[Meta] the area percent rule is doing its job and people should stop complaining
Housekeeping: the area percent rule is doing its job and people should stop complaining.
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.
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.
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.
I will update this if the picture changes rather than quietly leaving it up.
best — the order this archive was captured in
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.
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Right, and net peptide content is where the counterion and the water go. Both numbers can be honest and very different.
Does the lot on the vial match the lot on the page?
ask for the storage condition the stability statement was written against
Sent one vial to VendorInvestigate and PeptideMeter. 97.8% and a claimed 97.0%. Cost me more than the vial and I would do it again.
Is water content on the document? It changes your concentration arithmetic.
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.
What does the method line actually say? Not the technique — the column and the gradient.
a total of 0.4% tells you less than four named impurities adding to 0.6%
the lot on the vial has to match the lot on the page
identity and quantity are two questions and most documents answer one
Not quite. Two labs with different gradients can resolve a close-eluting impurity differently and both be reporting honestly.
area% is not mass%, and the difference is the whole game
Ask what the stability statement was written against — temperature, light, and whether it was the lyophilised powder or the reconstituted solution. Those are three different claims.
Small fix — 214nm, not 280nm. It matters here because the impurity you are asking about barely absorbs at the longer wavelength.
HPLC-UV and LC-MS answer different questions
net peptide content and purity are two different numbers on the same vial
Left up and flaired Explainer. This is the answer we keep linking, written properly.
Area percent or net peptide content? They answer different questions.
That is not what the method line says. It names the technique; it does not certify the operator.
The PDF creation date being later than the analysis date is usually a re-issue or a re-export. It is worth one question and it is not evidence of anything on its own.
Correction: that is net peptide content, not purity. Different number, different meaning, and the document says so if you read the row label.
That is the catalogue certificate rather than the batch-specific one. Same layout, completely different status.
the related substances line is where the story is
Correction: that is net peptide content, not purity.
This is the sentence to take away. The rest of the thread is footnotes to it.
chromatogram or it did not happen
Are the related substances itemised or is it a single total?
Yes. The structure of the document matters at least as much as the number printed on it.
- 1identity and quantity are two questions and most documents answer one7 comments in this branch · started by u/alcohol_aversion