How to read a peptide Certificate of Analysis

A Certificate of Analysis (COA) is the closest thing a research chemical has to a receipt for quality. It's a lab report, generated after a batch is tested, stating what was found in the vial and how confident the lab is in that finding. Reading one carefully is a basic research-hygiene skill, not a formality.

The HPLC purity figure is usually the headline number. High-performance liquid chromatography separates a sample into its component peaks; the purity percentage is the area of the main peak relative to everything else detected. A 99% figure means 1% of whatever eluted wasn't the target compound — that could be a synthesis byproduct, a degradation product, or residual solvent. It does not, by itself, tell you what that 1% is.

Mass spectrometry (MS) confirmation is the complementary check. Where HPLC tells you how much of a peak there is, MS tells you what that peak weighs — confirming the molecular weight matches the expected compound, rather than a similarly-eluting impurity with a passable purity number. A COA with HPLC purity but no MS confirmation is answering a different question than one with both.

Batch identifiers matter because purity is a property of a specific production run, not of the compound in general. A COA from a different batch — even of the identical molecule and supplier — describes a different sample. If a researcher wants to cite documentation for material actually in hand, the batch ID on the vial and the batch ID on the COA need to match.

None of this is a substitute for reading the actual document. A COA is a factual record, not a marketing claim — treat any supplier that can't produce one on request with the skepticism that deserves.