On a COA, what is the difference between identity, content and purity?
Updated: 2026-09-09 · Buying, quality and COA
Short answer
They are three separate questions. Identity: is this the molecule it claims to be? Confirmed by mass spectrometry. Content or assay: how much material is in the vial? Purity: what share of the detected material is the main compound? A certificate can answer one and stay silent on the other two.
In plain English
- Identity ≈ mass spectrometry: confirms the expected molecular mass.
- Content ≈ quantitative assay: how many milligrams are actually there.
- Purity ≈ HPLC: the main peak's share of what was detected.
- A vial can be 99% pure and still contain less material than the label claims.
What we know
- HPLC and mass spectrometry are standard techniques answering different questions.
- For approved medicines, identity and content are mandatory batch-release controls.
What we don't know yet
- You cannot tell which tests were skipped if the certificate does not list them.
- A COA describes the sample tested, not necessarily the whole batch.