Understanding peptide purity percentages — 98% vs 99% clinical significance

DanielChem_CHI said:

They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.

My vendor review relevant to certificates of analysis: ordered 4 times from my current compounder. Consistency has been excellent.

Shipping: 2-day priority with gel ice
Packaging: Professional, discreet, proper labeling with lot/batch numbers
Purity: 97.5% average across orders (Janoshik verified)
Customer service: Responsive within 4 hours
Price: $149/month for semaglutide 2.4mg equivalent

A narrower follow-up, since the general answer is now clear:

Why two laboratories report different purity for the same batch, and which number to believe?

dan_philly said:

My vendor review relevant to certificates of analysis: ordered 4 times from my current compounder.

Mass spectrometry confirmation for certificates of analysis: beyond HPLC purity, mass spec tells you the molecule's exact molecular weight. This confirms identity — not just purity.

Why this matters: HPLC can show you a "pure" compound, but if it's pure of the WRONG compound, purity is meaningless. Mass spec at m/z 4113.58 Da confirms it's actually semaglutide. My Janoshik report includes both HPLC and mass spec.

If your testing provider only offers HPLC, consider upgrading to include mass spec for at least your first batch from a new vendor.

Closing the loop on my own question.

Purity and content being different measurements was the piece I was missing. The vial was overfilled and pure, which is a very different problem from the one I thought I had.

sarah_TO said:

Mass spectrometry confirmation for certificates of analysis: beyond HPLC purity, mass spec tells you the molecule's exact molecular weight.

Agreed, and mass spec answers a question purity does not: identity. High purity tells you one thing dominates the chromatogram, not that it is the molecule you ordered. A mass within about a dalton of expected is the identity check.

Correct me if the detail matters more than I have assumed.