Inter-lab variability in peptide testing — July 2024

RetaRick_CA said:

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

I am more sceptical of certificates as a category than this thread is. The document tells you about a sample somebody chose to send, at a moment in time, and nothing about the vial in your hand. Independent purchase-and-test is a different quality of evidence and everything else is inference.

JennaRN said:

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

I tested my certificates of analysis batch through Janoshik — 97.6% purity. That's within acceptable range for compounded peptides. Turnaround was 10 business days.

The test costs ~$132 but gives you actual HPLC chromatography data plus mass spec confirmation. For the peace of mind alone, it's worth every penny. I test every new batch from a new vendor.

Dr.GastroMayo said:

I am more sceptical of certificates as a category than this thread is.

Community-verified vendors for certificates of analysis: I maintain a spreadsheet of Janoshik-verified test results submitted by community members. Across 56 submitted tests:

• 85% tested at ≥98% purity
• 10% tested at 95-98% purity
• 5% tested at <95% purity (concerning)

The vendors with consistently high purity results are listed in the Vendor Directory. We update monthly based on new community submissions.

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

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

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.