How to read a peptide certificate of analysis

The single most common mistake is treating a certificate as a property of the product name. It is not. A CoA describes one synthesis lot, analysed on one date, by one laboratory. A second lot of the same compound is a different material with a different certificate, and the two can differ measurably in purity, water content and impurity profile.

This is why the lot number is the first thing to check, and why it has to match the vial rather than the listing. A certificate that cannot be tied to the physical container in front of you is decorative. Before anything else, confirm that the lot on the vial label, the lot on the certificate, and the lot recorded on your inbound paperwork are the same string.

Date matters for the same reason. An assay performed at release tells you about the material as it left the analytical lab. It does not describe material that has since spent an unknown period at an unknown temperature. The certificate is a starting point for the material's history, not a permanent guarantee.

Purity on a peptide CoA almost always means chromatographic purity by reversed-phase HPLC with UV detection, usually at 214 nm because that wavelength responds to the peptide bond itself rather than to aromatic side chains. The number is an area percentage: the area of the main peak divided by the total integrated peak area.

That definition carries a consequence worth internalising. The figure is relative to what the method can see. Anything that does not absorb at the detection wavelength, does not elute under the gradient used, or co-elutes underneath the main peak is not counted. Residual solvent, inorganic salt, and water are all invisible to this measurement. A material can be 99% pure by HPLC and still be a substantial fraction non-peptide by mass.

So read purity as a statement about peptide-related impurities — deletion sequences, truncations, oxidised or deamidated variants, incompletely deprotected species — and not as a statement about how much of the powder is the compound. The second question is answered elsewhere on the certificate, and often is not answered at all.

Purity tells you the main peak is one thing. It does not tell you that thing is the compound you ordered. Mass spectrometry closes that gap by measuring the molecular weight of the material in the main peak and comparing it to the theoretical mass calculated from the sequence.

On a certificate you will typically see a theoretical or expected mass and an observed mass. For peptides, the observed value is often a multiply-charged ion — [M+2H]²⁺ or [M+3H]³⁺ — because electrospray ionisation tends to put several protons on a molecule of that size. A certificate showing an observed mass at roughly half or a third of the expected value is usually reporting a charge state, not a fragment, though the charge state should be stated rather than left for you to infer.

Agreement within a few tenths of a mass unit for a standard peptide is unremarkable. Systematic offsets are more interesting than random ones: a discrepancy near 16 daltons suggests oxidation, near 1 dalton suggests deamidation, and a shift matching a known residue mass suggests a deletion sequence that the chromatography did not resolve.

Water content, usually by Karl Fischer titration, tells you how much of the mass in the vial is water. Lyophilised peptides are hygroscopic and single-digit percentages are normal; a high figure means you are weighing out proportionally less compound than you think, and it also predicts poorer storage stability.

Counter-ion or salt content matters for the same reason. Peptides purified by reversed-phase HPLC with trifluoroacetic acid in the mobile phase come off as TFA salts, and the TFA is a real, weighable fraction of the powder. Where a certificate reports peptide content or net peptide, that figure — not the HPLC purity — is what governs how much compound is actually present. This distinction is common enough to deserve its own treatment.

Appearance is a cheap, honest check that costs nothing to use. A certificate describing a white lyophilised powder, next to a vial containing something tinted or visibly collapsed, is a discrepancy worth raising before the material goes anywhere near an assay.

A certificate produced by the same organisation that synthesised the material is a self-report. That is not automatically untrustworthy, but it is structurally weaker than analysis performed by an independent laboratory with no interest in the result, and the certificate should make clear which it is.

Look for the analysing laboratory's name, the method reference or a description of the gradient and column, and the instrument. Certificates that state a number without stating how it was obtained are asking to be taken on faith. A method section costs the issuer nothing and is the difference between a measurement and an assertion.