FitDadDave said:

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

Vendor quality matters when discussing certificates of analysis. I've tested my last 4 batches through Janoshik and purity has ranged from 99.1% to 100.5%. Consistency is key.

For anyone new: always request the COA, and consider independent verification. The $300 for a Janoshik test is worth the peace of mind.

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

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

josh_phd_bmore said:

Vendor quality matters when discussing certificates of analysis.

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

• Janoshik: Gold standard for peptide community, ~$147, 10-14 day turnaround
• Valitech: US-based, ISO 17025 accredited, ~$227
• Eurofins: Full pharmaceutical testing suite, more expensive but comprehensive
• ChemTox: Basic identity and purity testing, ~$127

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.

OP back with an update, since a thread like this is useless without one.

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.

anna.melb_AU said:

Third-party testing services for certificates of analysis besides Janoshik: Janoshik: Gold standard for peptide community, ~$147, 10-14 day turnaround…

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.