Inter-lab variability in peptide testing — July 2024

TrialNerd_Beth said:

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

I am more sceptical of certificates as a category than this thread is. The document tells you about a sample somebody chose to send, at a moment in time, and nothing about the vial in your hand. Independent purchase-and-test is a different quality of evidence and everything else is inference.

PedsEndoPhilly 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…

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: 15.4 minutes (consistent with semaglutide reference standard)
Peak purity: 99.7%
Related substances: 0.6% (single impurity at 9.1 minutes — likely a synthesis intermediate)
Mass spec: m/z 4113.6 (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.

HPLC_Greg said:

I am more sceptical of certificates as a category than this thread is.

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 ~$126 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.

One thing that is still open after nancy_portland’s answer:

Why two laboratories report different purity for the same batch, and which number to believe?

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.