16 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



of the element or compound to be determined can be incorporated in the 

 sample to be analyzed, then a quantitative determination can be made 

 without requiring a quantitative separation. To illustrate this, let us 

 assume that a sample is to be analyzed for sulfate by the usual barium 

 sulfate procedure: 1000 counts/min of labeled sulfate is added before pre- 

 cipitation, and the final precipitate collected nonquantitatively is found 

 by the usual methods to contain 1 mg sulfate and 500 counts/min. 

 Assuming that the labeled sulfate added no significant mass of sulfate to 

 the sample, we know immediately that ^^Hooo X 100, or 50 per cent, of 

 the sulfate was lost and that the true content of the sample must have 

 been 2 mg sulfate. When the labeled element or compound adds no 

 significant mass to that being analyzed, the general relationship is obvious 

 from the above. When this is not the case, it is necessary to employ the 

 specific-activity relationship, a useful form of which follows: 



where M = unknown mass of substance in sample 



M* = known mass of labeled substance added to sample 

 SA* = known specific activity of labeled substance added to sample 

 SA = measured specific activity of mixture of sample and added 

 labeled substance 

 This equation is easily derived from the fact that the added radioactivity 

 is constant before and after mixing, and therefore the added radioactiv- 

 ity = {SA*){M*) = (SA){M* + M). 



A practical illustration of the use of this method has been given by 

 Rosenblum (60) in connection with the difficult analysis of vitamin Bio in 

 feed supplements. Cobalt-60-labeled vitamin B12 in the amount of 

 26.6 Mg was thoroughly mixed with 100 g of sample, which was then 

 extracted with water, and the extracts were further purified by reextrac- 

 tion and chromatography to give a product showing the typical absorp- 

 tion spectrum of vitamin B12. The amount of vitamin B12 recovered at 

 this point was 43 Mg, as estimated from the extinction coefficient. The 

 amount of radioactivity associated with this 43 Mg vitamin B12 was found 

 to be 14.6 per cent of that originally present in the added labeled vitamin 

 B12. Substituting in Eq. (1-8), 



(1 /or* p \ 



146/43 ~ / ^ ^^^ 



In this procedure it is necessary only to isolate an amount of the sub- 

 stance to be estimated sufficient to allow chemical and radioactive meas- 

 urement with the desired precision. Gravimetric, volumetric, colorimet- 

 ric or other usual methods can be used for the chemical determination. 



