Most peptide buyers will never run a high-performance liquid chromatography instrument, and they do not need to. What they need is to read the paperwork the instrument produces. A certificate of analysis built on HPLC is the most common quality document a peptide buyer will see, and it is also the one most often misread. This guide walks through the five blocks of a typical COA and tells you which numbers actually carry the weight.
Start with the header. A legitimate COA names the compound, the lot number, the date of analysis, and the laboratory that performed the work. If any of those four fields is missing or generic, treat the document as marketing, not measurement. The lot number matters more than most buyers realise: purity claims that cannot be traced to a physical batch are not claims about anything you can hold in your hand.
Next, find the assay value, usually expressed as a percentage. For research peptides sold into the quality end of the market, expect values at or above 98 percent, with the method named explicitly. A number without a method is decoration. Reputable suppliers also report water content and counter-ion information where the salt form makes that relevant, and they do not hide the column type or detection wavelength in footnotes.
The third block is the impurity profile, and it is where serious suppliers separate from the rest. A strong COA lists individual related substances rather than a single aggregate figure, because aggregate impurity numbers can hide one large contaminant behind several small ones.
Fourth, look at who signed it. In-house quality control is fine as far as it goes, but stronger documentation pairs internal COAs with periodic third-party confirmation from an independent laboratory. Chain-of-custody language, even a single sentence describing how samples move from production to test bench, tells you the supplier expects to be audited.
Finally, check consistency across time. One excellent COA proves one excellent lot. Ask for the same document across three recent lots and compare the assay values. Tight clustering, within roughly half a percentage point, indicates a controlled process. Wandering values indicate a process that produces a good batch the way weather produces a good day.
None of this requires a chemistry degree. It requires patience and the willingness to read past the first number on the page. Suppliers that publish complete, lot-traceable documents make that patience easy instead of asking you to trust them. Ask for the paperwork. The good ones are proud of it.