What actually differs between suppliers in this category, and what to check before you order from anyone.
Documentation is the product
Two vials of the same sequence from two suppliers are not the same material. What separates them is whether the analytical package is real, lot-specific, and reproducible on request.
What we publish
Each released lot has a Certificate of Analysis carrying its lot number, the analytical method, and the measured result. The certificate is published here at the same time the lot is released — not on request, and not as a generic specimen document reused across lots.
What we do not publish
We do not publish figures we cannot evidence. Where this site shows no number, it is because the data does not yet exist. A supplier that quotes an average purity across lots it has not yet sold is quoting an aspiration.
What to check anywhere
Ask whether the certificate names the lot you are buying. Ask which method produced the purity figure and at what wavelength. Ask whether the quoted molecular weight matches the sequence actually supplied — a mass that matches the parent protein rather than the fragment is a common and diagnostic error.