BASIC DIFFICULTIES IN TRACER METHODOLOGY 77 



at high temperatures, the isotopic constitution of copper atoms diffused 

 into the silver sulfide was found to be 71.2 per cent Cu®* and 28.8 per cent 

 Cu^^ (52). This indicated a higher rate of diffusion for the lighter isotope. 

 The practical application of this behavior is seen in the large-scale sep- 

 aration of the uranium isotopes. 



The possibility of isotope effects with carbon is of major interest. 

 There are six isotopes of carbon: C^", C'\ C'\ C'\ C'\ and C'^ How- 

 ever, the discussion will be limited to C^^ and C'^ which are stable iso- 

 topes normally present in nature in a ratio of about 90: 1, respectively, and 

 to C^'*, which is the long-lived radioisotope most widely used experimen- 

 tally. It has been shown that living organisms, plants, and animals, 

 together with their geological remains, contain less C^* than does lime- 

 stone (53). Also algae have been found to contain 2.97 per cent less C^' 

 than did the CO2 in the solution in which the plants grew (54). In 

 studies of photosynthesis with barley seedlings, it was definitely shown by 

 California workers that the assimilation of C^^02 was about 17 per cent 

 slower than that of C'-02 (55). Buchanan et al. (56), in a series of 

 detailed experiments in which biological material was grown for about 

 3 yr exposed to a known specific activity of C^^, were able to confirm the 

 general finding that plants tended to reject the heavier isotope in favor 

 of C^-. The differences in isotope concentration were approximately 

 twice those found with C^^ as estimated from kinetic studies during the 

 growth of algae. These results were in disagreement with those of the 

 California group, who reported much higher C^^ effects. It was also of 

 particular interest that, whereas the organic matter of snails appeared to 

 reject the heavier isotope, the carbonate of the shell tended to concentrate 

 C^"*. This is consistent with the results on the natural occurrence of C^'*, 

 as discussed in the section on radiocarbon dating in Chap. 1. Although 

 there is little evidence to indicate the mechanism involved, it seems clear 

 that there is an isotope effect in nature by means of which plants discrim- 

 inate against the heavier isotopes of carbon. 



The effect of isotope substitution on organic reaction rates has been 

 reviewed by Ropp (57). These studies have led to some understanding 

 of the mechanism of carbon isotope effects as well as to quantitative 

 expressions for the magnitude of these effects. The effects may be clas- 

 sified as intermolecular or intramolecular. An example of an intermolec- 

 ular process is as follows: 



kit 



HCi^OOH > CO2 + H2O 



H^so« (2-8) 



HCi^COOH^^ C^^02+ H2O 



H2SO4 



The symbol k represents the specific rate constant for the reaction as 

 designated. The magnitude of the isotope effect may be expressed as 



