Inter-lab variability in peptide testing — July 2024

Dr.AddMedPHL said:

The provenance questions are more useful than the numbers.

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.

Correct me if the detail matters more than I have assumed.

Dr.GastroMayo said:

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

Community-verified vendors for certificates of analysis: I maintain a spreadsheet of Janoshik-verified test results submitted by community members. Across 57 submitted tests:

• 78% tested at ≥98% purity
• 15% tested at 95-98% purity
• 7% 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.

DataDave said:

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

I tested my certificates of analysis batch through Janoshik — 98.4% purity. That's within acceptable range for compounded peptides. Turnaround was 10 business days.

The test costs ~$136 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.

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

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

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.