the certificate against the lot printed on the container before any experiment begins. A mismatch is not a clerical detail — it is a chain-of-custody failure.

Reputable suppliers also retain a sample of each lot for re-analysis on request. Ask. The willingness to re-test is a meaningful signal of confidence.

Red flags researchers should look for

A handful of patterns reliably signal a problem: a COA with no lot number, no method conditions, no chromatogram, no mass spectrum, or a single document reused across multiple batches. A COA issued by the supplier with no third-party verification path is not invalid by definition, but it places the entire burden of trust on the supplier’s internal practices.

Logos, signatures, and dates can be fabricated. The substantive proof lives in the chromatogram and the mass spectrum themselves — the data, not the letterhead.

How to compare suppliers

When comparing suppliers, ask for the most recent COA for the lot you would actually receive. Compare the depth of reporting, the clarity of method conditions, and whether identity and purity are both addressed. Reputable vendors publish a COA library that lets researchers verify documentation before placing an order.

See our piece on choosing a supplier for a fuller checklist, and the COA Library for an example of what credible documentation looks like.