We get asked fairly often what separates a research peptide that's worth buying from one that isn't. Most of the answer happens before a single vial ships: manufacturer vetting.
When a new supplier comes across our desk, we start with documentation. We want to see a clear certificate of analysis tied to a specific lot, not a generic spec sheet pulled from a catalog. The CoA should report mass by HPLC, identity by mass spectrometry, and a measured purity figure, not just a claim.
If the paperwork holds up, we order an unmarked sample and send it to an independent lab, not the one the manufacturer uses. We re-run identity and purity, and we look at the impurity profile. A 99% purity number is meaningless if the remaining 1% is an unknown compound the supplier can't account for.
Only after two independent test rounds across two separate lots do we add a manufacturer to our active list. It is slower and more expensive than spot buying, but it is the only way we have found to keep what's on the label consistent with what's in the vial.