XVI. STABLE ISOTOPES AS TRACERS o73 



phthalimide procedure of Gabriel or by the catalytic reduction of 

 a-keto acids according to Knoop and Oesterlin rather than by more 

 usual methods (^4, P- 224). 



For deuterium compounds, a very useful synthetic method con- 

 sists of the hj^drogenation of double bonds. Examples of this process 

 are the synthesis of propionic acid from acrylic acid, stearic acid 

 from linoleic acid, butyric acid from crotonic acid, etc. {2Jf). These 

 and other useful syntheses for numerous compounds containing 

 deuterium are included in a recent bibliography {17, p. 142). Hot 

 concentrated deuteriosulfuric acid is valuable in introducing deute- 

 rium on the ct-earbon atom of fatty acids, and it may also be used for 

 amino acids. For some of the fatty acids it is necessary to use a 

 platinum catalyst in the presence of heavy water. Examples are 

 palmitic, lauric, capric, caprylic, and isocaproic acids. But with a 

 platinum catalyst, the deuterium atoms are equally distributed 

 among all the carbon atoms. 



A widely applicable preparative method consists of feeding plants 

 or animals simple labeled substances and allowing them to synthesize 

 the desired compounds. Apart from the fact that the yields obtained 

 are often quite low, there is also a further dilution of the isotope with 

 respect to any given location in a molecule, since the atomic label 

 may appear in many positions. This method defeats its own purpose 

 in many cases since the label may not be sufficiently specific. Where 

 animals are permitted to drink heavy water, for instance, practically 

 all of the chemical constituents of the animal body will contain deu- 

 terium. But the deuterium may appear in several positions in each 

 of various molecules. A more direct biological synthesis can some- 

 times be employed by letting an organism convert one isotopic com- 

 pound into a second substance that would otherwise be much more 

 difficult to synthesize in the chemical laboratory. One example of 

 this is the direct conversion of stearic acid containing deuterium into 

 deuterio-oleic acid within the body of a mouse {24, P- 224). 



Information about the synthesis of compounds containing carbon 

 isotopes can be obtained from contemporary research articles on 

 either C", C'^, or C*. Two rather recent summaries have been fur- 

 nished {17, pp. 153-160; 22a) but only frequent contact with the cur- 

 rent periodical literature can keep one up-to-date (see 3, p. IGl). A 

 similar remaik applies to research with sulfur, in which much of the 

 work has been done with the radioactive S^^. Some syntheses with 

 isotopic sulfur have also been discussed by Kamen {17, pp. 205-212). 



