How to read a peptide batch test report

A certificate of analysis is a record of what a laboratory measured on one specific batch. It is not a general statement about a product line, and two batches of the same peptide can carry very different numbers. Reading one properly means knowing which questions each section answers.

The header: what was tested

The top of a report identifies the material and the batch. Look for the product name, the lot or batch number, the date of analysis and the identity of the laboratory. The batch number is the part that matters most, because it is the only thing tying the document to the vials in front of you. A report whose lot number does not appear on the label is a report about something else.

Reports in the library on this site are indexed by batch, and the lookup accepts a lot number printed on a kit. Where a batch has been tested by an independent laboratory, that report is published with the laboratory's own verification key. Where it has not, the manufacturer's quality-control certificate is published instead and labelled as a manufacturer certificate, because those are different classes of evidence and mixing them up defeats the point of publishing either.

Purity by HPLC

The chromatogram is usually the largest item on the page: a baseline with one dominant peak and, if you look closely, several small ones. The stated purity is the area of the main peak as a percentage of all integrated peak area.

Check a few details rather than assuming them. The detection wavelength, typically 214 nm or 220 nm for peptides, tells you what the detector could see: 214 nm responds to the amide bond itself, so it registers essentially any peptide-related material, while 280 nm responds mainly to aromatic residues and misses sequences that have none. The gradient and column tell you how well things were separated; a short, steep gradient can bury a close-eluting impurity under the main peak. Whether the integration excluded the solvent front matters too, since including it inflates the denominator.

None of this makes a 98% figure wrong. It makes it a measurement with conditions attached, and the conditions are printed on the same page.

Identity by mass spectrometry

The mass spectrum answers a different question: is this the right molecule? The report gives an observed mass, and it should sit within a fraction of a Dalton of the theoretical mass calculated from the sequence. For a peptide of a few thousand Daltons, an agreement to one decimal place is a strong result.

Watch for two things. Multiply charged ions are normal in electrospray ionisation, so a peptide of 3357.9 g/mol may appear as a series of peaks rather than one; the reported figure should be the deconvoluted mass. And a mass match confirms composition, not sequence order. Two peptides built from the same residues in a different arrangement have identical masses. Sequencing methods exist for that question, and they are rarely part of a routine reagent certificate.

Water content and residuals

Freeze-dried peptides are hygroscopic, and the water they pick up is part of what you weigh out. Karl Fischer titration measures it directly, and a figure in the low single-digit percent is ordinary for a lyophilised powder. Where the laboratory reports it, the number appears on the certificate; where it does not, the water content is simply unknown rather than zero.

Residual solvents and counter-ion content are reported less often. Their absence from a certificate means they were not measured.

Verifying that a report is genuine

Independent laboratories issue reports with a verification key or report identifier that can be checked on the laboratory's own website. Doing that check takes a few seconds and is the only step that confirms the document was issued by the laboratory whose name is on it. A PDF or a photograph proves nothing on its own. Files are trivially editable, and a report that cannot be traced back to its issuer is an unverified claim.

Where a report on this site carries a verification key, the key is shown next to it. Where a batch has only a manufacturer certificate, that is stated plainly on the report card rather than left for you to work out.