What Should a Certificate of Analysis (CoA) for an Impurity Standard Contain?

The Certificate of Analysis (CoA) is what turns a vial of powder into a reference standard you can rely on. Auditors and regulators review it, your analysts calculate results from it, and it is the first document anyone asks for when a result is questioned. Here is what a complete CoA for a pharmaceutical impurity standard should contain, and what each section tells you.

1. Product identification

• Product name: chemical (IUPAC-style) name and any common or pharmacopoeial name (for example "Impurity B").
• Chemical structure: drawn structure, including stereochemistry.
• CAS number: where one has been assigned.
• Molecular formula and molecular weight: including the salt or solvate form, if any.
• Batch / lot number: links the certificate to the exact material you received.

2. Identity tests

Identity proves that the material really is the stated structure. Typical techniques are:

At minimum, expect ¹H NMR plus MS. For impurities that are isomers of each other, NMR is essential because MS alone cannot tell them apart.

3. Chromatographic purity

Purity is usually determined by HPLC (or GC for volatile compounds) and reported as area %. A good CoA states the method conditions or attaches the chromatogram, so you can judge whether minor peaks were separated. Remember: area % assumes every component has the same detector response, so it is an estimate of purity, not an absolute content.

4. Water content and residual solvents

• Water content: typically by Karl Fischer titration (or estimated by TGA). Impurities are often hygroscopic.
• Residual solvents: by GC headspace or estimated from the ¹H NMR spectrum.
• Inorganic residue: residue on ignition or sulphated ash, where material quantity allows.

These values matter because they reduce the actual amount of impurity in every milligram you weigh.

5. Assay or potency

For quantitative use you need to know how much of the impurity is really in the vial. Two common approaches:

• Mass balance: potency ≈ (100 − water − residual solvents − inorganic residue) × chromatographic purity ÷ 100.
• Quantitative NMR (qNMR): direct measurement against a certified internal standard.

If the CoA states only chromatographic purity, apply the water and solvent values yourself before using the standard for quantification.

6. Physical description

Appearance (for example "off-white solid"), and where relevant, solubility and melting point. A change in appearance on receipt can be an early sign of degradation.

7. Storage conditions, retest date and handling

• Storage: temperature (room temperature, 2-8 °C or −20 °C), protection from light and moisture.
• Retest date or expiry: the date until which the stated values are considered valid under the stated storage.
• Handling advice: for example "allow to reach room temperature before opening".

8. Approval and traceability

The CoA should be dated and approved by the quality function, and carry the supplier's name and address. Spectra and chromatograms referenced on the CoA should be available on request. This traceability is what auditors look for when they review your reference standard records.

Sample CoA checklist

How we supply documentation

Every impurity standard from Pharma Impurity and Chemicals is supplied with a Certificate of Analysis, and characterisation data such as NMR, MS and HPLC can be shared on request. Our operations are GMP certified with an ISO 9001:2015 quality management system. See our Quality page.

Next, read How to choose the right impurity reference standard, or browse our product catalogue.