RADIOISOTOPES — AEBERSOLD 229 



salt such as the carbonate or nitrate, or as the oxide. For most bio- 

 log:ical tracer experiments, however, it is necessary to incorporate the 

 radioisotope in some complex compound. If an investigator wants 

 to use a radioisotope, say carbon 14, in trying to find out what happens 

 to a sugar or an amino acid or a vitamin in a plant or animal process, 

 he must first incorporate the radioisotope into the compound being 

 studied. Sometimes these labeled or tagged compounds can be made 

 by the chemist in the laboratory. Frequently, however, it is necessary 

 to make them by biological means, that is, the radioisotope in some 

 simple form is injected into an animal and subsequently extracted 

 from the blood, urine, or tissues of the animal as the desired complex 

 compound. 



The tracer technique to a greater extent, however, derives its power 

 from a combination of extreme sensitivity and unique specificity. So 

 sensitive are the methods for measuring the radiations from radioiso- 

 topes that it is possible to detect the presence of atoms with millions 

 to hundreds of millions times the sensitivity possible with other ordi- 

 nary physical and chemical means now known. It is not difficult to 

 detect radioisotopes that have been diluted as much as a billion or 

 10 billion times, while dilutions of more than a trillion are attainable 

 (pi. 4, fig. 1). This means that in a tracer experiment in biology it 

 would be possible to detect one-hundred-millionth of an ounce of radio- 

 active material after it had become distributed in an animal as large 

 as a 1,000-pound cow. Or to put it another way, it would be possible 

 to detect 1 ounce of radioactive material, say radioactive sugar, mixed 

 uniformly in 100 million tons or in 2 billion 100-pound sacks of 

 nonradioactive or ordinary sugar. 



When we say that the tracer method has a unique specificity, we 

 mean simply that radioisotopes provide scientists with the ability to 

 follow a specific batch of atoms through a complicated system irre- 

 vspective of all the chemical processes that may be going on. For ex- 

 ample, it would be possible to trace an isotope in a soil nutrient through 

 a plant grown on the soil, through a cow fed on the plant, and finally 

 through a rabbit fed on milk obtained from the cow. Even though 

 the isotope would pass through a number of complex processes, its 

 telltale radiation would permit its positive identification throughout. 



Radioactive tracer atoms have allowed us to increase our power 

 of perception. They have permitted measurements and analyses at 

 concentrations far below those hitherto permissible. Equally im- 

 portant, they have permitted us positive identification of products and 

 processes. Their value as research tools can perhaps be best described 

 by noting what they have meant to the field of biology. 



In the seventeenth century the invention of the microscope marked 

 the beginning of our understanding of the importance of individual 

 cells and their relations to the whole organism. The discovery of 



