10 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



cannot be used until the investigator is certain by experiment that con- 

 stant specific activities do obtain in the system. 



In certain cases the equihbrium may be attained by repeated adminis- 

 tration of the isotope over long periods of time. Van Aliddlesworth (50) 

 has used this technique in a study of thyroid function in rats on an 

 extremely low iodine intake. Because of the low iodine levels in the diet 

 and in the body, it was not practical to analyze for the various iodine 

 fractions in the blood. By incorporation of radioiodine in the diet to be 

 used over the experimental period, it was considered that after a few days 

 all the iodine fractions in the animal's body would reach an equal specific 

 activity. It must be remembered that the specific activities of the diet 

 and body iodine were all decreasing at the same rate owing to radioactive 

 decay. Thus the various iodine fractions of the blood could be separated 

 in the usual way and contained enough radioiodine to be counted and to 

 give a measure of the stable iodine variations due to the experimental 

 treatment. This general procedure may become very valuable, especially 

 in radioisotope studies with calcium and phosphorus where the exchange 

 of ions between blood and bone interferes with single-dose studies (see 

 Chap. 2). 



Isotope Dilution. Isotope-dilution methods and variations thereof rep- 

 resent most valuable procedures in biochemistry and analytical chemistry, 

 since they provide data that are not at all or only with difficulty available 

 from other procedures. In principle, the method consists in incorporating 

 uniformly a small amount of the labeled test substance in the material to 

 be analyzed, isolating some of the test substance from the mixture, and 

 determining its isotopic content. The amount of dilution that has 

 occurred is a function of the amount of the test substance in the original 

 material. 



Volume Determinations. If 1000 counts/min of radioactivity in a 

 negligible volume is mixed into a volume of water that then measures 

 10 counts/min/ml, it is obvious that the volume of water was 100 ml. 

 The general statement may be derived simply from the fact that the 

 amount of radioisotope in the system is constant regardless of dilution. 

 Thus, if a known amount of radioactivity, let us say A counts/min in 

 B ml solution, is thoroughly mixed with an unknown volume, V ml, and 

 a small sample is taken which measures S counts/min/ml, then the fol- 

 lowing is true: 



A A 



V = -rj — B which reduces to V = tt (1-2) 



when B is small compared with V, as is often the case. 



The greatest advantage of this technique will be in situations where it is 

 impossible to measure the unknown volume directly or to disturb the 

 system in which it occurs. This is the case in humans and animals for the 



