[PSA] purity is not what most of this community thinks it is
purity is not what most of this community thinks it is. Two minutes of reading now saves an argument later.
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".
Happy to answer the boring questions. Those are usually the ones worth asking.
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.
Right, and net peptide content is where the counterion and the water go. Both numbers can be honest and very different.
area% is not mass%, and the difference is the whole game
post the whole document, not the headline number
axis labels included, always
axis labels included, always
Agreed, and the practical version is: ask for the itemised table. If it exists, you get it.
a certificate with no method line is decorative
Did you get the chromatogram or only the summary line?
Sent one vial to Medutest and PeptideMeter. 98.8% and a claimed 98.0%. Cost me more than the vial and I would do it again.
a total of 0.4% tells you less than four named impurities adding to 0.6%
ask for the storage condition the stability statement was written against
HPLC-UV and LC-MS answer different questions
HPLC-UV and LC-MS answer different questions
Disagree on this specific point. Two honest labs can resolve that impurity differently and report different numbers.
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.
Yes — and the reason to ask for the whole page is that the interesting parts are the ones people crop out to fit a screenshot.
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.
This. Area percent answers "what fraction of what the detector saw was the peak I wanted", and people read it as "how much peptide is in the vial".
This.
freya_baptista has the distinction right — one is a fraction of the detector signal, the other is a fraction of the mass.
Does the lot on the vial match the lot on the page?
chromatogram or it did not happen
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.
Small fix — 214nm, not 280nm. It matters here because the impurity you are asking about barely absorbs at the longer wavelength.
Left up and flaired Explainer. This is the answer we keep linking, written properly.
Not quite. Two labs with different gradients can resolve a close-eluting impurity differently and both be reporting honestly.
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.
Pushing back a little: a missing field is usually presentation, not concealment. Ask for the full document before drawing conclusions.
That is the catalogue certificate rather than the batch-specific one. Same layout, completely different status.
Yes. The structure of the document matters at least as much as the number printed on it.
Correction: that is net peptide content, not purity. Different number, different meaning, and the document says so if you read the row label.
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.
- 1Did you get the chromatogram or only the summary line?8 comments in this branch · started by u/sig_figs_sam
- 2Left up and flaired Explainer. This is the answer we keep linking, written…8 comments in this branch · started by u/coa_janitor