166 



ISOTOPIC TRACERS 



Table 12-2. Some Commonly-used Tracer Isotopes 



almost as readily as K, so the Rb can be valuable in studies on cell 

 membrane permeability. Sr**^ is also important biologically because of its 

 chemical similarity to another element, calcium. Several biological mecha- 

 nisms tend to concentrate the Sr"^ which might occur in fall-out from 

 nuclear weapons. Iodine collects in the thyroid; therefore P^^ has been 

 used, not only as a tracer, but for radiation therapy in disorders of the 

 thyroid. 



Available Forms of Tracer Materials: Radioisotopes formerly were 

 available chiefly as some salt containing the tracer element. C^^, for ex- 

 ample, was delivered as BaC^^Os. The carbonate could be converted to 

 C^'*02 by treatment with acid and ultimately converted into any of 

 several chemical compounds. Now producers of chemical and biochem- 

 ical compounds offer long lists of organic chemicals labeled with any of 

 several tracer isotopes. It is even possible to specify which atom (or 

 atoms) will be labeled. Acetic acid, for example, can be purchased as 

 C^^HaCi-OOH, C^-HsC^^OOH, or C^^HsC^^OOH. Most of the sugars, 

 most of the amino acids, and a variety of other compounds are available, 

 labeled with H", H^ C'^ C'\ N^^ 0'\ or assorted other tracers. The 

 availability of these compounds considerably simplifies the execution of 

 tracer experiments. 



