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PurityPaulOR said:

Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.

Third-party testing services for certificates of analysis besides Janoshik:

• Janoshik: Gold standard for peptide community, ~$146, 10-14 day turnaround
• Valitech: US-based, ISO 17025 accredited, ~$246
• Eurofins: Full pharmaceutical testing suite, more expensive but comprehensive
• ChemTox: Basic identity and purity testing, ~$116

Janoshik is the community standard but any ISO-accredited analytical lab can verify peptide identity and purity. The key is independence — don't rely solely on the vendor's own testing.

One thing that is still open after Dr.PulmRoch’s answer:

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

Dr.AddMedPHL said:

Third-party testing services for certificates of analysis besides Janoshik: Janoshik: Gold standard for peptide community, ~$146, 10-14 day turnaround…

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: 99.3% purity, 0.1% total related substances. Clean chromatogram with single sharp peak at expected retention time. Exactly what you want to see.

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.

NurseKim_ATL said:

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

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.

If somebody has the primary source to hand I would rather cite it than paraphrase it.