Your COA Says 99% Pure. But Is It the Right Molecule?

A single number on a Certificate of Analysis gets treated as the whole verdict on quality more often than it should. Ninety-nine percent pure sounds definitive. It answers exactly one of three separate questions a research-grade peptide actually needs to clear, and it’s worth knowing which one.

Purity: what percentage is actually the target peptide

High-performance liquid chromatography, HPLC, separates a sample into its parts and measures what proportion of everything detected corresponds to the target compound. A 99% purity result means 99% of what showed up on the chromatogram matches the expected retention profile for that peptide. It’s a real, useful measurement, and it’s also the easiest one to look at without actually confirming anything about the molecule’s identity.

Identity: Is it actually the molecule on the label

This is where mass spectrometry does the job HPLC can’t. LC-MS measures the molecular weight of the compound directly, confirming whether it matches the expected mass for the labeled sequence. A sample can return a clean 99% HPLC purity result and still be 99% pure of an entirely different peptide, a wrong sequence, a truncated version, a synthesis byproduct that happens to co-elute at a similar retention time. Purity and identity are answering two different questions, and a purity number alone can’t stand in for an identity confirmation.

Contamination: what’s still in there that shouldn’t be

Neither purity nor identity testing catches bacterial endotoxin, a byproduct of gram-negative bacteria that can survive standard purification if manufacturing conditions weren’t tightly controlled. This gets tested separately, through the LAL assay under USP <85> or its European and Japanese equivalents, and it’s measuring something categorically different: not what the peptide is, but what’s riding along with it. A sample can pass purity and identity cleanly and still carry an endotoxin load worth knowing about before it’s used in any research application involving living systems.

Three tests, one honest picture

Put together, these three tests answer three separate questions: what’s in the sample, what is the molecule, and what else is present that shouldn’t be. A single purity figure on a product page answers the first and implies nothing about the other two. This is exactly the gap a lot-specific, third-party Certificate of Analysis is supposed to close, one that documents chromatography, mass spectrometry, and endotoxin results independently rather than leading with a single headline number.

None of this is a claim that a high purity figure is meaningless. It’s a claim that it’s incomplete on its own, and that a supplier willing to publish all three results, from an independent lab rather than in-house self-reporting, is answering a fundamentally different level of question than one publishing a purity percentage and calling it done.

References

Compendial endotoxin testing standards: United States Pharmacopeia <85>, European Pharmacopoeia 2.6.14, Japanese Pharmacopoeia 4.01.