In European Pharmacopoeia, 5.12 reference standard, it is mentioned that we can establish primary reference standard when sufficent tests have been done to identify and quantity the API purity and impurities.
So, in actual practice, how do we know what tests and how many of them should we do?
Can we perform full tests for an API in USP/BP/EP, then we determine potency :
Using the assay titration potency minus the water content, impurities mentioned in the monograph.
then use a validated chromatographic method to determine its assay purity without PRS and compare with the result above.