Sec. 24.3] CRITIQUE OF BIOLOGICAL APPLICATIONS OF ISOTOPES 507 



(H), i.e., protium (p or H l ), has been suggested for a possible therapeutic 

 role when accelerated to the high energies of the new giant cyclotrons. 



From the foregoing discussion it may be seen that radioactive and stable 

 isotopes have applications both in tracer problems and in the study of radia- 

 tion effects. No exclusive distinction can thus be made between them on 

 either basis. However, for the most part radioactive isotopes are more 

 versatile and, except in a few cases, are the important tracers. By reason 

 of their great variety of radioactive properties, they also offer far more 

 diversified techniques for study of radiation effects. 



24.3. Isotopes as Tracers. Hevesy [Gen 76] has recently made an exhaus- 

 tive and valuable survey of the literature on the tracer application of radio- 

 active isotopes in biology. No such recent study exists for stable species. 

 Since isotopes in biological tracer work are tools for the elucidation of physi- 

 ological processes and for biochemical analyses rather than objects of study 

 in themselves, a somewhat artificial separation' of isotopic and nonisotopic 

 studies of the same phenomena would be effected by a perpetuation of 

 Hevesy's comprehensive approach. In reviewing tracer applications, the 

 author has sought rather to cite for the isotopes of each element enough 

 representative studies to illustrate their known range of usefulness; to these 

 can be added in many cases speculation as to additional applications. This 

 approach is essentially followed here in the chapters on individual elements 

 and groups of elements. 



Radioactive species that are adaptable to biological studies exist for most 

 elements. Exceptions to this include He, Li, B, and Ne, of which the known 

 unstable forms are so short-lived (40 sec or less in half-life) that it seems 

 unlikely that they will ever find biological application. Certain other ele- 

 ments are greatly limited in this regard by the shortness of half-life of their 

 longest lived radioactive isotopes; these include N, O, Mg, and Al, for the 

 isotopes of which the half-lives are 10.2 min or less. Fluorine and silicon are 

 somewhat unsatisfactory as their best isotopes have half-lives of only about 

 2 hr and 3 hr, respectively. The longest lived known isotope of Fa has a 

 half-life of only 21 min. But, aside from these 11 elements, all of which are 

 of low atomic weight except francium, the other 85 of the periodic system 

 have radioisotopes of reasonably or very convenient half-lives for biological 

 work. 



The use of stable isotopes as tracers for the 60 elements of which more than 

 one species exists is limited by two factors: (1) the expense and technical 

 difficulties involved in concentrating a given species from the isotopic mix- 

 ture characteristic of an element having more than one stable nuclear form; 

 and (2) the ready availability, convenience, and as a rule, greater accuracy 

 of application of at least one radioactive species for most elements. Only in 



