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About assay & purity

What is the differnce between Assay & Purity?
I want its calculation Formula?
What is the diffarance between External standard & internal standard?
What is the diffarance between Primary standard & secondary standard?
I can’t get a sufficient answer which make me satisfied.

My comments are as follows :-
1.Difference between Assay and Purity :- Assay Methods are not always specific and sometimes even impurities or degradation products also get determined e.g.Titrometric Methods or even Spectrophotometric methods.In such cases purity has to be determined by Specific Methods such as HPLC which are Stability Indicating Methods.
2.Generally there is no separate calculation.In some monographs Limits for both Assay (which may not be specific)and for Chromatographic Purity which takes care of Imp.& Degradabnts are separately prescribed.
3.External Std.and Int.Std.:- Generally, they are used for Chromatograpic Methods e.g.GC/HPLC.External stds.are injected separately while Int.Stds.are added quantitativly in Test Solns.Areas obtained in both are used for calculation of Content in the Test Product.Int.Std.used should not react with test product and should be stable enough.It should have different RT and peaks should not get merged.For more inf.pl.refer any Chromatographic book.
4.Primary Stds.are Ref.Stds.supplied by USP/EP/BP.These should be used for preparing Secondary Stds. which are used every time in analysis of Test Product.In some cases even Primary Stds.are also used as refree samples.
I think that this information es quite sufficient.
Dr.Barve