Purity assay versus content assay: two different numbers

On the subject in the title: Purity assay versus content assay: two different numbers Working notes rather than a conclusion.

Asking about purity assay versus content assay directly, because I have read four threads on it and each answered a slightly different question.

The version I want answered is the narrow one: given the method stated below, is the result within what anyone else has seen? I am not asking what it means yet.

Method, numbers and the two assumptions I am aware of making are below. If the assumptions are wrong that is more useful to me than agreement.

Peptide content and chromatographic purity are separate determinations and a certificate that reports only one has answered only one question. Content is the one that tells you how much is in the vial.

The signature block matters more than it appears to. A named individual with a date is a different kind of document from an unsigned one, whatever the numbers say.

I have deliberately not rounded that, because the rounding is where the argument starts.

Coming back to the opening post, because the follow-up matters more than the original answer.

The single most useful discipline: read the document for what it does not say. The omissions are more consistent between suppliers than the numbers are.

That is the version I would defend. It is not the version I started with.

That is the distinction I keep failing to hold on to. Written down now.

Counter-ion form is crucial: peptides are isolated as salts, most often trifluoroacetate from reversed-phase purification or acetate after salt exchange. The counter-ion is part of the mass in the vial and is not part of the peptide. Two vials of the same purity, one as TFA and one as acetate, contain different amounts of peptide per milligram.

Old habit: I write down the expected answer before I calculate it.

Water content (residual moisture) matters: lyophilised peptide is not dry. A few percent water is normal. The difference between a TFA salt and an acetate salt can be several percent. Content calculations need the water content to be accurate.

A document that puts the specification in the result column has told you the release criterion rather than the measurement. Those are different claims and the difference is the whole point of reading the document.

Adding this to the thread rather than to the wiki, because I am not confident enough for the wiki.

Adding the measurement that post #6 says would settle it.

Sterility testing is separate: chromatographic purity, mass spectrometry, and endotoxin testing all say nothing about whether a solution is sterile. If sterility matters, it needs to be tested and reported.

A partial answer, offered because a partial answer beats none.

Post #8 describes the usual case. This is about the unusual one.

Water content by Karl Fischer is worth having on a lyophilised product because residual water is both a mass contributor and a stability factor.

Posted with less confidence than the sentence structure implies.

Where two documents for the same lot disagree, the useful question is which was measured and which was transcribed. Most such disagreements are the second.

Noting that I have skin in this question and have tried to discount for it.