Understanding peptide purity percentages — what worked for you?

LondonLisa said:

The provenance questions are more useful than the numbers.

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

• Janoshik: Gold standard for peptide community, ~$131, 10-14 day turnaround
• Valitech: US-based, ISO 17025 accredited, ~$241
• Eurofins: Full pharmaceutical testing suite, more expensive but comprehensive
• ChemTox: Basic identity and purity testing, ~$121

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.

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

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

bri_stats said:

Third-party testing services for certificates of analysis besides Janoshik: Janoshik: Gold standard for peptide community, ~$131, 10-14 day turnaround…

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: 98.2% average across orders (Janoshik verified)
Customer service: Responsive within 6 hours
Price: $130/month for tirzepatide 10mg equivalent

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.

andrew_nyc 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.