Chromatography Forum

Hi all,
Now I am performing an impurity method validation. There are three isomers of one of known impurity, and they are separated very well in the LC analysis. Do I need to monitor all of them? or only major one? Do I need to do Accuracy, linearity etc for all of these isomers?
The three purity of the isomers in the material is ~40%, 20% and 10%.
Thank you very much in advance.

Any suggestion?

There's no way to answer that without knowing a *lot* more about the API and the impurities (such as their relevance to the synthesis of the compound and their biological effects).

The most general answer is that they are individual compounds and should be accounted for individually. If a plausible argument can be made that the ratios should be constant (e.g., from the synthesis or degradation pathway) then you *might* be justified in combining them, but if I were reviewing the method, I'd want to see validation data demonstrating that the ratios are, in fact, constant.

Thank you very much, Tom.
The API is hydrocortisone and the impurity is hydrocortisone symmetrical dimers, a synthesis process impurity. if the ratio is constant, when I do accuracy/recovery test, for example, 0.1% level, Do I need to treat them as "one" compound or three compounds?

They *are* three compounds. If you could demonstrate (as part of the validation) that the ratios are indeed constant, then *I* would accept reporting the total. The catch is that I'm not the one you need to convince: it's the person reviewing the method or the results.

This is the kind of situation where an SOP is invaluable: it gives you an ironclad answer if a reviewer nitpicks.

Thank you very much. I appreciate it.