purity — 6 things I got wrong before I got it right
Something I keep coming back to: purity — 6 things I got wrong before I got it right.
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.
How to ask for a batch record without it turning into a negotiation.
One email, three requests: the batch-specific certificate for lot X, the method line behind it, and the storage condition the stability statement was written against. No preamble, no explanation of why you want it.
What comes back is the signal. A full document with the column and gradient printed on it, inside a day, tells you there is a real quality system behind the operation. That is the test I run before I order from anyone new, and it costs nothing.
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".
Corrections welcome, especially the pedantic ones. Pedantry is how this board earns its reputation.
best — the order this archive was captured in
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.
Spent four months confusing area% and mass% in every post I made here. One explainer thread and it clicked permanently.
Same view. WWB prints the column and the gradient, which means somebody could actually reproduce the run.
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 HJ 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.
Your name and address edited out of the screenshot above. The batch details are untouched and should stay.
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.
a certificate with no method line is decorative
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.
HPLC-UV and LC-MS answer different questions
water content is on there and everyone skips it
Sent one vial to Medutest and PeptideMeter. 98.3% and a claimed 98.0%. Cost me more than the vial and I would do it again.
Does the lot on the vial match the lot on the page?
Agreed on asking for the storage condition. It is one line and it changes how you read the entire stability claim.
net peptide content and purity are two different numbers on the same vial
Yes. The structure of the document matters at least as much as the number printed on it.
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.
- 1Your name and address edited out of the screenshot above. The batch details…11 comments in this branch · started by u/coa_collector