kate.chem said:

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

Community-verified vendors for certificates of analysis: I maintain a spreadsheet of Janoshik-verified test results submitted by community members. Across 49 submitted tests:

• 78% tested at ≥98% purity
• 15% 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.

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?

Dr.NateNeph said:

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

My vendor review relevant to certificates of analysis: ordered 5 times from my current compounder. Consistency has been excellent.

Shipping: Same-day local pickup
Packaging: Professional, discreet, proper labeling with lot/batch numbers
Purity: 99.2% average across orders (Janoshik verified)
Customer service: Responsive within 6 hours
Price: $146/month for semaglutide 1.7mg equivalent

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.

Dr.SportsMedIN said:

My vendor review relevant to certificates of analysis: ordered 5 times from my current compounder.

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.

Happy to go further on any of that.