176 



RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



Calibration Curves and Standards. Determination of the true dis- 

 integration rate of a radioactive sample is not generally required in bio- 

 logical studies except where quantitative data are needed on the radiation 

 dose to the tissues. The true disintegration rate can be estimated from 

 the observed counting rate with an end-window counter, but the complex- 

 ity of such absolute beta counting precludes the use of these methods in the 

 ordinary laboratory (37 to 39) . If absolute values are needed, it is recom- 

 mended that known standards be obtained from the National Bureau of 

 Standards or elsewhere which can then be employed to calibrate the 



1000 2000 3000 4000 5000 6000 7000 8000 9000 



Counts per minute 



Fig. 5-6. Typical calibration curve for comparison of amounts of radioisotope in 

 standard and sample. 



specific-counting assembly by simple measurements under normal condi- 

 tions. Manov (40) has summarized the status of standardization of 

 radioisotopes as of 1953. Working standards are available for C'*, Co^**, 

 ji3i^ P^-, and Ra D-E. Additional experimental work needs to be done 

 on Fe^^, Fe^^ Au^**^, Na-*, and H^. Progress on the establishment of 

 British standards has been recorded by Perry (41). 



The primary problem is to evaluate the amount of radioactivity in a 

 given sample in terms of the amount that was originally introduced into 

 the biological system. One way of doing this is described here. With a 

 new counting assembly or tube, a series of dilutions of the radioisotope 

 should be measured to cover the normal counting range, as in Table 5-2. 

 A linear plot of some function of the radioactivity vs. the counting rate 

 should result in a straight line passing through the origin, as illustrated in 

 Fig. 5-6. If such a straight line is obtained with the deviations at the 

 higher counting rates within an acceptable error, then a single point can 



