British Pharmacopoeia drug assay methods

A method for the estimation of the total alkaloids and of non-phenolic alkaloids (emetine fraction) is given in the British Pharmacopoeia, 1932, Addendum VI, which requires the drug to contain not less than 2 per cent, of alkaloids, of which at least 55 per cent, must be non-phenolic bases, calculated as emetine. The British Pharmacopoeia also gives an assay process for emetine in emetine bismuth iodide, the form in which the drug is chiefly used in medicine it is required to contain not less than 25 and not more than 28 per cent, of emetine. In the United States Pharmacopoeia, XIII, both Cephcelis Ipecacuanha and C. acuminata are recognised and must contain not less than 2 per cent, of ether-soluble alkaloids. 

An interesting example is die assay of clonidine hydrochloride in injections and tablets (British Pharmacopoeia, 1980). In 0.01 Mhydrochloric acid, clonidine exhibits two sharp maxima ne - 272 nm and 279 nm, which are not suitable for precise measurement. However, clonidine forms an ion pair with bromothymol blue, and this can be readily extracted into chloroform for subsequent measurement of the broad maximum near 420 nm. Because of the intiinsic variabihty of reagents used in such methods, a pharmacopoeial reference standard is employed for cahbration. A similar policy is adopted for assays involving chemical modification of the drug, as in the tetrazolium assay for corticosteroids, the assay for folic acid involving hydrolysis, diazotisation, and coupling with N-(l-naphthyl)ethylenediamine, and the reaction of penicillins with imidazole and mercuric salts. 

In the UK and Europe the Beer-Lambert equation tends to be used in what is called the absolute method of assay. In this procedure the absorbance is measured experimentally and the Beer-Lambert equation is solved for c, the drug concentration. For this reason, the British Pharmacopoeia and European Pharmacopoeia quote A values in drug monographs. 

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