amsterdam_pete said:

They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.

My vendor review relevant to certificates of analysis: ordered 6 times from my current compounder. Consistency has been excellent.

Shipping: 3-day with temperature monitor
Packaging: Professional, discreet, proper labeling with lot/batch numbers
Purity: 98.3% average across orders (Janoshik verified)
Customer service: Responsive within 1 hours
Price: $155/month for tirzepatide 10mg equivalent

Following on from jim_asheville — 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:

My vendor review relevant to certificates of analysis: ordered 6 times from my current compounder.

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

• Janoshik: Gold standard for peptide community, ~$138, 10-14 day turnaround
• Valitech: US-based, ISO 17025 accredited, ~$228
• Eurofins: Full pharmaceutical testing suite, more expensive but comprehensive
• ChemTox: Basic identity and purity testing, ~$108

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.

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.

newstart_MO said:

Third-party testing services for certificates of analysis besides Janoshik: Janoshik: Gold standard for peptide community, ~$138, 10-14 day turnaround…

That is correct as far as it goes, and here is where it stops going. 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.