Sec. 24.3] CRITIQUE OF BIOLOGICAL APPLICATIONS OF ISOTOPES 511 



agents it is possible to put molecular groupings into a system such that they 

 behave essentially as do similar groups already present, but are nevertheless 

 distinguishable by virtue of slight mass differences in the case of stable 

 isotopes or of radioactive decay phenomena in the case of radioactive isotopes. 



The use of an isotope as a tracer for any given purpose involves (1) its 

 availability in a suitable form or its incorporation into the material to be 

 labeled; (2) its administration in a suitable manner to the biological system 

 to be studied; (3) its recovery and' measurement, or in some cases its measure- 

 ment directly in vivo. 



With radioisotopes, all these steps must be carried out with precautions 

 against the overexposure of personnel to ionizing radiation. This involves 

 avoidance not only of irradiation at the time of experimentation but also of 

 contamination by, or assimilation of, radioactive materials, and hence con- 

 tinued irradiation. Where human beings are the biological systems studied, 

 care must also be taken against exceeding the safe tracer dose — usually given 

 as 0.1 rep of total body irradiation per day. Safe handling of radioactive 

 isotopes has already been discussed in Chap. 19. 



The preparation of isotopically labeled substances involves the conserva- 

 tion of valuable materials. Therefore, special synthetic processes, in which 

 yields are as high as possible, often must be evolved. In some cases it is 

 possible to obtain labeled materials through biosynthetic processes; in such 

 cases an isotope is introduced into a living system and the desired labeled 

 material isolated later. 



The administration of labeled tracer substances presents no unusual 

 features other than those associated with health protection against irradiation 

 in case of radioisotopes. 



The measurement of isotopes involves a number of specialized techniques. 

 Quite different procedures must be applied to stable as opposed to radioactive 

 species. At the present time stable isotopes may be measured only in the 

 mass spectrometer except for deuterium and heavy oxygen, for which addi- 

 tional techniques are available. The latter depend either on measurements 

 of density or on measurements of refractive index of highly purified mixtures 

 of water and deuterium oxide ("heavy water") (see Chaps. 8 and 9 on stable- 

 isotope measurement). 



The radioactive isotopes may be detected or measured by three principal 

 techniques: in vitro, in vivo, and autoradiographic. The in vitro and in 

 vivo methods involve the use of the Geiger-Miiller counter, an electroscopic 

 counter, or an ionization chamber; the autoradiographic method, the use of a 

 sensitive photographic emulsion. 



The in vitro techniques of measurement are carried out on samples removed 

 from the biological system under study and are highly varied in procedure. 

 The choice of method depends upon the manner in which a given isotope 



