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Tracer Techniques 



I. Introduction 



Tracer methods have come into vogue since the later 1930's. In theory, 

 they are extremely elementary. Essentially, they consist of putting an 

 unusual isotope of an element into a biologically important metabolite 

 (or foodstuff) and following the progressive reactions of this metabolite 

 by determining the fate of the tracer isotope. The use of tracers has 

 been made possible by the development of methods to prepare and con- 

 centrate isotopes other than the commonly occurring ones. The pro- 

 duction and detection of these isotopes are possible only through the use 

 of complex physical equipment. Accordingly, tracer methods have 

 been included in this text among the specialized physical tools used in 

 biology. 



Most chemical elements consist of more than one isotope. All isotopes 

 of the same element have the same chemical properties but different 

 atomic weights. The atomic weight of an isotope is approximately 

 equal to the sum of the number of neutrons plus the number of protons 

 in the nucleus. In contrast, the chemical properties are determined 

 only by the number of protons, that is, the atomic number. (Owing to 

 their different atomic weights, two isotopes of the same element may 

 have different rates of reaction.) Often, the isotope number is shown by 



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