64 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



when it is measured in the original sample, (c) The radiation character- 

 istics of the daughter serve to identify the parent, (d) The original sam- 

 ple can be analyzed many times by allowing time for equilibrium to be 

 reached between successive chemical separations of the daughter. The 

 chemical separation and radioassay of Y^° from the samples containing 

 Sr^o-Y^'' is a good illustration of the advantages of this procedure. This 

 method also allows differentiation between Sr^° and Sr*^, the latter having 

 no radioactive yttrium daughter. 



In a few cases a process known as isomeric transition (abbreviated IT) 

 occurs in which a radioactive daughter is isotopic with the parent. 

 These types of contamination can be anticipated from the published decay 

 schemes for the radioisotope to be used and are noted in the descriptions 

 of the specific radioisotope in Chap. 6. 



Detection of Radioactive Impurities. Knowledge of the nature of the 

 target material may give leads as to possible radiocontaminants. For 

 example, sodium, rubidium, or cesium activities might well be antic- 

 ipated in irradiated potassium. In many cases the determination of the 

 half-Hfe and the absorption curves (see Chap. 5) may provide information 

 as to the presence of extraneous activities. Obviously these methods are 

 of no value if the impurities have half-lives or energies of radiation close 

 to those of the principal activity. 



Special tests may be devised to ascertain the absence of radiocontam- 

 inants that may be of particular significance. For example, if a sample 

 of radiocobalt as produced by cyclotron bombardment of iron were to be 

 used in a hematopoietic study, it would be important to be sure that no 

 iron activity was present. This could be determined by incorporating 

 some stable iron in an aliquot of the isotope preparation and then per- 

 forming a chemical separation to recover the iron. If there were no 

 radioactivity in the recovered iron, this would be evidence that there was 

 no significant radioiron in the original radiocobalt preparation. The 

 procedure could then be repeated for manganese, nickel, or any other 

 element that might be a probable and important contaminant. 



A demonstration of how a combination of physical and biological 

 factors led to the detection of radiocontaminants in pile-produced radio- 

 copper was reported by Frierson et al. (14). These workers administered 

 Cu^^, which showed the expected half-life of 12.9 hr, to swine and rats. 

 The activity collected in the urine consistently gave a half-life of about 

 28 hr. Paper chromatography and chemical fractionation allowed the 

 identification of two major radiochemical impurities, Zn^^ and Ag'^". 

 The urine measurements were made about twenty half-lives after 

 removal from the pile, so that by this time the ratio of the short-lived 

 Cu^^ to the radiocontaminants had decreased merely by physical decay. 

 In addition, it was evident that the animals caused a decrease in this ratio 



