Understanding peptide purity percentages — what worked for you?

paige_pharma said:

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

Same experience, arrived at from the opposite direction.

Following on from Dr.PeteFamMed — and this may be the naive question:

Did your prescriber agree with that reading, and if not what was their objection?

PeptideChemSF said:

HPLC results interpretation for certificates of analysis: when you get your Janoshik report back, here's what the numbers mean: Purity %: Should be…

Community-verified vendors for certificates of analysis: I maintain a spreadsheet of Janoshik-verified test results submitted by community members. Across 56 submitted tests:

• 85% tested at ≥98% purity
• 10% tested at 95-98% purity
• 5% tested at <95% purity (concerning)

The vendors with consistently high purity results are listed in the Vendor Directory. We update monthly based on new community submissions.

PeptideChemSF said:

HPLC results interpretation for certificates of analysis: when you get your Janoshik report back, here's what the numbers mean: Purity %: Should be…

My HPLC chromatogram analysis for certificates of analysis — I sent my batch to Janoshik and here's what the results showed:

Main peak retention time: 14.8 minutes (consistent with semaglutide reference standard)
Peak purity: 98.6%
Related substances: 0.5% (single impurity at 8.6 minutes — likely a synthesis intermediate)
Mass spec: m/z 4113.58 (matches expected molecular weight)

This is a clean result. The mass spec confirmation is particularly important — it proves the molecule is actually semaglutide and not a substitution.

Moderator note: leaving this open. It is being argued well and the disagreement is the useful part.