Purity, identity and quantity are three separate tests. A document proving one is routinely presented as though it proved all three.
what a coa actually is
A certificate of analysis is a lab report. Someone ran a sample through analytical equipment — usually high-performance liquid chromatography, sometimes paired with mass spectrometry — and this is the printout. It is a record of one test on one sample, not a warranty covering everything in the box.
three separate questions
Identity asks whether the molecule is the one named on the label, and that is mass spectrometry's job. Purity asks what fraction of the material is the target compound versus related impurities and truncated sequences, which is what the HPLC percentage describes. Content asks how many milligrams are actually in the vial, which is a different measurement again. A vendor can show you a 99% purity figure for a vial containing half the stated quantity of the wrong peptide, and technically nothing on that certificate is false.
Purity, identity and quantity are three separate tests.
what it never covers
Sterility. Endotoxin load. Residual solvents from synthesis. Heavy metals. None of these appear on a standard purity COA unless they were specifically commissioned, and they usually were not. For anything intended to cross skin, that omission is the whole ballgame — a chemically pure powder contaminated with bacterial endotoxin will still make someone very ill.
the batch question
The certificate must carry the same lot number as the vial in your hand. A single COA displayed on a product page, covering a batch synthesised two years ago, tells you nothing about current stock. Ask whether the lab was independent, and whether the sample was submitted anonymously or hand-picked and sent in by the vendor. Those two details separate a real quality programme from a marketing asset.