It's an easy assumption to make: a Certificate of Analysis showing 99%+ purity must mean the material is also sterile. The two are actually separate properties, tested for with separate methods, and a CoA that reports one doesn't automatically speak to the other. Knowing the difference matters for reading any CoA correctly.
Two different questions
Purity testing (typically RP-HPLC, as covered in our purity-testing article) asks how much of what's in the vial is the target peptide versus synthesis by-products. Sterility testing asks a completely different question: whether viable microorganisms are present in the sample at all.
A sample can be extremely pure by HPLC — meaning very little of it is anything other than the intended peptide — while still not having been tested for microbial contamination, because that requires an entirely separate assay looking for something HPLC purity analysis was never designed to detect.
Why the two aren't automatically linked
Purity is a chemical measurement of composition; sterility is a microbiological one. Neither test implies the other's result, because they're looking for entirely different things using different instruments and methods — an HPLC trace has no way to register the presence or absence of microorganisms.
Reading a CoA with this in mind
The practical takeaway is to read a CoA for exactly what it states rather than what it might imply. If sterility testing was performed, it should be listed as its own parameter with its own result — not inferred from a purity figure. Where a CoA doesn't mention sterility at all, that's worth taking as face value: it wasn't the test being reported.