Saw a COA doing the rounds with exactly 100.00% purity on it. Worth explaining what that number actually measures, because it's a lot narrower than it sounds.
HPLC purity is a comparison, not a headcount. The machine looks at everything it detected, then works out how much of that was your peptide. If nothing else showed up big enough to count, the software prints 100.00%. That's a legitimate result — it just means "we didn't find anything else worth listing," not "there's nothing else in here."
Four things it misses:
Impurities too small for the method to bother reporting
Anything hiding underneath the main peak
Stuff the detector can't see at the wavelength it was set to
Everything that isn't peptide at all — water, salts, leftover solvent
That last one is the big one. Freeze-dried peptide powder is usually only 75–90% peptide by weight. The rest is water and salts. HPLC purity doesn't touch that, because it's only looking at the peptide.
So a vial can be 99% pure and still hold noticeably less peptide than the label says. Both numbers are honest. Only one gets printed on the website.
Some labs won't print 100% at all, capping at 99.8% because that's what their method validation supports. Others will print it when nothing else turned up. Neither is wrong. It's a difference in how the lab validated its method, not how good your vial is.
Short version: read 100.00% as "nothing else showed up under this method." Then look for peptide content or assay on the same COA. That's the number that tells you what you actually bought.
Has anyone seen a COA that reports both? They seem rare, and it's the pair that actually matters.
(Prompted by a discussion @Krysia830073 was having on X.)