Coming back to: Purity assay versus content assay: two different numbers

The arithmetic in post #13 is right; the assumption feeding it is the part to check.

Practical experience of Purity assay versus content assay, offered as one case with the conditions stated, not as a general finding. Conditions first, because they are what make it interpretable.

I would be cautious about generalising from the Purity assay versus content assay example above. It is a good example. It is one example.

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.

That is all I can say without guessing.

The distinction between release testing and characterisation is one industrial documents make and retail ones usually do not. It is the difference between "we tested every lot for this" and "we established this once".

Nothing above should be read as advice about what anyone else should do.

Amino acid analysis gives content: it hydrolyses the peptide and quantifies the residues. It measures the peptide content independent of chromatographic purity. Very few supplier certificates report it.

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.

I would put the burden of proof on the interesting explanation, not the dull one.

Post #19 describes the usual case. This is about the unusual one.

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.

Happy to be the one who is wrong here if it settles the question.

Adding the measurement that post #19 says would settle it.

I think the Purity assay versus content assay question is answerable and has not been answered, which is a more optimistic position than most of this thread.

Adding a null result on Purity assay versus content assay. I looked, carefully, and found nothing, and null results deserve posting precisely because they never are.

A chromatogram supplied as a small image is legible for peak shape and not for baseline detail. That is enough to sanity-check an integration and not enough to reproduce it.

Adding the caveat now so it does not have to be extracted later.

Clear enough that I do not think I have a follow-up, which is unusual.

When a document puts the specification in the result column, it has told you the release criterion, not the measurement. Those are different claims and the second is weaker.

Purity assay versus content assay has been discussed here with more heat than it deserves, mostly because two definitions have been in play the whole time.

Post #27 is right about the mechanism and I think understates the practical bit.

The distinction between release testing and characterisation is one industrial documents make and retail ones usually do not. It is the difference between "we tested every lot for this" and "we established this once".

I am aware this is the third time this month I have made this point.

What would change my mind on Purity assay versus content assay is a second dataset collected by someone with no stake in the first. Until then I hold it loosely and I would rather say so than pretend to more.

This is the sort of exchange that makes the archive worth searching.