What a certificate of analysis actually proves, and what it does not

A certificate of analysis is the single most-cited document in research-peptide sourcing, and also the most frequently misread. Researchers see a purity percentage near the top, usually 98% or higher, and treat the whole document as a pass or fail stamp. That number is real, but it is answering a narrower question than most people assume.

What the purity number is actually measuring

A COA's purity figure typically comes from high-performance liquid chromatography (HPLC), which separates a sample into its component compounds and measures what fraction of the total signal belongs to the intended peptide versus everything else, degradation products, synthesis byproducts, residual solvents. A 99% HPLC purity result means 99% of what the instrument detected matches the target compound's expected retention profile. It does not, on its own, confirm that the detected compound is structurally correct.

That confirmation comes from mass spectrometry (MS), which verifies molecular weight and can catch structural errors, a missing amino acid, an incorrect sequence, a synthesis mistake, that HPLC purity alone would not necessarily flag. A COA that reports HPLC purity without a corresponding MS identity check is verifying that the sample is clean, not that it is the right molecule. Both tests are checking different things, and a complete COA states results for both, not just one.

The questions a purity percentage does not answer

• Who ran the test. An in-house lab result is not the same claim as third-party verification. The manufacturer testing its own product has no independent check on the result.
• When it was run. A COA generated at the time of synthesis says nothing about the compound's condition after shipping, handling, or storage between then and when it reaches a researcher.
• Which batch it covers. A COA is batch-specific. A lot number printed on the vial should trace back to the exact certificate for that production run, not a generic template reused across batches.
• What else was screened for. Heavy metals, endotoxins, and residual solvents are separate tests from purity and identity, and not every COA includes them.

Third-party testing is the part most worth checking for

In-house testing is not worthless, but it is the manufacturer grading its own work. Independent third-party verification, a separate lab with no financial stake in the result, is the closest thing this field has to an audit. When comparing sourcing options, the presence of a named third-party lab on the COA is a stronger signal than the purity number itself, because it means someone other than the seller checked the claim.

A COA is a starting point, not a verdict

None of this means purity numbers are meaningless, they are a necessary baseline. It means treating a single percentage as the entire due-diligence process is a mistake. A COA worth trusting names its testing method (HPLC and MS, not one or the other), identifies who ran it, ties to a specific batch and lot number, and ideally comes from or is verified by a source independent of the seller. Anything short of that is a partial answer dressed up as a complete one.