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

One more thing on Purity assay versus content assay that took me far too long to see: the two figures people quote are not measuring the same quantity. Once you notice that, the apparent contradiction disappears.

The most useful reply I ever got about Purity assay versus content assay was a request to state my units. It sounds like pedantry and it has saved me twice.

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.

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.

That matches what I have seen, for whatever a single anecdote is worth.

The arithmetic in post #41 is right; the assumption feeding it is the part to check.

Two claims get bundled together under Purity assay versus content assay and they need separating. The descriptive one — this is what was observed — is usually well supported. The causal one — this is why — usually is not.

Almost every disagreement in threads like this one dissolves once you say which of the two you are making.

Understood. Thank you for being specific about the limits of it.

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.

I looked this up rather than remembered it, which is the right order.

Picking up post #74: that is the part I would want checked first.

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.