Selectivity Magnesium Hydroxide

Good afternoon.
We are preparing a validation protocol for the raw material Magnesium Hydroxide (method from USP 41) paste and in the item selectivity of the item content we were not doing it and justifying to the regulatory agency that the method has limitations for selectivity. Studying the subject we saw that it is possible, in addition to justifications, to put a complementary method to it. This information comes from ANVISA Brasil which is similar to ICH. Now I don’t know what this method would be and if this method would be used only for validation or if it would be an integral part of the raw material analytical method.