[Meta] the purity rule is doing its job and people should stop complaining
the purity rule is doing its job and people should stop complaining. Disagreement welcome, but bring a concrete alternative wording.
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.
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.
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.
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
Reminder: a question about a document is not an accusation about a supplier, and posts that blur the two get retitled.
Small fix — 214nm, not 280nm. It matters here because the impurity you are asking about barely absorbs at the longer wavelength.
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.
Agreed on asking for the storage condition. It is one line and it changes how you read the entire stability claim.
Correcting myself upthread: I said the date was missing and it is on the second page, which I had not scrolled to.
That is the catalogue certificate rather than the batch-specific one. Same layout, completely different status.
Sent one vial to Medutest and Janoshik. 99.6% and a claimed 99.0%. Cost me more than the vial and I would do it again.
post the whole document, not the headline number
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.
Is water content on the document? It changes your concentration arithmetic.
Batch number and date of analysis, are both on there?
Agreed — the itemised related substances table is the difference between a document and a receipt.
Agreed — the itemised related substances table is the difference between a document and a receipt.
Adding the bit that always gets missed: the lot on the vial has to match the lot on the page, or none of this applies to your material.
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.
water content is on there and everyone skips it
Careful — "the metadata looks odd" is a reason to ask a question, not a conclusion. Re-issues are ordinary.
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.
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.
Not quite. Two labs with different gradients can resolve a close-eluting impurity differently and both be reporting honestly.