what does a certificate of analysis actually tell you?
COAs get cited like they settle things and I'm slowly realising they're narrower than people think.
What one typically reports: identity (often mass spec, confirms the molecular weight matches) and purity (usually HPLC, what fraction of the material is the target compound). Sometimes water content.
What that establishes: a tested sample was mostly the stated compound.
What it doesn't: that your specific vial came from that batch, what the impurities actually are (98% pure says nothing about the other 2%), sterility or endotoxin, or that the analysis was independent.
The identity vs purity distinction is the one I'd underline. Deliberately not getting into who provides what, that's a different topic and not one for this thread. Have I got this right?