BASIC DIFFICULTIES IN TRACER METHODOLOGY 61 



Hamilton (2) in Fig. 2-1. It is apparent that the percentage of collection 

 decreased as the dosage contained increased amounts of iodine and that 

 in any given experiment the results would be primarily a function of the 

 mass of iodine in the range indicated. In this type of study the dietary 

 and body status of iodine would also have to be taken into account. For 

 example, Gorbman (3) showed that almost ten times as much I'^* was 

 required to destroy the thyroids of mice on diets containing moderate 

 amounts of iodide as was recjuired to destroy the thyroids of mice on a 

 low iodide diet. In some experiments, especially those involving toxicity, 

 large amounts of the element must be used. This presents no problem, 

 since the dosage can be increased by using additional inert element. 

 Difficulties usually arise when it is necessary to keep the dosage at a 

 minimum. 



Physiological concentrations of salts and normal acidities should be 

 used if possible, particularly when solutions are to be administered to 

 animals parenterally. Some substances are difficult to maintain in solu- 

 tion under such conditions, and these are discussed under the specific 

 elements in Chap. 6. 



RADIOCHEMICAL PURITY 



In practically all tracer studies it is essential that the radioactivity 

 measured in a system be derived originally from a single known atomic or 

 molecular species. It is necessary that any radioactivity other than that 

 of the principal species be recognized and eliminated. 



Extraneous Elements. All component atoms of the target material 

 may become more or less radioactive. Likewise, in isotope production, 

 nuclear reactions may occur other than the one that leads to the principal 

 activity. The producer of radioisotopes minimizes radiocontamination 

 by choosing a target material which has minimum chemical impurities 

 and which is of such a nature that atomic species prone to activation are 

 not present (e.g., use of the metal or the oxide). The contribution of each 

 atomic species is not proportional to the amount present, because each 

 species has its characteristic efficiency of production (cross section). 

 Thus a 1 per cent impurity of sodium in a potassium target will result in 

 a 13 per cent contamination after pile irradiation. Similarly 0.01 per 

 cent calcium phosphate in target CaCOs may result in a 5 per cent con- 

 tamination of Ca^* with P^-, which may lead to difficulties because of the 

 increased sensitivity of counting P^- (4). 



Chemical State. Any radioactive atoms that are not in the same 

 valence state or chemical form as the element that is to be followed must 

 be considered as radiocontaminants. Bombardment of the target mate- 

 rial may often produce different forms of the principal activity. When 

 the target material is processed, however, it usually results in a product 



