What a purity test proves — and doesn't

A certificate of analysis — a COA — is a lab report on one sample. The number everyone quotes from it is purity: the percentage of the material that is the intended peptide, usually measured by HPLC, a technique that separates a mixture into its components and measures how much of each is present. 99% purity means 99% of what is in the vial is, chemically, the right molecule.

Here is what that number does not tell you. It does not tell you how much is in the vial. A vial labelled 5 mg could contain 3 mg of 99%-pure peptide, or 7 mg. That is a separate measurement — net content, or assay — and it fails independently of purity. In independent testing of grey-market products, purity is usually fine and it is the *quantity* that varies, sometimes by 20–40% in either direction.

It also does not tell you the peptide is the one on the label. Purity says "99% of this is one substance"; identity — confirmed by mass spectrometry, which weighs the molecule — says which substance. A vial can be 99% pure and 99% the wrong peptide. Good COAs report both.

And it says nothing about sterility or endotoxins — bacterial fragments that survive filtration and cause fever when injected — or about heavy metals and residual solvents from synthesis. Those are separate tests, often not run.

Finally, a COA describes one sample from one batch, on one day. A vendor's own COA describes the sample the vendor chose to send. An independent test describes a sample someone else bought. Neither says anything about the next batch. The honest reading of any COA is: "this particular vial, on this date, had these properties." Everything beyond that is inference.

None of this bears on whether the compound *does* anything — that is the evidence matrix's job. A perfectly pure, correctly dosed, sterile vial of a grade-D peptide is still a grade-D peptide.

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