ISOTOPE TECHNIQUE 



an unchanged particle having a mass very nearly the same as that of 

 the proton. A particle containing 6 protons and 6 neutrons will be 

 a carbon nucleus, v^hich has a charge of 6. Using the notation of the 

 nuclear physicists, the particle will be denoted as Cg^ the superscript 

 indicating its mass and the subscript its nuclear charge. If we add 

 one more neutron to the nucleus, we obtain Cj^. This is an isotope 

 of carbon, for we have merely increased the mass and not the nuclear 

 charge. Introduction of a proton into the Cg" nucleus would yic^ld 

 Ny^, an isotope of nitrogen, for both the mass and the nuclear charge 

 have been increased by one unit. The particle Cg^ exists in nature, 

 for naturally occurring carbon is a mixture containing one atom of 

 C'l^ for every ninety-nine atoms of Cg". Introduction of one more 

 neutron into the nucleus of Cl^ would give rise to Cg^, and subtraction 

 of one neutron from Cf would form Cl^. Both these particles are 

 carbon atoms and both have been artificially prepared. Neither one 

 exists in nature, since each is radioactive, with a half-life of about 24,000 

 years and 21 minutes, respectively. Other carbon isotopes may be 

 visualized, such as Cl, Cl^, or Cg^, but presumably these are radioactive 

 and very unstable, i. e., their half-life times must be very short. 



A study of the stable isotopes suggests that a nucleus is stable 

 only if it has approximately the same number of protons and neutrons 

 in its structure. The forces which bind the nucleus seem to be inter- 

 actions between pairs of protons and neutrons. 



In the last ten years, the physicists ha\'e prepared artificially 

 radioactive isotopes of practically every element. During the same 

 period, chemists (16, 20, 28, 42-45), notably Urey, concentrated and 

 obtained in more or less pure form the heavy isotopes of hydrogen, 

 oxygen, nitrogen, carbon, and sulfur. At the present time, the bio- 

 chemist has available for his investigations isotopes, either stable or 

 radioactive, of every element of biological interest. Table I lists some 

 of these isotopes with their natural abundance, if stable, and their 

 half-life time, if radioactive. The elements chosen were those of bio- 

 logical interest, and it is fortunate that for every element there is avail- 

 able either a concentrate of a heavy stable isotope or a suitable radio- 

 active isotope. 



The choice of the particular type of isotope to be used for any 

 specific problem depends on the isotopes available and the type of 

 problem. In the study of phosphorus metabolism by the isotope 



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