A certificate of analysis, or COA, is a lot-specific quality record that reports what a testing laboratory measured on one particular batch. It is not a marketing sheet and not a permanent property of the compound; it describes the material in front of you at the time it was tested. Reading one well means knowing which fields to expect, what each means, and where the common errors and ambiguities hide.
What a COA is for
The document exists to link a physical lot to a set of measured attributes so that a receiving laboratory can decide whether the material matches what was ordered. Every figure on it is a measurement with a method and a date behind it. That framing matters because it sets the reader’s job: not to trust the numbers because they are printed, but to check that the identity fields agree with the compound ordered and that the quality fields were produced by methods appropriate to them.
The example fields discussed below are illustrative of what a peptide COA typically carries. They should be read as the shape of the document rather than as a specific real lot; the values on a genuine certificate belong to the batch it was issued for.
The fields a peptide COA should carry
| Field | What it reports | How to read it |
|---|---|---|
| Product name and code | The material the lot claims to be | Match against your order and the identity fields below, not on its own |
| CAS number | Registry identity of the substance | Confirm it is the exact number for the compound, including complex versus free-peptide forms |
| Lot or batch number | The unique handle for this production run | Record it; every other figure is scoped to this lot |
| Appearance | Physical description of the solid | Should match expectation, for example a white or coloured lyophilized powder |
| Purity (HPLC) | Percent by peak area from chromatography | Note the method and that it is an area percent, not an absolute assay |
| Identity (MS) | Mass-spectrometry confirmation | Observed mass should agree with the expected value for the sequence |
| Water or solvent content | Residual moisture or solvents | Relevant to net peptide content and storage |
| Counterion or salt content | Acetate or other counterion level | Explains why net peptide mass differs from gross powder mass |
| Test date and analyst | When and by whom the lot was tested | Anchors the record in time and to a responsible party |
Identity fields come first
Before any quality number matters, the identity fields have to agree. The product name is the least reliable entry because names travel loosely, so the CAS number and any structural identifiers are the real check. For a compound supplied as a salt, confirm the counterion, since a net-peptide-content figure can differ from the gross powder mass by the mass of the associated acetate or other counterion. A certificate that omits identity fields, or lists a name without a registry number, is incomplete rather than merely brief.
Purity and identity are two different questions
Purity by HPLC answers how much of what is in the vial is the target species, expressed as a percentage of chromatographic peak area. Mass-spectrometry identity answers whether that main species is the right molecule. A material can be highly pure and still be the wrong compound, or the right compound at lower purity, which is why both fields belong on a complete certificate. The companion note on HPLC purity and mass-spec identity covers how each measurement is produced and read.
Treating certificate data as a sample, not a promise
One framing point underlies everything else on the document. A certificate reports a measurement made on a specific lot at a specific time; it is not a standing property of the compound and not a guarantee that a future lot will read identically. This is why example certificate figures, including the kind of purity values that appear in a catalogue, should be read as illustrations of the fields the document carries rather than as a fixed grade attached to the material forever. When a real lot arrives, its own certificate is the record that governs, and the sensible practice is to file that certificate against the lot number so the two never drift apart.
The same caution applies to any certificate presented without the provenance fields that tie it to a physical batch. A polished-looking document with no lot number, no test date, and no responsible analyst is a template, and a template describes no particular material. Reading a certificate critically means asking, at every field, whether the entry is a measurement of the thing in your hand or a generic placeholder, and treating the two very differently.
Where COA errors and ambiguities hide
Several recurring problems are worth watching for. A purity figure with no stated method is hard to interpret, since area percent from one gradient is not directly comparable to another. A missing lot number detaches every measurement from the physical batch. A CAS number that matches a related but different form, for instance a complex versus its free peptide, is a subtle identity error that a quick glance misses. And any certificate presented without a test date or a batch reference should be treated as a template rather than a record of a real lot.
A practical reading pass, then, runs identity first, quality second, provenance throughout: confirm the CAS number and structural fields, read purity and identity as separate questions with methods attached, and make sure a lot number and test date tie the whole document to a physical batch. Example certificates for catalogue materials can be reviewed under lab results, the reasoning behind research-use framing is covered in the FAQ, and related quality-control notes sit in the lab standards archive.
Common questions
What is a certificate of analysis?
A lot-specific quality record reporting what a testing laboratory measured on one particular batch, including identity and purity fields. It describes the material as tested on a given date, not a permanent property of the compound.
Which COA field is most important to check first?
The identity fields, especially the CAS number and any structural identifiers. The product name travels loosely, so it should be matched against the registry number rather than trusted on its own.
Why can net peptide content differ from the powder weight?
Because a peptide is often supplied as a salt. The counterion, such as acetate, and residual water add mass to the powder, so the net peptide content is lower than the gross weight in the vial.