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
Sigma-Aldrich — Research-Grade Standards
Certified reference materials, analytical reagents, and research-grade standards for peptide verification. Trusted by laboratories worldwide.
Shop Reference StandardsClosing 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.