[Discussion] can we stop arguing about water content until somebody posts a number
Trying to get a straight answer on this: can we stop arguing about water content until somebody posts a number.
The document I was worried about turned out to be a re-issue with a later creation date. Asked, got the original, entirely ordinary.
Sent one vial to Medutest and Janoshik. 99.3% and a claimed 98.5%. Cost me more than the vial and I would do it again.
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.
Would rather be corrected in public than confident in private.
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.
post the whole document, not the headline number
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.
Area percent or net peptide content? They answer different questions.
Did you get the chromatogram or only the summary line?
area% is not mass%, and the difference is the whole game
Correcting myself upthread: I said the date was missing and it is on the second page, which I had not scrolled to.
Not quite. Two labs with different gradients can resolve a close-eluting impurity differently and both be reporting honestly.
a total of 0.4% tells you less than four named impurities adding to 0.6%
chromatogram or it did not happen
the related substances line is where the story is
Same view. Homopeptide prints the column and the gradient, which means somebody could actually reproduce the run.
identity and quantity are two questions and most documents answer one
Does the lot on the vial match the lot on the page?
Pushing back a little: a missing field is usually presentation, not concealment. Ask for the full document before drawing conclusions.
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.
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.
the lot on the vial has to match the lot on the page
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.
a certificate with no method line is decorative
Cosigning. The batch identifier is load-bearing, and a certificate that cannot be tied to the vial in your hand is not about your vial.
Are the related substances itemised or is it a single total?
water content is on there and everyone skips it
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.
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.
- 1Area percent or net peptide content? They answer different questions.9 comments in this branch · started by u/sofia_danquah