XVll. H A D 1 O A C T I V K T U A C E R S (333 



impurity. If they differ, repetition of the scavenging operation is in- 

 dicated to insure removal of all the impurity (5, p. 115). 



If the chemical nature of an active impurity is known, it is usually 

 feasible to add that element in inactive form and make a chemical 

 separation. The process of adding carrier for the active impurity 

 and separating can be repeated as often as necessary. For example, 

 if the chemical separation is only 80% effective, the first separation 

 will leave 20% of the original impurity, but the second will leave only 

 20% of that or 4%, and so on. Again the radioactivity enables one 

 to follow the process both qualitatively and quantitatively. 



If the presence of an impurity of shorter half-life than the product 

 activity is demonstrated, it may be po.ssible to allow the impurity to 

 decay out before using the tracer or before measuring the samples. 

 Large differences in absorbability of the radiation may be utilized if 

 the impurity has soft radiation and that from the product is hard. 

 Measurement through an aluminum absorber of proper thickness will 

 then cut out the soft radiation and permit the harder to pass through 

 and to be a measure of the amount of the product activity present. 

 If the energies of the radiations are reversed it may be possible to get 

 the amount of product by difference probably with some loss in ac- 

 curacy. A recent article on the subject of tracer purity is recom- 

 mended as a source of further information {45). 



In almost all biological work one deals with carrier and tracers 

 that are isotopic. The chemical yield through any given steps is 

 assumed to be identical to the yield of tracer or "radiochemical yield." 

 Thus let us say that in a given purification process it is determined by 

 weighing that an over-all chemical .yield of 70% of the originally 

 added carrier was obtained in the sample whose activity is measured. 

 Then, dividing the activity of the sample by 0.7 is assumed to give 

 the activity that would have been observed had the chemical recov- 

 ery been 100%. In those few cases of biological interest in which 

 isotopic carriers are not available for the tracer, e.g., 7.5 hour astatine 

 (element 85), no correction can be made for chemical recover3^ Thus, 

 although iodine can be used as a carrier for astatine it cannot be as- 

 sured that they behave identically, and indeed they do differ more 

 than expected {■^6). 



In summary, it must be said that before engaging in any large scale 

 program in handling tracers through chemical operations, purifica- 

 tions, and other processes, a reference work in radiochemistry or, bet- 

 ter, a qualified radiochemist should be consulted. 



