Dr.NutriCornell 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.
SkepticalSean said:Two reports on the same product from two laboratories, four percentage points apart, and no method stated on either.
Vendor quality matters when discussing certificates of analysis. I've tested my last 5 batches through Janoshik and purity has ranged from 98.4% to 99.6%. 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.
pete_manc_UK said:I am more sceptical of certificates as a category than this thread is.
Reconstituted peptide stability data for certificates of analysis: I had Janoshik test my reconstituted vial at day 1, day 14, and day 28 stored in the refrigerator.
Day 1: 100.7% purity
Day 14: 99.0% purity
Day 28: 98.1% purity
Minimal degradation over 28 days when properly refrigerated. This confirms the standard 28-day beyond-use date is appropriate. Beyond 28 days, I'd reconstitute a fresh vial to be safe.
Sigma-Aldrich — Research-Grade Standards
Certified reference materials, analytical reagents, and research-grade standards for peptide verification. Trusted by laboratories worldwide.
Shop Reference StandardsFollowing on from Dr.Martinez — and this may be the naive question:
Whether a purity figure and a content figure can disagree without one of them being wrong?
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.