D. RITTENBERG AND D. SHEMIN 



technique, radioactive phosphorus, Pja, must be employed, since only 

 one stable atomic species exists, namely, Pis- The radioactive isotope 

 has a convenient half-life (14 days), and the radiation it emits is easy 

 to measure. Furthermore, the preparation of Pis is relatively simple. 

 The choice for the case of oxygen is also clear; the stable isotope O^ 

 should be used since the radioactive oxygen isotope Og has a half-life 

 of only 126 seconds, a time too short to permit the carrying out of 

 almost any biological experiment. Similarly, the stable heavy hy- 

 drogen. Hi, is preferable to the radioactive isotope, Hi. The reason 

 in this case is not a restricted half-life but the fact that the radiation 

 emitted by Hi is so soft that it is extremely difficult to measure. Also, 

 in this particular case, the measurement of Hi concentrations is simpler 

 than that of the radioactivity. 



By this reasoning we conclude that the stable isotopes should be 

 employed for studies of hydrogen, nitrogen, and oxygen, and the radio- 

 active isotopes for sodium, phosphorus, calcium, iron, and iodine. 

 The choice for carbon and sulfur is more difficult. While Gg cannot 

 compete with Cl^, isotope C" can. The actual choice here depends 

 on highly technical points involving the analytical procedures and 

 the relative cost of preparation of the isotopes. It seems likely that, 

 at least for the next five years, Gg^ rather than Cl^ will be employed 

 but that thereafter G" may supplant it. The choice in the case of 

 sulfur is also not easy, since neither form (Sjg or SH) has a definite 

 advantage over the other. 



One of the problems which enters into the choice between the 

 stable and the radioactive isotope is the relative ease of the isotope 

 determinations. With the exception of hydrogen, all stable isotopes 

 require the use of a mass spectrometer for their measurement. The 

 apparent complexity and cost of the present type of mass spectrometers 

 have often acted as a deterrent to investigators who have contemplated 

 employing this technique. It is likely that, after the war, the mass 

 spectrometer will be greatly simplified and that its cost from some 

 commercial instrument maker will not exceed that of a good optical 

 spectrograph. 



The analysis of organic compounds for deuterium may be car- 

 ried out by the falling drop procedure (17). The apparatus involved 

 and the procedures used are so simple that no competent investigator 

 need have any hesitation about setting up such equipment. However, 



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