Certificates
Use the COA to compare the work, not the logo
Two certificates can both say “purity” and still describe different experiments. When you are choosing a laboratory, read the method line the way you would read a quote.
Start with the lot identifier and the laboratory name. Then separate three claims that get collapsed in marketing: identity, purity, and content. Identity might be a retention time, a UV spectrum, or a mass. Purity is usually an area percent. Content is a mass in the container. A laboratory that includes mass spectrometry in every report is offering a different base product from one that prices LC-MS as an add-on. Neither fact is a score. Both belong in the shortlist notes.
Next, look for the contamination lines your question actually needs, and for the method name. Endotoxin, a PCR screen, TAMC/TYMC, and a sterility method cited as USP <71> should not be written down as if they were one test. If the profile on this site says a method was not published, leave it blank in your notes too.
Finally, try the laboratory’s own check: a verify page, a QR code, or a report number. If the check exists only when the client opts in, that is a limitation, not a failure of the other laboratories. Do not run one laboratory’s PDF through another laboratory’s checker.
A longer teaching version of this reading order, with a recommendation attached, is on Peptide COA Guide. That site is operated by the same group. This directory does not carry the recommendation. The field list here is the one to use while you are on the comparison table.