What even IS a COA and why should I care — my results so far

Dr.MetabolicMD 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 98.9% 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.

A narrower follow-up, since the general answer is now clear:

Why two laboratories report different purity for the same batch, and which number to believe?

HPLC_Greg 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, ~$136, 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, ~$126

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.

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.

ingrid_STO said:

Third-party testing services for certificates of analysis besides Janoshik: Janoshik: Gold standard for peptide community, ~$136, 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.