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How to read a Certificate of Analysis
Which fields on a certificate of analysis decide whether an impurity or reference standard is fit for your method, and which ones get skipped.
Published 24 September 2026 · ImpurityLab technical team
What the certificate actually covers
A certificate of analysis is a batch-specific document. It describes one lot of one catalogue number, and it is only meaningful for that lot. If the vial in your hand carries a different lot number, the certificate is a description of something else.
For an impurity or reference standard, a complete certificate normally carries:
- Identity block — chemical name, catalogue number, CAS registry number, molecular formula, molecular weight, and the salt or hydrate form as supplied.
- Identification — the techniques used to confirm structure, typically IR plus 1H NMR, often with mass spectrometry. "Conforms" without naming the technique is not identification.
- Purity or assay value — and, critically, which of the two, measured by what method.
- Residual and counter-ion data — water by Karl Fischer, residual solvents by GC where relevant, and inorganic residue or elemental impurities.
- Handling — storage conditions, light and moisture sensitivity, and the re-test or expiry date.
- Traceability — who produced the material, under what quality system, and whether the assigned value is certified with an uncertainty statement.
A certificate is not a specification sheet and it is not a safety data sheet. A specification sheet states what future batches will be held to. A certificate states what this batch actually was.
Fields worth checking before you inject
Most of the certificate is administrative. The part that changes your calculation is small, and it is usually in the same four places.
| Field | What it decides | What gets skipped |
|---|---|---|
| Purity vs assay | Whether you have an area percentage or a mass fraction | Whether the stated method was HPLC, qNMR or mass balance |
| Salt / hydrate form | The molecular weight you divide by | Whether the value is expressed on the free base or the salt |
| Water content | How much of the weighed mass is not your analyte | Karl Fischer result on a hygroscopic solid |
| Impurity profile | Whether the stated purity hides a co-eluting peak | The chromatogram itself, and the detection wavelength |
| Re-test date | Whether the number still applies today | Materials opened and stored warm for two years |
If a certificate gives a single purity figure with no method stated, treat it as an area percentage until someone confirms otherwise. The difference between an area percentage and a mass fraction can exceed one percent on a small polar molecule, which is far larger than the tolerances most impurity methods are asked to defend.
Where it usually goes wrong
Area percentage treated as mass. HPLC purity by area assumes every component responds equally at the detection wavelength. They do not. A 98.5 % area figure and a 99.8 % assay figure can describe the same vial and both be correct, because one is a ratio of detector responses and the other is a mass fraction.
The salt form. A hydrochloride adds roughly 36.5 g mol−1; a calcium salt of a divalent acid changes the formula unit again. Weighing 10 mg of the salt while calculating as the free base introduces an error of the same order as the salt fraction — tens of percent, not fractions of a percent.
Hydrates and solvates. "Dihydrate" is not a footnote. Two waters add about 36 g mol−1 and shift every subsequent molarity calculation.
Isotope-labelled material. For a deuterated or 13C-labelled standard, chemical purity and isotopic enrichment are separate specifications. A material can be 98 % chemically pure and 99 atom % D at the same time, and the atom percent is what your internal standard calculation depends on.
Counter-ion scavenging. Small amounts of residual solvent or water in a lyophilised standard change the effective concentration of the solution you prepare, which matters most in exactly the low-level work where impurity standards are used.
A worked check
Suppose a certificate reports 97.5 % HPLC purity, 3.2 % water by Karl Fischer, and describes the material as a hydrochloride. You need a 1.00 mg mL−1 solution on a free-base basis.
- Read the form. The stated purity is on the salt, so the free-base fraction is not the purity figure.
- Correct for water. Of the mass you weigh, 3.2 % is water, so the fraction that is the salt is 96.8 %.
- Correct for the analyte content of that salt. Apply the stated chemical purity to the anhydrous salt mass.
- Correct for the salt-to-free-base ratio using the two molecular weights before calculating the target concentration.
Four lines of arithmetic, and any one of them skipped produces a systematic bias that no amount of replicate injections will remove. This is also why the certificate should state the molecular weight of the form as supplied, not only of the parent compound.
What to ask for, and when
- Always — the lot number of the material actually shipped, and confirmation that the certificate matches it.
- For quantitative work — the assay method and, for qNMR or mass balance, the uncertainty or the individual contributing assays.
- For related-substances methods — the chromatogram, so you can see whether a co-eluting peak is present and at what retention.
- For regulated filing — the quality-system context: whether the material is a certified reference material with a stated uncertainty, or a reference material without one, and under which accreditation.
- For deuterated or labelled standards — chemical purity and isotopic enrichment stated separately.
None of this requires a special request if the supplier builds the certificate to carry it. It only becomes a delay when it has to be chased after the order.
The short version
Read the form, read the water, read which purity number you were given, and check the date. If your method depends on the value, make sure the value has a method behind it. Everything else on the certificate is context.