How to Read a Certificate of Analysis

Start with the batch number

A certificate describes one specific batch. If the number on the document does not match the number on the vial, the document describes different material and proves nothing about what you hold. This is the first check because it is the one that invalidates everything else.

The sequence is the most informative field

It determines almost everything else. It tells you the expected molecular weight, which residues are vulnerable to oxidation or disulfide formation, whether 280 nm quantification will work, and which mass-spectrum satellites to expect. A certificate without a stated sequence is missing the field that makes the others checkable.

Check the fields against each other

This is the part most people skip and it costs nothing. Does the stated molecular weight match the stated sequence? Does the CAS number resolve to a compound of that mass? Internal contradictions are common and revealing: a certificate quoting a mass of 4,963 Da for a seven-residue peptide, or a CAS belonging to a parent protein alongside a fragment's sequence, is describing two different things at once.

Net peptide content versus gross weight

The most frequently missing number. Synthetic peptides are supplied as salts, usually trifluoroacetate, and the counter-ion is part of the powder weight. Net peptide content states how much is actually peptide, and without it a stated milligram figure overstates what you have. A certificate reporting both is describing this properly.

Independence of the issuing laboratory

A certificate from the supplier's own laboratory is weaker evidence than one from an independent facility. It is not worthless (the analysis may be perfectly good), but independence is part of what makes the document evidence rather than assertion, and you should know which you are looking at.

What a certificate cannot tell you

Whether the compound does anything. A certificate is a chemistry document establishing what is present and in what proportion. It says nothing about biological effect, and presenting analytical documentation as though it supported a claim about activity is a category error that appears constantly in commercial writing.

Quick reference