A certificate of analysis is a narrow document. It records the results of specific tests, run on a specific sample, on a specific date, by a specific laboratory. It is not a quality rating, it is not a guarantee about the vial in your hand, and it is not evidence about anything other than the material that was actually tested.
Most of the difficulty in reading one comes from that gap. A COA can be entirely truthful and still be close to uninformative — because the sample it describes cannot be connected to the product being sold, or because the number on it does not measure what the reader assumes it measures.
The fields that carry the weight
Work through a COA in this order. If any of the first four fields is missing or unresolvable, the rest of the document cannot be interpreted, no matter how impressive the numbers are.
| Field | What it establishes | What its absence means |
|---|---|---|
| Lot / batch identifier | Which physical production run was tested. | The result cannot be tied to any specific product. Treat the document as marketing. |
| Test date and report date | When the material was in the stated condition. | No way to judge whether the result predates reprocessing, repackaging or long storage. |
| Testing laboratory identity | Who is accountable, and whether they can be contacted independently. | Nobody is accountable. An unnamed lab cannot be asked to confirm the report. |
| Analytical method | What question was actually asked of the sample. | A bare percentage with no method is not a measurement, it is a claim. |
| Purity result and its basis | The number, plus what it is a percentage of. | See the section below — the basis matters more than the figure. |
| Identity confirmation | That the material is the compound named, not merely a pure something. | Purity without identity is compatible with a pure and entirely wrong substance. |
| Chromatogram or spectrum | That a human can check the summary against the raw output. | You are asked to trust a transcribed number with no way to audit it. |
Purity of what, exactly
This is where most misreadings happen. A reversed-phase HPLC purity figure is almost always a chromatographic area percentage: the area of the main peak divided by the total area of all detected peaks, usually with UV detection near 214 nm where the peptide bond absorbs.
That is a useful number. It is not the same as the fraction of the vial's mass that is the intended peptide. Two things sit outside it.
- Anything that does not absorb at the detection wavelength, or does not elute under the method conditions, is invisible to the measurement. Inorganic salts and many process residues fall into this group.
- Synthetic peptides purified by preparative HPLC are typically isolated as salts — trifluoroacetate is common — and retain bound water. Counterion and water together routinely account for a substantial share of the dry mass.
Identity is a separate question from purity
A purity result describes how much of the detected material is one component. It says nothing about which component that is. Identity requires its own evidence, and mass spectrometry is the usual instrument for it: an observed mass consistent with the expected molecular weight of the named sequence.
A complete identity claim shows the expected mass, the observed mass, and the charge states or adducts involved. A COA that reports only 'identity: conforms' with no observed value is asking to be taken on faith.
Four ways an accurate COA still tells you very little
- It is not lot-specific. A single representative COA republished for every order describes one historical sample, not the material shipped.
- It is a supplier's internal result presented as third-party verification. Both have a place; conflating them removes the independence that made the second one worth having.
- The method is stated but not specified. 'HPLC' without column, mobile phase, gradient and detection wavelength cannot be reproduced or compared between lots.
- It is a picture of a document. An image with no retrievable original, no lab contact and no lot lookup cannot be confirmed by anyone.
What to ask for
The reasonable requests are narrow and specific, and a supplier with real documentation can answer them without difficulty.
- The COA for the lot number physically printed on the vial you received, not a representative example.
- The chromatogram and the mass spectrum, not only the summary table.
- The full method parameters, so a second laboratory could run the same analysis.
- Whether the report came from the supplier's own laboratory or an independent one, stated plainly.
- Net peptide content, if the material is being compared on a mass basis.
What a COA cannot establish
A certificate of analysis speaks to the composition of a sample. It carries no information about safety, suitability, sterility unless a sterility test was actually performed and reported, or appropriateness for any particular use. It is not evidence of regulatory approval, and it does not convert an unapproved substance into an approved one.
Sources
- FDA — Analytical Procedures and Methods Validation for Drugs and BiologicsFDA guidance on what a defensible analytical procedure includes.
- ICH Q2(R2) — Validation of Analytical ProceduresWhat specificity, accuracy and precision mean for a reported result, and what method validation actually covers.
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