Purity, in the pharmaceutical sense, is a specific claim: it says that a defined percentage of the mass in a container is the target molecule, and it implies that the balance — the impurities — has been characterized or at least bounded. For peptides sold outside the approved-drug supply chain, there is no regulator making that claim on the buyer's behalf. The claim, if it exists at all, is being made by the seller. Third-party purity testing is what separates a seller's assertion from an independently verifiable fact.

Where impurities come from

Peptides are manufactured by solid-phase peptide synthesis (SPPS), a process that builds the target sequence one amino acid at a time on a solid resin. The chemistry is well understood and, at industrial scale, capable of very high purity. It is also capable, when scaled poorly or run without appropriate quality controls, of producing recognizable classes of impurities: deletion sequences (where a coupling step failed and an amino acid was skipped), truncated sequences, oxidation products, aggregation products, and residual solvents.^[1]

None of these is exotic. They are the standard impurity profile that quality-control chemists in the pharmaceutical industry monitor and specify. Regulated drug manufacturers publish specifications with defined acceptance criteria for each named impurity class. Research-peptide sellers rarely do.

What 'purity' actually measures

A percentage-purity figure — '99.2% purity by HPLC' — is a specific measurement produced by a specific method. It typically reflects the fraction of UV-absorbing peak area attributable to the main peak in a chromatogram, integrated across a defined wavelength range.^[1] It is not a direct mass fraction; it is a chromatographic ratio, and its accuracy depends on the method used, the wavelength chosen, the peak-purity assessment, and whether impurities that do not absorb at the chosen wavelength have been separately controlled for.

This matters because two 'purities' can be numerically similar and analytically incomparable. A seller reporting HPLC purity at 220 nm and a seller reporting purity at 254 nm are answering related but not identical questions. A serious CoA describes the method well enough that the number can be interpreted; a decorative CoA reports the number and hopes it will not be interrogated.

The counterfeit and substitution question

Independent testing of grey-market peptide products, when it has been performed, has repeatedly found products that did not contain the labeled compound at all, contained it at a fraction of the labeled amount, or contained substituted molecules of related structure.^[2] This is a category of failure that identity testing (mass spectrometry) directly addresses and that visual inspection cannot address at all. A white powder is a white powder. A mass spectrum is either consistent with the labeled sequence or it is not.

Counterfeiting is not evenly distributed across sellers and it is not evenly distributed across categories, but its existence is a stable feature of an unregulated market. Third-party identity testing is the specific control that catches it.

Contamination categories that matter for injectables

For peptides intended to be reconstituted and injected, three contamination categories carry specific injection-related risks. Bacterial contamination — addressed by sterility testing under USP <71> or an equivalent framework — is the most familiar.^[3] Bacterial endotoxin — addressed by USP <85> or equivalent — is a heat-stable component of Gram-negative cell walls that survives filter sterilization and can provoke a systemic response.^[4] Residual solvent — often from synthesis and lyophilization steps — is a third category, controlled under ICH Q3C in regulated manufacturing.

None of these is exotic. All are standard specifications for injectable pharmaceuticals. Their absence from a research-peptide CoA is a specific silence about specific risks, not a general assertion of quality.

Independence is the property that matters

In-house testing is not worthless; a seller that runs an internal quality lab and publishes internal results has done more than one that publishes nothing. But an in-house CoA has an obvious structural conflict — the same organization that would lose revenue from a failing result decides whether to publish it. Independence eliminates that specific conflict. An ISO/IEC 17025-accredited third-party lab with a named contact, a report number, and no financial stake in the outcome is a different order of evidence.

What testing cannot do

A full third-party purity and contamination panel does not answer whether the compound is safe or effective for any specific use in any specific person; it answers what is in the vial and what is not. That is a necessary condition for any meaningful downstream conversation about the compound. It is not a sufficient one. Reading testing carefully is the price of admission to that downstream conversation, not a shortcut around it.