net peptide content vs purity — two different numbers, same vial
Posting this as a discussion rather than a claim: net peptide content vs purity — two different numbers, same vial.
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".
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.
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.
Happy to answer the boring questions. Those are usually the ones worth asking.
best — the order this archive was captured in
Area percent is the peak area of your compound over the total integrated area, at one wavelength, on one gradient. It is a relative measure of what the detector saw, not a mass fraction of the vial.
Right, and net peptide content is where the counterion and the water go. Both numbers can be honest and very different.
Right, and net peptide content is where the counterion and the water go.
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.
Area percent or net peptide content? They answer different questions.
The document I was worried about turned out to be a re-issue with a later creation date. Asked, got the original, entirely ordinary.