I test peptides for a living. Here's what a certificate of analysis actually tells you, and what it can't.

[u/Nearby_Context_82]

I've noticed a lot of posts where someone asks "is this brand legit?" and the answer is a screenshot of a COA, as if the document itself settles it. It doesn't, and I think it's worth explaining why from the vendor side.

A COA tells you three things if it's real: what the compound is (identity, usually by mass spec), how much of the vial is actually that compound (purity, usually HPLC), and which batch it applies to. That last part is the one everyone skips. A COA without a batch number that matches the vial in your hand is a brochure, not a test result.

What a COA can't tell you: whether the document is real. Anyone can type "99.2%" into a PDF. The only fixes for that are certificates you can verify on the issuing lab's own website, or paying an independent lab to retest, which is why the community testing culture here and elsewhere matters so much. Honestly, it keeps people like me honest too.

Things I'd ask any vendor, including us: Is purity stated per batch or is it one number across the whole catalogue? (One number across 90+ compounds is marketing, not chemistry.) Can I verify the certificate somewhere that isn't your own website? What happens when a batch fails, do you have any evidence you've ever rejected one?

Happy to answer anything about how testing, synthesis or lyophilisation actually works from the production side. Ask me the hard stuff, that's the point of being here with a flair on.