Impurity thresholds: where the common numbers come from

Impurity thresholds: where the common numbers come from — setting out what I have, and where I think it stops being reliable.

I have spent a fortnight trying to pin impurity thresholds down and I want to set out where I have got to, because I suspect the honest answer is duller than the thread this will produce.

What I have: a consistent observation across a small number of cases, collected the same way each time. What I do not have: any controlled comparison, or any reason to think my cases are representative.

The specific question is whether the pattern survives once the obvious confounder is removed. I cannot remove it with what I have.

Coming back to post #3, because the follow-up matters more than the original answer.

Answering the impurity thresholds question as asked, then the question I think is meant. As asked: yes, with the qualification below. As meant: it depends on how the first measurement was taken.

What I would want before treating impurity thresholds as settled: the method, the sample, and whether anyone tried to find the opposite result. Two of the three are usually missing.

Trifluoroacetate is a counter-ion rather than an impurity, and it appears in the mass balance rather than in the chromatogram. Conflating the two accounts for several confused threads here.

Take the reasoning and check the arithmetic; I do not always get it right.

A supplier that can tell you what its principal impurity is has answered a harder question than one that can tell you its purity, and the answer is more useful.