A certificate of analysis runs to a page or two, but five numbers decide whether it means anything. This is a plain checklist, written for buyers rather than chemists.
Number one is the assay value, the purity percentage. For research peptides at the quality end of the market, expect 98.0 percent or better by HPLC-UV. The method must be named. A purity figure with no method listed is a marketing number, not a measurement.
Number two is the impurity profile. USP <233> governs elemental impurities, but the organic impurities section is where suppliers show their honesty. A serious COA lists individual related substances. A lazy one reports a single aggregate, which can hide one large contaminant behind a row of small ones.
Number three is water content, usually by Karl Fischer titration. Hygroscopic peptides drift on this number lot to lot, and drift predicts stability failures later. Tight clustering across three lots, within about half a percentage point, indicates process control.
Number four is the counterion or salt form, most commonly acetate or TFA. Trifluoroacetate residuals matter for any application touching cell work. The number should be present, method-referenced, and consistent.
Number five is the accreditation of the testing laboratory. ISO/IEC 17025:2017 is the baseline. The lab name on the COA should resolve to an accredited scope that actually covers the method used.
One honest limit: a certificate describes the lot that was tested, not every vial that ships. A supplier could document well and still ship badly, so treat a COA as a starting point, not proof.