A certificate of analysis is a record of what specific tests, performed on a specific quantity of material on a specific date, returned. It is not a description of a product, a guarantee about future material, or a statement of what the material may be used for. Reading one well is mostly a matter of keeping those distinctions in view.
The lot is the subject
The single most important field is the lot number. Everything else on the page is scoped to it.
A lot is the quantity produced in one process run and treated as a unit for testing and traceability. Two lots of nominally the same peptide are two different materials: they were synthesised separately, purified separately and tested separately, and they can differ in impurity profile, content and appearance while both conforming to the same specification. A certificate carried over from one lot to another tells a researcher nothing about the vial in hand.
This is why every analytical figure published on this site sits against a lot number, and why the vial label carries that number in a form that resolves to the record.
Identity
The identity section answers one question: is this molecule the intended molecule?
The standard evidence is a mass measurement compared with the mass calculated from the intended sequence and its modifications. A certificate that reports an observed mass without the calculated mass leaves the comparison to the reader; one that reports both, with the method, states the finding completely.
Mass alone does not settle everything. Species that share the intended mass — a diastereomer produced by racemisation, for instance — are invisible to an intact-mass measurement and appear, if at all, as a resolved peak in the chromatogram. Identity and purity are therefore read together rather than separately.
Purity, and what the figure covers
Chromatographic purity is normally reported as the main peak as a share of total detected peak area, with impurities summed the same way; reference-standard work describes this explicitly as the sum of HPLC impurities expressed as a percentage of total detected area, with the main peak determined from replicate injections from a single container 1.
Three fields qualify that number and should appear beside it:
- The method: column chemistry, gradient and run conditions determine what is separated from what.
- The detection wavelength: 214 or 220 nm responds to the backbone amide, 280 nm to aromatic side chains. Species that do not absorb at the wavelength used are not counted.
- The number of injections: a figure from replicate injections carries different weight from a single measurement 1.
A purity percentage quoted without those qualifiers is not directly comparable with any other purity percentage.
Content or assay
Where a certificate reports content — sometimes labelled assay — it is answering the mass question rather than the composition question: how much of the weighed solid is peptide, excluding water and counter-ions 2.
The route matters, because the routes are not interchangeable. Amino acid analysis hydrolyses the peptide and quantifies the released residues, with published conditions using 6 M hydrochloric acid at 110 °C for 18 to 24 hours and reported intra- and inter-day coefficients of variation at or below 10% 3. An HPLC assay determines content against a characterised reference material whose own purity was established first 1. A certificate that names the route and the reference material allows the figure to be interpreted; one that gives a bare percentage does not.
Related substances and other tests
Beyond the main-peak figure, a certificate may enumerate individual impurities, report water content, identify residual solvents, or give inorganic residue. These are the components that make up the non-peptide balance of a lyophilised solid: water, counter-ions such as acetic or trifluoroacetic acid, residual solvents and inorganic impurities 1.
Where a material is intended for work in biological systems, an endotoxin result may appear, expressed in endotoxin units per milligram with the method named. As with every other figure, the method and the reference standard against which the units are defined determine what the number means.
Dates and the testing laboratory
Three dates commonly appear, and they are not interchangeable.
The test date fixes the moment the results describe. The manufacture date fixes the age of the material. The retest date is the point at which the lot should be re-examined against its specification rather than assumed still to conform; it is a different instrument from an expiry date, which marks the end of an assigned period outright.
The testing laboratory should be named, and whether it is the manufacturer or an independent party should be visible. Neither arrangement is inherently better evidence, but the two are different claims, and a certificate that leaves the distinction implicit is weaker than one that states it.
Questions a certificate does not answer
A certificate is silent on several things that are sometimes read into it.
It does not establish that a material is suitable for any particular application; suitability is determined by the requirements of the work, not by the document.
It does not describe material other than the lot tested.
It does not remain true indefinitely: the storage conditions and elapsed time between the test date and the moment of use govern how far the description still holds, which is the subject of the stability page.
And it does not authorise anything. A certificate records analytical findings; regulatory status is a separate matter recorded separately, per jurisdiction and per compound.
How verification works here
Every released lot on this site carries a certificate identifying the laboratory, the test date, the methods and the per-compound results, and the lot number on the vial resolves to that record through the batch verification page. Blends carry one line per component, because a single combined figure would not say which compound it described.