PeptideChemSF 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.

RetaRick_CA said:

Agreed, and mass spec answers a question purity does not: identity.

That reframing is the part I needed. Sending this to two other people who asked me the same thing last week.

PeptideChemSF said:

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

Community-verified vendors for certificates of analysis: I maintain a spreadsheet of Janoshik-verified test results submitted by community members. Across 62 submitted tests:

• 82% tested at ≥98% purity
• 12% tested at 95-98% purity
• 6% 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.

sean_dublin said:

I have a certificate reporting 99.2% purity and 6.1mg content on a vial sold as 5mg, and I cannot work out whether those two numbers are consistent or…

Reconstituted peptide stability data for certificates of analysis: I had Janoshik test my reconstituted vial at day 1, day 14, and day 28 stored in the refrigerator.

Day 1: 99.0% purity
Day 14: 98.5% purity
Day 28: 98.4% purity

Minimal degradation over 28 days when properly refrigerated. This confirms the standard 28-day beyond-use date is appropriate. Beyond 28 days, I'd reconstitute a fresh vial to be safe.

Moderator note: the sourcing question belongs in the vendor section and has been split out.