How to Read a Certificate of Analysis (COA)
Updated · CATO technical documentation
A certificate of analysis is the document that decides whether a reference standard can carry the weight your method places on it. Most laboratories read the purity figure and stop there. That is where problems begin, because the number at the top of the certificate is the least useful part of it unless you know what was measured, how the value was assigned, and what basis it is expressed on. This guide works through a certificate section by section, in the order the information actually matters. It sits alongside our wider guides library.
The administrative block
Start with the identifiers, because everything else depends on them being right.
- Product name and catalogue number, matched against your own method or purchase record.
- Lot or batch number. This is the identifier you cite in your report; a certificate without one cannot be traced back to a physical vial.
- CAS number, molecular formula, and molecular weight. Check the formula against the structure you need, not just the name.
- Storage conditions and, where stated, shipping conditions.
- Date of analysis, date of release, and either an expiry date or a re-test date.
- Intended use statement.
Two of these deserve more attention than they usually get. The re-test or expiry date tells you how long the material may be used, provided it is stored as specified; a certificate with no date is incomplete. And the CAS number is a check, not a guarantee: a single CAS number can cover a compound supplied as a free base, a salt, or a particular hydrate, so the certificate must state which form this lot is.
Identity and characterisation data
Identity is established by orthogonal techniques. A certificate that shows a single retention time match from an HPLC method has demonstrated that one peak behaves like the expected compound; it has not demonstrated structure. Stronger evidence looks like this:
| Evidence | What it supports |
|---|---|
| NMR spectrum | Skeleton and substitution pattern |
| Mass spectrum with molecular ion | Molecular weight and formula |
| Infrared spectrum | Functional groups, useful for salt and polymorph questions |
| Elemental analysis | Formula confirmation, if reported |
| Chiral or isomeric data | Configuration, where relevant |
For an impurity standard, the structure is the product. If the certificate does not reference the spectral data, ask for it. Ask whether the supplier can produce the spectra behind the certificate. A supplier that characterised the material itself is normally able to discuss the data rather than only forward a document. Where an assignment rests on comparison with a literature spectrum, that should be visible as well.
Purity, assay, and the content basis
This is the section most often misread, because three different quantities are commonly reported under similar headings.
- Chromatographic purity describes the proportion of the detected peak area that belongs to the main component, usually by HPLC or GC. It says nothing about anything the method cannot see, such as water, residual solvents, or inorganic residue.
- Assay or assigned content describes how much of the vial's mass is the compound of interest, and it is the value you use when you weigh material to prepare a stock solution.
- Uncertainty qualifies the assigned content. If the value is given as a range or with an uncertainty term, that term belongs in your measurement uncertainty budget.
Where a content value is obtained by mass balance, the certificate should show the components subtracted: organic impurities, water, residual solvents, and inorganic residue. Where it is assigned against another standard, the certificate should say which. A common failure is a certificate that reports high chromatographic purity and no assigned content at all; using such material as if it were 100 percent pure introduces a bias that no amount of instrument care will remove. Look for a certificate that states the assigned content and its basis rather than a purity figure alone.
Pay attention to the basis as well. Values expressed on an "as is" basis, a dried basis, or an anhydrous and solvent-free basis are not interchangeable, and neither is a content value for a salt form versus the free base. If your calculation assumes one and the certificate states another, your result will be wrong by a fixed factor.
Common pitfalls
- Using chromatographic purity as the content value when preparing standards from a weighing.
- Overlooking the salt, hydrate, or solvate form that the reported content refers to.
- Accepting a certificate with no lot number, no re-test date, or no storage instruction.
- Treating a single chromatographic technique as proof of structure.
- Ignoring the stated uncertainty, then reporting results to more significant figures than the standard can support.
- Assuming a new lot behaves like the old one. Compare certificates across lots, and see our supplier selection checklist for what to look for in lot-to-lot consistency.
For the underlying definitions, what is an impurity reference standard explains how identity and content are established in the first place, and how a working standard differs from a primary one. Our quality documentation sets out the evidence CATO retains for each lot.
Request a quote
If you need a certificate that answers these questions rather than a summary, send the CAS number, compound name, and the documentation your method file requires to [email protected], or request it through the contact page. CATO issues identity, purity, and content data with its standards and can supply the supporting spectra on request. All materials are supplied for laboratory and research use only, and they should be stored and handled as the certificate specifies.