46 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



The important thing is that, after radioisotope B has been added, any 

 losses from the sample do not affect the results, since A and B are lost in 

 the same proportion and the value of C can be determined despite such a 

 loss. 



This application may be illustrated by the work of Saylor and Finch 

 (105) on the absorption of iron. In a conventional study the isotope is 

 given orally to an animal and the absorption measured by analysis either 

 of the carcass or of the excretions. In human beings or large animals, 

 analysis of the carcass is either impossible or impractical, and the quan- 

 titative collection of excretions is always difficult. By the application of 

 the double-isotope principle it was possible to determine the absorption of 

 orally administered radioiron merely by analyses of a blood sample. A 

 known amount of Fe^^ was administered to a rat by stomach tube, and a 

 short time later a known amount of Fe^'' was intravenously injected into 

 the same animal. After several days a blood sample was taken and 

 analyzed for both isotopes. The percentage of absorption of the orally 

 administered Fe^^ was calculated as follows: 



100 X Fe^^ p-^ 

 % Fe" absorbed = ^^ ' (1-39) 



where Fe^^ = amount of Fe^^ in blood sample 



Fe^" = amount of Fe^^ injected intravenously 



Fe^^ = amount of Fe^^ in blood sample 



Fe^^ = amount of Fe^^ given orally 

 This calculation is based on the assumption that iron is removed from 

 the blood in the same way whether it reached there by direct injection or 

 by absorption from the gastrointestinal tract. Values obtained by this 

 procedure were in agreement with results from the direct analyses of the 

 carcasses of the rats used. The double-labeling principles were also used 

 in the direct-carcass analyses to correct for manipulative errors. Since 

 no large amounts of intravenously administered iron are lost from the 

 body by excretion, the recovery of the injected Fe^" from the whole car- 

 cass was assumed to represent also the recovery of Fe^^ from the carcass 

 and was used to correct the observed Fe^"" value. Thus absorption from 

 whol^carcass analysis was calculated as follows: 



Fe^^ injected 



Absorbed Fe" = recovered Fe^^ f^rrs j (1-40) 



Fe^^ recovered 



An example of how two radioisotopes of different elements are used to 

 label the same substance is given in the work of Keston et al. (63) on the 



