A certificate of analysis is a record of what was measured on one specific batch, by one laboratory, on one date. It is not a description of the product in general, and it is not a guarantee about the vial in your hand unless the lot numbers match. Most of the skill in reading one is knowing which fields carry evidence and which are simply transcribed from the order.
The panels, and what each one proves
A complete certificate answers four separate questions. They are genuinely separate: a batch can pass any one of them and fail another.
| Panel | Question it answers | Typical method |
|---|---|---|
| Identity | Is this the right molecule? | Mass spectrometry (ESI or MALDI) |
| Purity | How much of the peptide present is the target sequence? | RP-HPLC, area percent at 214 nm |
| Content | How much peptide is in the vial by mass? | HPLC quantitation against a standard |
| Safety attributes | What else came along? | Endotoxin, water, residual solvent, counter-ion |
The common failure is a document that reports purity and nothing else. Purity is a proportion, and a proportion of an unidentified substance is not informative.
What does identity confirmation actually prove?
That the observed mass matches the theoretical mass of the sequence, within the instrument's tolerance. That is strong evidence and it is not proof of sequence. Mass spectrometry weighs the molecule; it does not read it left to right. Two peptides with the same amino acid composition in a different order have identical masses, and a swapped pair of residues is invisible to a mass check. Where order genuinely matters, sequencing by tandem MS or Edman degradation is the method that settles it, and most routine certificates do not include either. Knowing which question your certificate answered is the point.
Is a purity figure without a chromatogram worth anything?
Much less than it appears. The figure is the area of the main peak as a percentage of total integrated area, and every step in that sentence involves a choice: the gradient, the wavelength, the run length, where the integration baseline was drawn, which peaks were excluded as solvent front. A trace lets you see whether the main peak is symmetrical, whether anything co-elutes on its shoulder, and whether the run was long enough for late-eluting material to appear at all. A number alone asks you to trust all of those decisions without seeing any of them. Reading the trace takes about thirty seconds.
The fields people skim
Four fields carry most of the traceability, and all four are easy to read past.
Lot or batch number. This is the only field that connects the document to the vial. A certificate for the compound is not a certificate for your material. The number printed on the vial label and the number on the document have to be the same string — ours are searchable on the lot verification page, and the full library sits at Certificates of Analysis. Whether a supplier can produce that document for your lot is the first thing worth checking about them. The label itself carries more than the lot number.
Date of analysis. Tests are run on a sample at a moment. A certificate issued eighteen months ago describes the batch as it was then. Nothing on the document speaks to what has happened since, which is why storage history is not a detail.
Method, stated specifically. "HPLC" is a category. "RP-HPLC, C18, 0.1% TFA in water/acetonitrile gradient, 214 nm" is a method. The second can be compared across laboratories; the first cannot.
Acceptance criteria. A result without a specification is a measurement, not a pass. "Purity 98.4%" says something; "Purity 98.4%, specification ≥98.0%, Conforms" says the same thing plus who decided what good looked like and when.
Why does the lot number matter more than the compound name?
Because synthesis is a batch process and batches differ. Two lots of the same sequence from the same manufacturer can differ in purity, in impurity profile, in water content and in counter-ion. That is not a defect; it is what batch manufacturing is. The certificate exists precisely to describe one batch, and cross-matching a document to a different lot discards the only thing it was for.
What is the difference between a certificate of analysis and a test report?
A test report states results. A certificate states results against specifications, and adds a disposition — conforms or does not conform — signed by someone accountable. The distinction matters when a figure sits near a limit: a report leaves the judgement to you, and a certificate records that the judgement was made and by whom. Both are useful. Only one is a release document.
What should make you look harder
None of the following proves anything is wrong. Each is a reason to ask a question.
- A round number. Instruments produce 98.7%, not 99%. A figure rounded to the unit has usually passed through a person.
- 100% purity. Area normalisation cannot produce 100% unless nothing else was integrated. It reflects a detection limit, not an absence of impurities.
- No date, no lot, no signature. A document that cannot be tied to a batch, a moment and a person is a marketing asset.
- A purity figure with no content figure. The two answer different questions, and the missing one is usually the more consequential.
- An image rather than a record. A certificate that exists only as a screenshot, with no independent way to check it, asks for trust that a verification code would make unnecessary.
Should a certificate ever report 100% purity?
No, and a certificate that does is describing the method rather than the material. Purity by area normalisation is the main peak divided by everything the detector integrated. Reporting 100% means nothing else crossed the integration threshold — which is a statement about sensitivity, not about the sample. Real certificates report 97–99% and something plausible in the remainder.
Does a certificate of analysis expire?
The document does not; its relevance does. It describes a sample on a date. A retest date or a shelf life is a separate claim requiring stability data, and most peptide certificates do not carry one. Absent that, the sensible reading is that the certificate describes release condition, and everything after release is a function of how the material was stored and handled.
Why are purity and content on the same document if they mean different things?
Because both are needed to know what is in the vial, and neither implies the other. Purity is a proportion of the peptide present; content is a mass. A batch can be 99% pure and hold substantially less peptide than the label suggests, because water and counter-ion make up the difference — the arithmetic is worked through in what a 5 mg vial actually contains. For co-formulated material the requirements are stricter again, and a blend certificate has to do more work.
All products referenced here are supplied for laboratory and research use only. They are not drugs, foods, supplements or cosmetics, and are not for human or veterinary use.
