Question of the month - March 2020
My active pharmaceutical ingredient (API) is present in the hydrochloride form.
Does that mean I can only use impurity reference standards which are also in the hydrochloride form?
No. As far as we can tell, from our own experience and that of our customers, it does not matter in what salt – or even free base – form your impurity reference standards are present.
If an API is present as an HCl salt, the impurities in it might be present as an HCl salt or a free base, maybe even as another salt. No matter which, in HPLC the mobile phase determines in what solvated form each impurity passes the column/detector, so retention time, UV spectrum and also mass spectrum will be similar, regardless of the form in which the impurity is present in the tested API sample. Therefore it also does not matter in what form your impurity reference standard is present. But, of course, you do need to know exactly which form this is, in particular if you want to quantify with that impurity standard.