Dr.BariatricHTX said:

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

HPLC results interpretation for certificates of analysis: when you get your Janoshik report back, here's what the numbers mean:

• Purity %: Should be >97%. Below 95% is concerning.
• Retention time: Should match reference standard for the peptide
• Related substances: Degradation products should be <2% total
• Single impurity: No individual impurity should exceed 1%

My last batch: 99.2% purity, 0.0% total related substances. Clean chromatogram with single sharp peak at expected retention time. Exactly what you want to see.

Following on from james_edin — and this may be the naive question:

Whether a purity figure and a content figure can disagree without one of them being wrong?

PeptideSynthNJ said:

HPLC results interpretation for certificates of analysis: when you get your Janoshik report back, here's what the numbers mean: Purity %: Should be…

My vendor review relevant to certificates of analysis: ordered 3 times from my current compounder. Consistency has been excellent.

Shipping: Next-day with cold pack
Packaging: Professional, discreet, proper labeling with lot/batch numbers
Purity: 98.4% average across orders (Janoshik verified)
Customer service: Responsive within 2 hours
Price: $156/month for tirzepatide 10mg equivalent

Reporting back.

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.ObesityMed said:

My vendor review relevant to certificates of analysis: ordered 3 times from my current compounder.

Agreeing with Dr.ObesityMed, and the qualification matters more than the agreement. One addition: an unstated method makes a report close to uninterpretable. "99% pure" without the technique, the standard and the conditions is a claim, not a measurement.