Two questions, two methods
- Identity is established by LC‑MS, which measures the mass of the molecule and compares it against the expected value.
- Purity is read from a chromatographic trace, usually HPLC with UV detection, as the target peak’s area divided by total peak area.
Why purity figures are close to useless as a comparison
Nearly every supplier in this category advertises a purity in the same narrow range. When every number is the same, the number stops distinguishing anyone. What still distinguishes suppliers is structural:
- Is the certificate per lot, or one document reused across products?
- Is the laboratory independent of the seller?
- Is the chromatogram included, or only the summary figure?
- Is measured fill published, including when it comes in under the label?
The failure mode identity catches
Purity being slightly lower than advertised is a quality issue. The vial containing a different compound entirely, or nothing active at all, is a different category of problem. Identity testing is what catches that, and it is the reason a certificate without a mass confirmation trace is worth much less than it appears.
What to ask for
Not “what is your purity”, which every supplier answers the same way. Ask “can I
see the certificate for the lot I would receive, with the trace on it”. The answer to that
one varies enormously.