Dr.CardioMD said:

Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself…

This answered a question I did not know how to ask.

Adding the clinical framing, because it changes how the question reads.

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.

anna.melb_AU said:

Vendor quality matters when discussing certificates of analysis.

Community-verified vendors for certificates of analysis: I maintain a spreadsheet of Janoshik-verified test results submitted by community members. Across 55 submitted tests:

• 75% tested at ≥98% purity
• 18% tested at 95-98% purity
• 7% 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.

mona_PHX said:

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

I tested my certificates of analysis batch through Janoshik — 97.3% purity. That's within acceptable range for compounded peptides. Turnaround was 10 business days.

The test costs ~$120 but gives you actual HPLC chromatography data plus mass spec confirmation. For the peace of mind alone, it's worth every penny. I test every new batch from a new vendor.

Moderator note: a post naming a supplier has been edited. Discuss suppliers in the review sections, not here. Tagging this one for the weekly digest.