“≥99 %” is the most repeated number in this category and the easiest to misread. Purity is not an intrinsic property of a molecule name; it is the result of a specific measurement on a specific batch, and the method changes what the number means.
Purity is measured, usually by HPLC
For peptides, purity is typically reported as the area percentage of the main peak in a reversed-phase HPLC chromatogram. “99.4 %” means the target peak accounts for 99.4 % of the detected area; the remaining fraction is related substances and impurities. A purity figure without a method is incomplete.
What the number does not tell you
- Identity - purity says how much of one thing is present, not that it is the right thing. That is what LC-MS confirms.
- Batch - purity belongs to the batch that was tested. A number copied from an older batch is decoration.
- Other attributes - endotoxin, sterility and heavy-metal results are separate tests, not implied by a purity percentage.
How to read it
Treat a purity claim as a triplet: number, method, batch. If any of the three is missing, or the batch on the certificate does not match the vial, the figure cannot be verified.