A Certificate of Analysis, or COA, is the most important document that comes with a chemical. It is the supplier's statement of what was tested on the specific lot you received, against which limits, and with what results. Reading it properly takes a minute and can prevent weeks of troubleshooting. This guide walks through a typical COA and highlights what to look for.
COA versus specification sheet
A specification sheet, sometimes called a product specification or technical data sheet, lists the limits a product is designed to meet. A COA reports the actual results for one lot. A document that shows only limits, or only the word complies without numbers, tells you much less than a COA with measured values. For critical applications, ask for numerical results.
Header: identity and traceability
The top of a COA should identify:
- Product name and, ideally, common synonyms
- CAS number, which identifies the exact substance, including whether it is a salt or hydrate
- Molecular formula and molecular weight, which you need for concentration calculations
- Grade and specification reference, such as ACS, a pharmacopoeia and edition, or an in-house specification number
- Lot or batch number, which must match the container label
- Manufacturing date and retest or expiry date
Check first that the lot number on the COA matches the label. A mismatch is the most common documentation error and, in regulated environments, a reason to quarantine material until resolved.
The results table
The body of a COA is a table of tests. Each row should show the parameter, the specification limit, the result and, ideally, the method. A typical organic compound COA might look like this:
| Test | Specification | Result | Method |
|---|---|---|---|
| Appearance | White to off-white powder | White powder | Visual |
| Identification | IR conforms to reference | Conforms | FTIR |
| Assay (HPLC, anhydrous basis) | ≥ 98.0% | 99.4% | HPLC-UV |
| Water | ≤ 0.5% | 0.12% | Karl Fischer |
| Residue on ignition | ≤ 0.1% | 0.03% | Gravimetric |
| Largest single impurity | ≤ 0.5% | 0.21% | HPLC-UV |
| Residual solvents | Meets ICH Q3C limits | Complies | Headspace GC |
Assay and its basis
Assay values are meaningless without their basis. An assay on the anhydrous or dried basis excludes water or volatiles, so the as-is content is lower. If a material has 99.4% assay on the anhydrous basis and 5% water, the as-is content is about 94.4%. When preparing standards, correct for this.
The method also matters. A titration measures all species that react with the titrant, so a titrimetric assay can exceed 100% or hide impurities that react similarly. HPLC assay against a reference standard is specific but depends on the purity assigned to that standard. Area-percent purity by HPLC or GC is not the same as assay. It ignores water, inorganics and anything that does not respond to the detector.
Identity tests
Identity is usually confirmed by IR, NMR, HPLC retention time against a standard, or classical chemical tests. For reference standards and impurities, expect stronger evidence: NMR and mass spectrometry, often with spectra attached.
Impurities
Reagent COAs often list specific inorganic impurities, such as chloride, sulfate, iron and heavy metals. Pharmaceutical COAs list related substances, sometimes with named impurities, and residual solvents. Values reported as less than a number, such as < 0.05%, mean the impurity was below the reporting or quantitation limit of the method.
Water and volatiles
Water by Karl Fischer is specific to water. Loss on drying captures water plus any other volatiles. Knowing which was used helps explain differences between suppliers.
Dates
A retest date means the material should be re-examined before use after that date. It can often be extended if retesting shows compliance. An expiry date is a firm end of use. Both assume storage as directed on the label. A COA with no date information for an unstable product is a warning sign.
Signature and issuer
A COA should state who issued it, usually the quality control or quality assurance function, and should be signed or electronically approved. If the COA was issued by a distributor, it should make clear whether results come from the manufacturer, from the distributor's own testing, or from an independent laboratory.
Red flags
- Lot number does not match the label, or the COA is undated.
- Results are identical to the specification limits, or every result is simply complies.
- Assay is given without basis or method.
- The same COA appears for different lots.
- Tests that matter for your application are missing.
- Units are inconsistent, such as ppm and percent mixed without explanation.
Using the COA in your own quality system
In regulated laboratories, a supplier COA is usually accepted only after the supplier has been qualified, and even then the receiving laboratory typically performs at least an identity test on each lot. File the COA with your receipt records, note the retest date in your inventory system, and make sure the COA travels with any material you subdivide or transfer.
What to do if something looks wrong
Contact the supplier with the lot number and a specific question. A competent supplier can explain the method, provide chromatograms or spectra, or confirm the result with the manufacturer. If you are qualifying a supplier for regulated use, compare several COAs over time and verify at least some results in your own laboratory.
FAQs
Is a COA the same as a specification?
No. A specification lists limits the product is designed to meet. A COA reports measured results for a specific lot against those limits.
Why is assay reported on the anhydrous basis?
It expresses purity excluding water, so that results are comparable regardless of moisture. Correct for water content when weighing for quantitative work.
What does a result of less than a value mean on a COA?
It means the impurity was below the reporting or quantitation limit of the method used, not that it is absent.