Chapter IVA 



ASSAY METHODS 



There are three important types of assay methods which may, theoret- 

 ically at least, be developed for each of the B vitamins: (1) chemical 

 (or physical-chemical) methods; (2) microbiological methods using 

 bacteria, yeasts and molds; and (3) biological (animal assay) methods. 

 In addition, it might be desirable, of course, to use combinations of these 

 methods. Other organisms, such as higher plants or lower animals that 

 have potentialities for vitamin assay work, may be considered in connec- 

 tion with gaining the broadest picture. Animal or plant tissues may be 

 used in some cases. Even human beings can be used for vitamin assay 

 and the newest (probable) member of the B vitamin family, the anti- 

 pernicious anemia principle, was assayed for exclusively in this manner 

 until a useful microbiological test was finally developed. 



There are three general purposes which underlie the performance of 

 vitamin assays: (1) they may be performed for purposes of exploration 

 with respect to distribution in nature, in foods and in food products; (2) 

 they may be used as a guide in isolating, purifying and determining the 

 functions of a principle; (3) they may be used for the assay and control 

 of commercial concentrates and of synthetic products. The method to be 

 used must be selected on the basis of its availability and applicability 

 to the intended purpose. A chemical or physical-chemical method, for 

 example, might be extremely valuable in testing nearly pure commercial 

 concentrates or synthetic mixtures, and yet might be of no value what- 

 ever in connection with the assay of materials where the principle exists 

 only in traces. 



Other things being equal, the chemical and physical-chemical methods, 

 when applicable, are most advantageous because of their accuracy, speed 

 and definition. In general, however, such methods become progressively 

 less applicable as we pass from preparations which are relatively pure 

 to those in which the substance tested for is present, for example, to the 

 extent of only a few parts per billion. In the latter cases, chemical or 

 physical-chemical tests are likely to be far too insensitive. When such 

 tests are sensitive enough for the assay of a natural mixture, they are 

 likely to be interfered with by extraneous substances, which, however, it 

 may be feasible to remove. 



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