A buyer sends a one-line request: "we need 99% GHK-Cu, 500 g". Before we quote, we ask which method produced that 99%. Most specification disputes we see start here, not at the price.
Two suppliers, both 99%, different numbers
HPLC purity is not one universal measurement. Area normalisation reports the main peak as a share of all detected peaks; an external-standard assay compares it with a weighed reference. The first says little about non-UV-active impurities or moisture, the second raises a mass-balance question. Both can honestly be printed as 99%.
A worked example you can recompute. Assume a COA states 99.0% by area normalisation on the dried portion and 1.8% water by Karl Fischer. In 100 g of powder there are 98.2 g of dry material, and 99% of that is about 97.2 g of active - not the 99 g you would get by ignoring the water line. These are stated assumptions, not a real COA, but the arithmetic is the point: two lines on one document move the charge by about 1.8 g per 100 g.
Five fields to write into the purchase specification
1) Purity method: HPLC column type, gradient, detection wavelength, and whether the figure is area normalisation or against an external standard.
2) Identity: MS molecular weight against the theoretical value, plus comparison with a named reference standard. A purity number alone cannot tell you which peptide is in the drum.
3) Water and residual solvents: state the method and the limit. Both change the mass you actually weigh.
4) Impurity reporting: which related substances are controlled individually, and what the reporting threshold is.
5) Batch traceability: the lot number on the COA must match the label on the pack and the packing list.
What to ask for before the first order
Ask for a redacted COA from a recent batch, the chromatogram behind it, and a short method description. A supplier who can send those three items usually has a working release process. A supplier who can only send a marketing spec sheet is asking you to approve a number that nobody has to reproduce.
We also ask buyers to run one check in their own lab on the sample. The point is not to catch us out - it is to make sure both labs integrate the same peaks the same way before a 10 kg order is released.
Where the price difference comes from
A tighter purity window, individual impurity limits and a validated method all cost time in purification and analysis. If the brief only needs a cosmetic-grade active at a low inclusion level, a pharmaceutical-style impurity profile may not change the finished product at all. Tell us the claim you need to support and we will say plainly whether the tighter grade is worth paying for.
A short checklist
Method, identity, water, impurities, traceability. Send those five lines with your enquiry and the quotations you get back become comparable between suppliers - which is the only way a price comparison means anything.