Purity vs identity: why one number isn’t enough

Ask most people how “good” a peptide is and they’ll quote a purity figure — 98%, 99%, 99.9%. It’s a useful number, but on its own it can be quietly misleading. Purity and identity answer two different questions, and you need both.

Purity answers “how clean?”

An HPLC purity percentage tells you how much of the sample is a single dominant component versus everything else. A 99% result means 99% of the material eluted as one main peak. What it does not tell you is what that peak actually is.

Identity answers “clean — but is it the right thing?”

Here’s the trap: a sample can be 99% pure and still be the wrong compound — a different sequence, a truncated version, or a similar molecule entirely. HPLC would happily report a clean single peak for any of them. Only mass spectrometry, which measures molecular weight against the theoretical value for the intended sequence, can confirm that the pure thing is the correct thing.

Purity without identity is a confident answer to the wrong question. “99% pure” is only meaningful once you know 99% pure of what.

Why both, every time

Run together, they close each other’s gaps — which is why a proper Certificate of Analysis reports both, and why we treat “verified” as meaning a batch passed purity, identity and endotoxin, not just one of them.