Quality and Testing
What a purity figure actually certifies, and the difference between purity and net peptide content.
Almost every peptide sold carries a purity figure, and almost nobody reads it correctly. Purity describes the proportion of the peptide present that is the correct sequence. It does not describe how much peptide is in the vial. Those are two different measurements, produced by two different assays, and a product can score highly on one while carrying substantially less material than the label weight suggests.
The gap between them is not fraud. It is trifluoroacetic acid left over from synthesis, residual bound water, and residual solvent, all of which are normal consequences of how peptides are manufactured. But the gap is real, it is typically 15 to 25% of the mass, and it is the single most useful thing to understand when comparing two products that look identical on paper.
This section covers what each number on a certificate of analysis means, which fields carry information and which are decoration, and what to ask a supplier for when the certificate does not say.