BASIC DIFFICULTIES IN TRACER METHODOLOGY 



65 



by preferential absorption and urinary excretion of the radiocontam- 

 inants. The difficulty was eliminated by the electrolytic purification of 

 the original Cu*'^ preparation. These findings emphasize the possibility 

 of unexpected interference from radiocontaminants and the precautions 

 necessary when very large amounts of a 'principal activity are used to dem- 

 onMratc very small accumulation in some parts of a system. 



Removal or Elimination of Radioactive Impurities. If the impurity 

 has a half-life much less than that of the principal activity, then inter- 

 ference is eliminated by allowing a lapse of time sufficient for the impurity 

 to remove itself by decay. Except for parent-daughter isotopes, this 

 period of waiting can come at any time during the experiment just so 

 long as it occurs before the final measurement of the sample. In the case 

 of a short-lived daughter, this method can also be used except that the 

 period of waiting must occur between the time of final sample preparation 

 and the time of measurement. 



To illustrate the parent-daughter relationship, let us consider the couple 

 20-yr Sr^^-GO-hr Y''", in which the daughter is relatively short-lived as 



Biological System 



Samples 



after being 



taken 



Samples 



after standing 



21 days 



(Sr^o-Y^") 



equilibrium 

 mixture 



Fig. 2-2. Schematic illustration of the method of handling parent-daughter isotopes 

 in which the daughter is relatively short-lived, by allowing a lapse of time between 

 taking the sample and the radioassay. 



compared with the parent. In an equilibrium mixture one normally 

 measures the energetic beta ray of Y^"^, which is then a direct measure of 

 the Sr'J" content. If one has pure Sr^", it will take about 21 days for equi- 

 librium to be attained, after which the Y''" will be decaying at the same 

 rate as it is being produced. After this mixture is placed in a biological 

 system, as illustrated in Fig. 2-2, let us postulate that the Sr^" gQgg ^q 

 tissue A and that the Y'^^ goes to tissue B. As the Sr^" deposited in tissue 

 A gives rise to Y^°, the latter may either stay there or be continually 

 removed for deposition in tissue B. After sample A is taken and proc- 

 essed, there can be no further removal of Y^" by translocation, and after 

 waiting for about 21 days, there will again be a Sr^^-Y^" equilibrium, and 

 the measurement of the Y-*" will indicate how much Sr^" was present in 

 sample A. Likewise, after sample B is taken and set aside for 21 days, 

 the Y^° activity will have decayed, and the measurements will indicate 



