How to read a Certificate of Analysis — complete guide
Pinned because it affects plans people have already made, not because it is dramatic. Everything below is what is confirmed as of today.
This post is the record, and it will be edited as things change — with the change noted rather than substituted. If you have something firmer than what is here, a document or a date or a first-hand account, post it and it goes in with credit.
Read it this way:
- What is stated is confirmed; what is uncertain is marked as uncertain
- Anything time-sensitive is worth verifying yourself before you act on it
- The replies below carry the corrections, so read them before asking
Nothing here is advice about your situation, and the situation is still moving.
COA_Karl said:How to read a Certificate of Analysis — complete guide Pinned because it affects plans people have already made, not because it is dramatic.
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.
COA_Karl said:How to read a Certificate of Analysis — complete guide Pinned because it affects plans people have already made, not because it is dramatic.
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.
Sigma-Aldrich — Research-Grade Standards
Certified reference materials, analytical reagents, and research-grade standards for peptide verification. Trusted by laboratories worldwide.
Shop Reference StandardsShort answer first, then the reasoning. Inter-laboratory variation comes from real methodological differences, not sloppiness: which reference standard was used and how it was itself calibrated, the gradient and column, the detection wavelength, and where the integration baseline was drawn. A percentage point or two between competent labs is expected. What is not expected is a large gap, and the usual explanation there is that one report is not of the batch in your hand.
Dr.RenalNash said:Agreed, and mass spec answers a question purity does not: identity.
Same position here, arrived at the long way round. They are not contradictory, they measure different things, and this is the single most common misreading of a certificate. Purity is a relative area measurement: the main peak as a percentage of total peak area in the chromatogram. Content is an absolute mass measurement against a reference standard. A vial can be 99% pure and contain 6.1mg — pure material, overfilled — or 99% pure and contain 3mg, which is pure material and a short fill. Purity tells you what fraction of what is there is the right molecule; content tells you how much of it there is. You need both, and a certificate with only purity is telling you half the story.