A Certificate of Analysis is the document that converts instrument output into a structured claim about one specific batch of material. It is the only artefact connecting the vial in your hand to a measurement someone actually made. Everything else a supplier publishes — purity guarantees, “lab tested” badges, star ratings — ultimately points back to this page, which is why reading one properly is the most useful skill a research buyer can develop.
The four questions a complete certificate answers
Every credible COA addresses four separate questions, and it is worth keeping them distinct because they are measured by different instruments and can fail independently.
Identity. Is this molecule what the label claims? Answered by mass spectrometry, which compares the observed mass against the theoretical mass calculated from the stated sequence.
Purity. What fraction of the peptide present is the target compound rather than related impurities? Answered by reversed-phase HPLC, reported as area percent.
Content. How much actual peptide is in the vial, as opposed to counterion, residual water and solvent? Answered by a net peptide content assay — and frequently omitted.
Safety attributes. Endotoxin load, sterility, elemental impurities, residual solvents. Answered by four more techniques, none of which are chromatographic.
A certificate that answers only the first two is not wrong, but it is incomplete, and the gap is where most quality problems live.
Reading the header
Start at the top. A usable certificate carries a batch or lot identifier that matches the printing on the vial label, a manufacture or release date, the stated quantity per vial, and the name of the laboratory that performed the testing. If the lot number on the document does not match the lot number on the vial, the document describes different material and tells you nothing about what you have.
Watch the date. Certificates are snapshots at release. A document issued three years ago describes material as it was at release, not as it is after three years in a freezer — or, worse, three years in a warehouse.
Reading the purity section
The purity line is usually reported as something like “99.4% (area %, 220 nm)”. Three details matter.
The wavelength tells you what the detector was watching. Peptide bonds absorb around 210-220 nm, so detection in that range sees the backbone of essentially everything present. Detection at 280 nm only sees aromatic residues — tryptophan, tyrosine, phenylalanine — and will simply miss impurities that lack them.
Area percent means the target peak’s area divided by the total area of all peaks. It is a ratio between things the detector saw. Anything the detector did not see — inorganic salts, water, most counterions — is not in the denominator and does not reduce the number.
The chromatogram itself should be attached. A purity figure without the trace behind it asks you to accept a number with no way to check the integration, the baseline, or whether a shoulder was resolved or quietly folded into the main peak.
Reading the identity section
Mass spectrometry results should state both the theoretical average or monoisotopic mass and the observed mass. Agreement within a few tenths of a dalton for a small peptide is routine. What you are checking for is not just closeness but the absence of surprises: a mass 18 units low suggests dehydration, 16 units high suggests oxidation, and a mass short by a whole residue suggests a deletion sequence that co-eluted closely enough to escape the purity calculation.
What incomplete certificates leave out
Three omissions are common enough to be worth naming.
A purity number with no chromatogram. A certificate with no lot number, or a lot number that appears on every product. Identity confirmed by “HPLC retention time” alone — retention time is consistent with an identity, but it does not establish one.
Net peptide content is the most consequential omission. A vial labelled 10 mg containing 99% pure peptide can hold appreciably less than 10 mg of the peptide itself once trifluoroacetate counterion and adsorbed water are subtracted. If your working concentrations are derived from the label rather than from a content assay, that error propagates through every calculation downstream.
Using the certificate
Treat the COA as a record to keep, not a page to glance at. Store the PDF alongside the lot number in your inventory, and reference it in the methods section when the material is used. Reproducibility depends on being able to say precisely which batch produced which result — and a year later, the certificate is the only thing that still remembers.