How to read a certificate of analysis

A certificate of analysis is the only document that ties a specific batch of material to a specific set of measurements. Everything else on a product page describes a compound in the abstract. The COA describes the vial in front of you.

It is also the document most often skimmed. Below is what each line actually means, and which lines are worth arguing about.

The lot number is the anchor

Every figure on a COA is scoped to one lot. A purity number without a lot number attached is a marketing claim, not a measurement. If the certificate quotes a catalogue-wide figure rather than a batch-specific one, it is describing a product family, and there is no way to check whether the material you received belongs to the run that was tested.

The lot number should appear in three places that agree with each other: etched or printed on the vial, on the certificate, and in the supplier's archive when you ask for it later. If those three diverge, nothing downstream is traceable.

Purity, and what the number is measuring

Purity is almost always reported as area percent by reversed-phase HPLC. The sample is separated on a C18 column, a detector watches the eluent at a fixed wavelength, and the main peak's area is expressed as a share of total integrated area.

Two consequences follow, and both are routinely missed:

• Area percent is relative, not absolute. It tells you how much of what the detector saw was the main peak. It says nothing about material the detector did not see.
• Detection is wavelength-dependent. At 214 nm the detector responds to the peptide bond, which is why that wavelength is conventional for peptides. Species without strong absorbance there, counterions and residual salts among them, contribute little or nothing to the integrated area.

This is why a 99% area-percent figure and a 99% mass figure are different claims. A lyophilised powder can be 99% pure by HPLC area and still be a substantial fraction salt and water by weight.

Peptide content is the other number

Peptide content, sometimes called net peptide content, is the share of the dry mass that is actually peptide rather than counterion, residual solvent or bound water. It is determined by amino acid analysis or nitrogen determination, not by HPLC.

A rigorous certificate reports both. Purity tells you how clean the peptide fraction is; peptide content tells you how much of the powder that fraction represents.

Identity: mass, not retention time

Retention time alone does not establish identity. Two different species can co-elute under one method, and retention shifts with column age, temperature and mobile phase batch.

Identity confirmation comes from mass spectrometry. The certificate should list the theoretical monoisotopic or average mass, the observed mass, and the deviation between them, typically in parts per million or as an absolute mass difference. A deviation quoted without the theoretical value it was measured against is not checkable.

The chromatogram matters more than the summary

A COA that reports "≥ 99%" and nothing else has thrown away the evidence. The raw trace shows things the summary line cannot:

• Whether the main peak is symmetric or tailing, which speaks to column condition and sample behaviour
• Whether the impurity profile is one close-eluting shoulder or a scatter of small unrelated peaks
• Whether the baseline was integrated sensibly, or whether the integration window was drawn to flatter the result

Ask for the trace. A supplier who archives raw data can produce it; one who cannot has only ever had the summary.

Independent testing and what it is worth

A certificate produced by the manufacturer is a self-report. That is not automatically a problem, but it is a different category of evidence from a third-party result, where a sample of the same lot has been sent to a laboratory with no interest in the outcome.

Where both exist, they should be close. Small divergence between an in-house and an independent figure is normal, because methods differ. Large divergence, or a supplier who will only ever show one of the two, is worth a question.

A short checklist

None of this is exotic. It is the ordinary paperwork of analytical chemistry. The reason it is worth reading closely is that a certificate is the only part of a purchase that can be checked after the fact.