Reading a certificate of analysis properly
A specification limit and a measured value are different claims. Know which one you are looking at.
Every lot we ship carries a certificate of analysis. It is worth knowing how to read one, because certificates vary far more between suppliers than the products do.
Specification versus result
A certificate can report two quite different things:
- Specification limit - "Iron: ≤ 100 ppt". This says the lot passed.
- Measured result - "Iron: 12 ppt". This tells you the actual blank you will carry into your method.
For trace work, the measured result is what matters. If a supplier only prints limits, your blank could legitimately sit anywhere below that ceiling, and it may move between lots.
What to check before releasing a lot to a method
- Lot number matches the bottle. Obvious, routinely missed.
- Date of analysis, not just date of manufacture.
- The specific elements your method cares about. A general trace-metal panel may not include the one that interferes with you.
- Assay and concentration range - especially for aqueous acids, where "70%" may mean 67-70%.
- Expiry or retest date, and whether that assumes an unopened container.
Keep the certificate with the data
Regulated laboratories already do this. Everyone else should: when a result is questioned two years later, the reagent lot certificate is often the only way to reconstruct what happened.
Certificates for any Purity Fine lot can be requested from our team with the lot number, including for material bought some time ago.