BASIC DIFFICULTIES IN TRACER METHODOLOGY 71 



Giles and Bolomey (37) found that P'- and C" in solution produced 

 chromosomal changes in Tradcscantia at levels of 100 and 900 /zc/liter, 

 respectively. 



In retrospect, it must be emphasized that the radiation effect is impor- 

 tant here only inasmuch as it directly or indirectly affects the experimen- 

 tal observation. As a trivial example, if an insect labeled with a radio- 

 isotope is physically affected so that it will not cover its normal distance, 

 then this is important in flight-range studies. If it is not so affected, then 

 it makes little difference if the insect becomes sterile or undergoes other 

 metabolic changes, particularly if this occurs after the experiment has 

 been completed. Of course, with human beings any possible latent 

 effects must be avoided. It is apparent that generalizations are impos- 

 sible as to the radioisotope levels that can or cannot be used, since there 

 are too many variables. Some of the values quoted above will give indi- 

 cations as to when the danger levels may be reached. In practice, one 

 may iisnaUy determine whether radiation effects are significant by performing 

 replicate experiments using graded levels of radioactivity. In some cases 

 duplicate experiments in part can be undertaken with stable isotopes or 

 chemical indicators. 



The chemical effects of radiation have been reported in connection wath 

 the radiation decomposition of some C '^-labeled compounds (38). It is 

 recommended that frequent checks be made of labeled organic compounds 

 to exclude the possibility of the presence of decomposition products. 



EXCHANGE REACTIONS 



An isotopic exchange reaction as a special case of exchange may be 

 defined as a chemical reaction in which the atoms of a given element inter- 

 change between two or more chemical forms of the element. Wahl and 

 Bonner (11) may be consulted for a detailed treatment of this subject, and 

 Edwards (39) has reviewed chemical-exchange studies. References (40, 

 41) may be consulted for collections of papers and references on this 

 subject. 



Examples of isotopic exchange reactions are as follows: 



1. Simple exchange due to electrolytic dissociation (ionization): 



2PbCl2 + 2Pb*(X03)2 ;^ 2Pb++ + 4C1- + 2Pb*++ + 4N0i- 



jr 



PbCl2 + Pb*Cl2 + Pb(X03)2 + Pb*(N03)2 



(2-1) 



If lead chloride and labeled lead nitrate are dissolved in solution, the free 

 ions will be present. It is clear that, if lead chloride were separated from 

 such a solution, it would contain an equivalent amount of labeled lead 

 atoms that were present in the original lead nitrate. 



