the certificate of analysis question that gets asked weekly, answered properly
the certificate of analysis question that gets asked weekly, answered properly, which sounds obvious until you try to state the evidence for it.
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.
Sent one vial to VendorInvestigate and PeptideMeter. 99.4% and a claimed 98.5%. Cost me more than the vial and I would do it again.
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.
If somebody has the same thing measured a different way, post it next to mine and we will see whether they agree.
best — the order this archive was captured in
The four things a certificate must carry, and what each one is for.
Batch identifier ties the document to your vial; without it the document is about somebody else’s material. Date of analysis tells you when the number was true. Method — column, gradient, wavelength — is what makes the figure reproducible rather than asserted. Individual related substances give you the shape of what else is in there.
Everything beyond those four is welcome. Anything missing from them is a question to ask before you order, and in my experience the answer arrives quickly from the suppliers who have a batch record to draw it from.
axis labels included, always
ask for the storage condition the stability statement was written against
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".
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.
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.
I would not read a rounded figure as evasive. Plenty of documents round to one decimal as house style.
Careful — "the metadata looks odd" is a reason to ask a question, not a conclusion. Re-issues are ordinary.
Agreed — the itemised related substances table is the difference between a document and a receipt.
the related substances line is where the story is
water content is on there and everyone skips it
Does the lot on the vial match the lot on the page?
Batch number and date of analysis, are both on there?
Agreed on asking for the storage condition. It is one line and it changes how you read the entire stability claim.
Area percent or net peptide content? They answer different questions.
batch number, date of analysis, method, individual impurities — that is the minimum set
redact your own name, never the batch details
the lot on the vial has to match the lot on the page