How to read a certificate of analysis

A COA describes one batch. If the lot number on the document does not match the lot printed on the vial in your hand, the document tells you nothing about your material, it describes a different batch that may have been made months earlier by a different process.

This is the single most common way a real certificate gets used dishonestly: a supplier assays one good batch, then serves that same PDF alongside every subsequent order. Always check that the numbers match, and be suspicious of any supplier who only ever shows one certificate.

Purity on a peptide COA is almost always HPLC area percent, high-performance liquid chromatography separates the components of the sample, and the figure is the target peptide's peak area as a proportion of total peak area.

This matters because it is a relative measurement. It tells you what proportion of the peptide-like material is your target compound. It does not tell you how much peptide is in the vial. A vial can be 99.5% pure and contain half the stated milligrams. Those are two separate questions, and only one of them is answered by the purity figure.

You want to see the chromatogram itself, not just the number. A clean trace shows one dominant peak with a flat baseline. Several substantial peaks clustered near the main one usually mean truncated or deletion sequences, synthesis failures where the chain is missing residues. Those are the impurities that matter most, because they are structurally similar enough to be biologically active in ways you did not intend.

HPLC tells you how pure the sample is. Mass spectrometry tells you whether the thing you purified is the compound it is supposed to be. It measures molecular mass, which for a peptide of known sequence is a specific, predictable number.

This is the check that catches outright substitution, the wrong peptide sold under the right name. A COA with HPLC but no mass spec has confirmed that the vial contains one thing in high purity, without confirming what that thing is. Insist on both.

Compare the observed mass on the report against the theoretical mass for the sequence. They should agree closely. A discrepancy of a few hundred daltons is not a rounding error. It is a different molecule.

Peptide content (sometimes "net peptide content") is the proportion of the vial's mass that is actually peptide. The remainder is typically residual water and counter-ions left over from synthesis and purification, trifluoroacetate is the usual one.

This is normal and expected. It is also why a vial labelled 10 mg may contain somewhat less than 10 mg of peptide by mass unless the supplier has accounted for it. A good certificate states the content figure so you can do that arithmetic yourself.

Water content by Karl Fischer titration appears on more thorough certificates. It matters for stability: a lyophilised peptide holding significant residual moisture degrades faster in storage.

The most revealing thing you can do with any supplier is ask for the certificate matching the lot number on the vial you are about to receive, before you order. The answer tells you almost everything.

A supplier who assays properly will send it without friction, because it is their strongest argument. A supplier who does not will explain why they cannot, and the explanation is always more elaborate than "here it is".