516 ISOTOPIC TRACERS AND NUCLEAR RADIATIONS Chap. 25 



stable species deuterium, H 2 (or D), and the radioactive species tritium, 

 H 3 (or T). Deuterium was the earliest stable tracer used and has had wide 

 application. Tritium has so far had very limited use partly because of the 

 technical difficulties inherent in measuring its very weak beta rays, which 

 can be detected only in a gas counter, and partly because of its limited 

 supply until very recently. Accurate gas counters have now been developed, 

 and ample supplies are now available from the nuclear-pile reactors by the 

 reaction Li 6 (n, a)H 3 . This isotope should therefore have increasing applica- 

 tion, particularly in view of the fact that a considerable increase in sensitivity 

 of measurement is possible with compounds labeled with tritium over that 

 with compounds labeled with deuterium. Furthermore, its long half-life 

 (about 12 years) makes it available for complex syntheses and long-term 

 experiments. 



Deuterium has been used to label a wide variety of organic compounds. 

 These are summarized in a recent text by Kamen (Gen80). Whenever 

 deuterium may be introduced in a relatively stable position in an organic 

 molecule, i.e., attached to a carbon atom where it is generally stable bound, 

 it becomes essentially an auxiliary label for carbon. The first experiments 

 with isotopically labeled compounds were with deuterium-labeled fats 

 [Genl25]. Previous to this work it had been thought that depot or storage 

 fat was a biological energy store and was outside the general metabolism, 

 becoming active only in times of need. However with the deuterium label 

 the surprising fact was soon shown that these fats were in a state of fairly 

 rapid turnover and, furthermore, that fatty acids or fats of one type could be 

 converted into fats in which the acid moiety was changed by deletions or 

 additions to the carbon chain or by desaturation. It has been shown on the 

 other hand that such dietarily essential fatty acids as linoleic acid are not 

 formed from other fatty acids. 



Since the initial work with fats, there has grown up an extensive literature 

 on a number of classes of organic compounds labeled with deuterium at 

 stable positions. 



Deuterium has also been used, in the form of heavy water, in the study of 

 body-water content (by the isotope-dilution method) [D152,153,195], body- 

 water turnover in fish [D150,151] and man [D152,153], and in a number of 

 other problems in the movement of water in the vertebrate body [D3 1,7 2, 73, 

 etc.]. 



Tritium has had very limited application. The only work reported up to 

 the end of 1947 has been four papers: one on photosynthesis [T3], one on the 

 determination of total body water [T4] in a manner analogous to that in which 

 deuterium has been used, and two [Tl,2] on the use of tritium in showing the 

 conversion of phenylalanine to adrenalin. 



The use of deuterium and tritium as tracers, especially as auxiliary tracers 



