BASIC DIFFICULTIES IN TRACER METHODOLOGY 79 



than 15 per cent and usually less than o per cent. I'hus these eit'ects will 

 pass undetected in most biological experiments. Where the interest is 

 primarily concerned with the behavior of the radioisotope itself, it is not 

 necessary to take the isotope effect into account. This is the situation in 

 health-physics studies of the retention and excretion of radiotoxic sub- 

 stances. Also the isotope effect will usually be negligibly small when the 

 isotopic atom is not involved in bond making or breaking. Thus deute- 

 rium or tritium can be used satisfactorily in many cases for labeling large 

 molecules or for studies involving body-water determinations, and C^' 

 can be used for labeling almost any organic molecule provided the label 

 is located in a nonfunctional part of the molecule. It should be noted 

 that there are many organic reactions in which, contrary to expectation, 

 no isotope effects have been noted (57). As yet, empirical working rules 

 are not available, but as more data are accumulated, it may be possible 

 to predict the extent of isotope fractionation under specified conditions. 



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3. Gorbman, Aubrey: Functional and Structural Changes Consequent to High 

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