Dr.ObesityMed said:They are not contradictory, they measure different things, and this is the single most common misreading of a certificate.
Agreed, and mass spec answers a question purity does not: identity. High purity tells you one thing dominates the chromatogram, not that it is the molecule you ordered. A mass within about a dalton of expected is the identity check.
Ask again with the specifics and you will get a better answer than this one.
FitDadDave said:Right, and one flattering data point from a group buy is not consistency.
Saving this. It is the first explanation that did not require me to already understand it. Adding it to my notes with a link back to this thread.
Dr.ObesityMed 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.
Worth separating that from certificates of analysis, which this thread keeps folding into the same question. They behave differently and the advice does not transfer.
Sigma-Aldrich — Research-Grade Standards
Certified reference materials, analytical reagents, and research-grade standards for peptide verification. Trusted by laboratories worldwide.
Shop Reference StandardsPurityPaulOR said:Six orders from the same supplier over eight months, every one independently tested, and the consistency is the part worth reporting rather than any…
Vendor reputation tracking for vendor vetting: I maintain a personal spreadsheet with every order I've placed — date, vendor, batch number, Janoshik result, personal efficacy assessment (1-5 scale), and any issues.
After 14 orders across 5 vendors: average purity 97.2%, zero sterility concerns, 0 shipping issues (all resolved). This data gives me confidence in my current source and documentation if issues arise.
I encourage everyone to track their orders systematically. Community data aggregation makes us all safer.
Moderator note: two posts asking for a source have been merged into one. Please search the thread before asking again. Thread quality here is what the rules are for. Keep it up.