634 ADOLF F. VOIGT 



2. Special Problems in Using C 



14 



The synthesis of biologically important compounds containing 

 tracers is a research problem of considerable complexity. It is met 

 most often in the case of organic compounds containing C^^ although 

 the synthesis of organic sulfur, phosphorus, and iodine compounds 

 with those elements labeled is also important. One of the principal 

 difficulties in C^* synthesis (which applies equally well to syntheses 

 involving C^^) is that of getting the active or labeling atoms in a 

 proper compound as a starting point for the synthesis. For example, 

 C^'* was first made available as barium carbonate, which readily 

 gives C^^02, but this is very difficult to use as a starting material. 

 In addition, at a somewhat higher price, C^^HaOH, HC^^HO and 

 some carboxyl-labeled fatty acids are now sold by the Isotope Divi- 

 sion and other compounds will be available in the future. In addi- 

 tion, several commercial companies {88,95,97) have been authorized 

 to prepare other intermediates and to sell them to institutions ac- 

 credited by the Atomic Energy Commission. Correspondence with 

 these companies or wdth the Isotope Division is suggested as the 

 method of finding out what is available and its price. 



Once the starting material is obtained, a method of synthesis that 

 will label the compound in the proper position must be chosen with 

 some care. For many problems it will be necessary to have the com- 

 pound labeled in some specific position; for others, the tracer atoms 

 can be scattered uniformly through the compound. Synthesis has 

 been discussed in a number of recent review articles in which the ex- 

 perience using C^^ has been applied to the essentially similar problems 

 involved in use of C^^ {2, p. 161 ; 47; 48', see esp. 3h). Some of the basic 

 principles by which this differs from ordinary synthesis are : The reac- 

 tant containing tracer is vastly more valuable than any other react- 

 ant and conditions are so adjusted to give maximum yield on the 

 basis of this reactant. Dilution of the tracer is avoided to keep re- 

 covered tracer active enough for reuse. Adequate precautions are 

 taken against accidental loss of the tracer, contamination of the equip- 

 ment and laboratory, and overexposure of personnel. (Since one can 

 be much surer of these last points if the system is completely closed, 

 the use of a closed vacuum line type of apparatus is highly advisable.) 



In the use of labeled organic compounds the possibility of exchange 

 of tracer between its expected position and other molecules or other 

 positions in the same molecule cannot be overlooked. Experiments 



