558 Tracer Techniques /30 : 2 



a superscript. For example, C 14 is a carbon isotope with six protons 

 and eight neutrons. Other carbon isotopes such as C 11 , C 12 and C 13 

 all have six protons per nucleus but have 5, 6, and 7 neutrons, respec- 

 tively, in the nucleus. (Sometimes the atomic number is shown as a 

 subscript 6 C 12 , but this subscript is redundant.) 



Some naturally occurring isotopes are radioactive; these emit three 

 different types of particles. All of them emit either an alpha particle 

 (an He 4 nucleus) or a negative beta particle (a high-velocity electron). 

 In addition, many emit the third type, gamma rays, which are high- 

 energy electromagnetic photons. Few naturally occurring radioisotopes 

 are useful as biological tracers. (Two naturally occurring radioisotopes, 

 H 3 and C 14 , are widely used for tracer studies.) 



Most artificially produced radioactive isotopes emit beta particles; 

 some emit positive ones and others negative ones. Often, gamma rays 

 accompany the radioactive decay. In some cases, it is easier to detect 

 the gamma rays (photons) than the beta rays (electrons). Artificial 

 radioisotopes form the basis for most biological tracer studies. Rather 

 than attempting to produce a catalog of all isotopes, three selected radio- 

 active isotopes, C 14 , I 131 , and P 32 , are presented. These were chosen 

 because they are among the most common species used. Although they 

 have been employed in many types of experiments, only a few examples 

 are described. These are meant to be illustrative rather than complete. 



Nonradioactive isotopes can also be used for tracers, provided the 

 naturally occurring isotope ratios are varied. It is possible to measure 

 these ratios by means of mass spectrometer- type techniques. These 

 techniques are illustrated by the nonradioactive isotope N 15 , which 

 normally occurs at concentrations that are small compared to N 14 . 

 By concentrating the N 15 , one may prepare samples of metabolites with 

 an excess concentration of N 15 . 



In general, the tracer element is used in a form diluted with a carrier; 

 that is, radioactive C 14 is used only diluted in a large excess of non- 

 radioactive C 12 . These can then be studied in systems which are in 

 over-all thermodynamic equilibrium, that is, with respect to carbon 

 atoms, but are far from equilibrium for the tracer isotope. Tracer 

 techniques make possible the study of equilibrium biological processes 

 without altering the chemical equilibria. 



2. Radioactive Tracers 



Radioactive tracers continually disintegrate during an experiment. 

 The probability of one atom disintegrating appears to be a purely random 

 phenomenon ; the number of disintegrations per second is proportional 





