CHAPTER 2 



BASIC DIFFICULTIES IN TRACER METHODOLOGY 



Chemical Effects. Radiochemical Purity — Extraneous Elonients; Chomical 

 State; Radiocolloids; Parent-Daughter Relationships; Detection of Radioactive 

 Impurities; Removal or Elimination of Radioactive Impurities. Radiation Effects. 

 Exchange Reactions. Isotope Effects. 



Experiments with radiotracers will be valid and interpretations mean- 

 ingful only if careful consideration has been given to the behavior that 

 may result from the chemical, radiochemical, physical, and biological 

 properties of the radioisotope preparation used. The listing of precau- 

 tions may appear formidable indeed at first glance. However, in prac- 

 tice, the difficulties for the specific radioisotope and the specific experi- 

 ment are usually overcome rather easily. Also, once the necessary con- 

 ditions have been established for a given line of experimentation, they 

 may suffice over a considerable period of investigation. The important 

 point is that the possibilities for misinterpretation should be recognized at 

 the beginning of the investigation and that the experiment should be 

 planned so as to permit interpretations that are as unequivocal as possible. 



CHEMICAL EFFECTS 



Except for the principal element, there will usually be no problem of 

 chemical effects due to the radioisotope preparation. An indication of 

 possible contaminating chemicals may be gained from a knowledge of the 

 target material, which is usually analytical reagent grade or purer. If 

 there should be some question, control experiments can be run using 

 appropriate levels of nonirradiated target material. Particularly where 

 materials are to be injected into humans must care be taken to ensure 

 that no pyrogens are present, and normal sterilization procedures should 

 be employed. There are few reports in the literature of difficulties from 

 this source. However, Graham et al. (1) observed a toxic factor in a 

 particular batch of pile-produced F-^'- which increased the death rate of 

 embryos following injection into eggs for the production of labeled viruses. 

 Later preparations did not show this effect. 



The amount of the principal element introduced into the system may 



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