help me understand purity, I have read the wiki twice
Asking properly rather than in a comment on somebody else’s thread: help me understand purity, I have read the wiki twice.
LC-MS gives you identity; UV purity gives you a relative quantity. A document with the first and not the second, or the reverse, is answering half the question.
Asked GL Biochem for the batch-specific certificate and had the full document — batch, date, method line naming the column — inside six hours. That is now my baseline for everybody.
Sent one vial to Medutest and Janoshik. 98.8% and a claimed 98.0%. Cost me more than the vial and I would do it again.
That is everything I have. The rest is opinion and I have tried to keep it out.
best — the order this archive was captured in
Related substances itemised individually is a much stronger document than a single total, because it tells you the shape of the impurity profile rather than its size.
Does the lot on the vial match the lot on the page?
Spent four months confusing area% and mass% in every post I made here. One explainer thread and it clicked permanently.
UV at 214nm sees the peptide bond and picks up almost everything; at 280nm you are looking at aromatics only. Which wavelength the number came from is part of the number.
Right, and net peptide content is where the counterion and the water go. Both numbers can be honest and very different.
Small fix — 214nm, not 280nm. It matters here because the impurity you are asking about barely absorbs at the longer wavelength.
I would not read a rounded figure as evasive. Plenty of documents round to one decimal as house style.
Removed — cropped document. Post the whole page or do not post it; the fields people crop are the fields that matter.
Batch number and date of analysis, are both on there?
Batch number and date of analysis, are both on there?
This is the sentence to take away. The rest of the thread is footnotes to it.
Asked for the storage condition behind the stability statement. Got it in a sentence. It is a two-line email and almost nobody sends it.
Why the same vial can honestly produce three different numbers.
Purity by area percent is relative to what the detector saw on one gradient at one wavelength. Net peptide content subtracts counterion, water and residual solvent. Mass of peptide per vial is net content times fill weight, and fill weight has its own tolerance.
So a document reading 99% purity, 6% acetate and 4% water is describing a vial that is about 90% peptide by mass, and every one of those numbers is honest. The mistake is treating the headline figure as the answer to "how much peptide did I get".
My first certificate had a compound name, a percentage and a logo. I thought that was normal. It is not, and now I know what to ask for.
Net peptide content is the fraction of the powder that is actually peptide once counterion, water and residual solvent are accounted for. A 99% pure acetate salt can still be about 85% peptide by mass.
the lot on the vial has to match the lot on the page
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Area percent or net peptide content? They answer different questions.
Started keeping a folder of every certificate I have been sent. The ones that itemise impurities are a small group and they are mostly the same suppliers.
water content is on there and everyone skips it
post the whole document, not the headline number
Water content 3.8%, acetate 6.2%, purity 99%, net peptide about 90% of label weight. All four numbers honest, and I had to read it twice.
The document I was worried about turned out to be a re-issue with a later creation date. Asked, got the original, entirely ordinary.
- 1Net peptide content is the fraction of the powder that is actually peptide…10 comments in this branch · started by u/dead_space_doug