562 FRED M. UBER 



A. FUNDAMENTAL BASIS OF STABLE ISOTOPE 



METHODS 



1. Principles and Underlying Assumptions 



Nearly three hundred different Stable atoms have been discovered 

 and classified on the basis of their mass. Since there are less than a 

 hundred chemical elements in the periodic table, many elements must 

 comprise atoms with two or more mass types. Atoms that differ in 

 mass but not in their ordinary chemical behavior are called isotopes. 

 Substances that possess only one stable isotope each are not generally 

 useful for our present purpose. Some representatives of the twenty 

 odd elements in this group are : sodium of mass 23 (usually written 

 as Na^'), Al^^, P'^, and P". Biologically important elements that 

 consist of two stable isotopes each are hydrogen, carbon, nitrogen, 

 and chlorine. Among other elements useful in biological investiga- 

 tions, mention must be made of oxygen, which has three stable iso- 

 topes, and of sulfur, which has four. 



When one of the two or more stable isotopes of a chemical ele- 

 ment occurs naturally with a very low concentration, the possibility 

 exists of employing this rare isotope as an atomic marker or label. 

 It is necessary, however, that its natural concentration first be in- 



TABLE I 



Natural Concentration of the Stable Isotopes Most Commonly Employed in 



Biological Studies 



creased artificially. Since practical processes for segregating isotopic 

 components have been developed in recent years {1,3,6), this labeling 

 possibility has now been realized for the elements listed in Table L 

 The basis then for the use of stable isotopes as atomic labels with 

 which to observe the behavior of molecules consists of synthesizing 

 compounds in which the comparatively rare isotope of an element 

 occurs in a greater than normal concentration. 



