XVI. STABLE ISOTOPES AS TRACERS 563 



As an example, let us suppose that we wish to investigate the 

 source of the oxygen in the water obtained when ethyl alcohol reacts 

 with acetic acid to form ethyl acetate. For one possible approach, 

 ethyl alcohol in which the oxygen isotope of mass 18 occurred in ex- 

 cess of its normal concentration could be synthesized and then al- 

 lowed to react with ordinary acetic acid. An analysis of the water 

 derived from this reaction would then be made to determine its O^^ 

 concentration. If its 0^^ content were found to be normal, one would 

 conclude that the oxygen in the water came from the acetic acid and 

 not from the ethyl alcohol. A confirmatory analysis could be made 

 on the ethyl acetate. In this case, it has been established (15) that 

 the oxygen in the water splits off from the acetic acid and the reaction 

 is written as follows: 



C.HsO'^H + CH3COOH ;=± CH3COO'8C2Hb + H2O 



The biological use of the least concentrated isotope of an element 

 as a tracer is based on the assumption that organisms are unable to 

 distinguish between the two or more isotopes of a chemical element. 

 The plausibility of this assumption is favored by the relatively late 

 discovery of isotopes and l)y the extreme difficulties encountered in 

 their concentration in the laboratory. It has now been established 

 that organisms in the course of their normal metabolic processes are 

 incapable of discriminating between the various isotopes of an element 

 within rather wide limits (24)- The only noteworthy exception oc- 

 curs in the case of hydrogen. This is not surprising in view of the 

 fact that the heavy hydrogen isotope is just twice the mass of the 

 lighter one. Even with hydrogen no appreciable differential effect 

 is shown biologically except in high concentrations. When it is 

 realized that the stable isotopes used as labels are everywhere present 

 in nature, only in somewhat smaller concentration, it is clear that 

 nothing is being introduced that is essentially new or foreign to a 

 normal living organism. A theoretical discussion of the validity of the 

 use of tracers to follow chemical reactions recently appeared {24a) • 



2. Theory of Measurement 



With the possible exception of the hydrogen isotopes, whose mass 

 ratio of two is great enough to permit accurate determinations of 

 concentration by means of density measurements, the instrument of 

 choice for determining isotope concentrations is the mass spectrome- 

 ter. When electrically charged particles enter a uniform magnetic 



