"THORIUM B" IN BLOOD VOLUME STUDIES 585 



a correction by waiting for a few hours, until tlie TliC of the plasma, 

 which has a half-life of one hour, has decayed. But even in this case, 

 when comparing the activity of different samples, we have to bear in 

 mind that a decay of half of the ThB present takes place in the course 

 of 10.6 hours. 



In evaluating the results of the activity measurements, we can avoid 

 correcting for the change in the activity with time by comparing Ihe 

 activity of the sample secured with that of the standard sample, provided 

 both change their activity with time in the same way. If the secured 

 sample has an initial activity of 200 counts per minute, for example, 

 we will register, in the course of 10 minutes, almost 2000 counts. In fact, 

 as the activity has decreased in 10 minutes from 200 to 197.8 counts per 

 minute, we will measure only 1987 counts in the course of 10 minutes 

 or 198.7 per minute. We now measure for one minute the standard sample, 

 having 2000 counts. The ratio between the activity of the standard 



sample and that of the secured sample works out then as — = iq 07 



198.7 



If greater accuracy is wanted, we again measure for 10 minutes the 



secured sample which has now a mean activity of 196.4, and repeat this 



procedure. Thus the correct activity ratio between standard and secured 



sample works out as 



2000 2000 ^^j2 



198.7 + 196.4] 197.6 



2 



As the standard sample is prepared by diluting active with inactive 

 blood, a strongly active standard sample can be easily obtained. The 

 measurement of the activity of such samples takes, in contrast to the 



200 H 



iOO 



^ 



n 1 1 1 — 



36 15 I 2 



Ser.opHs Hours 



Fig. 1. Change in the activity of blood with time after injection 

 of thorium B labelled whole blood. 



secured samples, only very few minutes. When applying an automatic 

 sample changer, which shifts five secured samples and one standard 

 sample at 10 minute intervals, and permits reading the counts on six 

 independent telephone counters, the following correction for decay has 



