A certificate of analysis is not marketing. It is a record of what a laboratory measured on a specific batch of material on a specific day, and almost everything worth knowing about a supplier is visible in how much of it they are willing to publish.

Purity is not one number

Purity by RP-HPLC is the percentage of the material that came off the column as your peak rather than as something else. It is the figure everybody quotes. It is also the easiest one to make look good, because it says nothing about what the other percent is, and nothing at all about how much of the powder in the vial is peptide.

A chromatogram at 214 nm is reading the peptide bond. Salts, counter-ions and water are close to invisible to it. You can have a 99% pure peptide that is two-thirds salt by weight, and both numbers are true.

Net peptide content is the number that changes your arithmetic

Net peptide content is the fraction of the lyophilized solid that is actually peptide. The rest is trifluoroacetate from the purification, residual water, and counter-ions. It typically runs between 75 and 90 per cent.

This is the figure that decides your stock concentration. Reconstitute a 10 mg vial at 82% net content as though it were 10 mg of peptide and every concentration downstream is 18 per cent under what you wrote in the notebook. Most suppliers do not publish it. It is on every certificate here and on every product page.

Identity is a separate question from purity

Mass spectrometry answers a different question: is this the molecule you ordered. A pure sample of the wrong peptide passes an HPLC purity test perfectly. The observed mass should sit within a fraction of a unit of the calculated molecular weight, and any certificate that reports purity without reporting identity has answered the easier half.

Water, and why it is on there

Karl Fischer titration measures residual moisture. It matters for two reasons: water is part of the mass you are weighing out, and a lyophilized peptide that came out of the dryer wet degrades faster in storage than one that did not.

What a weak certificate leaves out

• No lot number, or a lot number that appears nowhere else — on the vial, on the packing slip, in any index you can search.
• Purity without identity. One assay, usually HPLC, and no mass spectrometry.
• No net peptide content, which is the single most useful number on the page for anyone actually pipetting.
• No date of analysis and no retest date, so you cannot tell whether the document describes material tested this year or three years ago.

Two laboratories, one lot number

The manufacturer tests its own material at release, which is normal and not sufficient: it is the one party with an interest in the answer. We draw a second sample from the lot that arrives here and send it to an independent laboratory at our cost. Both reports carry the same lot number, which is what makes them comparable at all.

The whole index is at certificates of analysis, superseded lots included.