GENERAL PROCEDURES FOR RADIOASSAY 189 



reasonable sensitivity of measurement. For example, the minimal 

 detectable activity of Co*^" with an (Mul-window counter, which measures 

 mainly the beta rays, was found to he 1 .8 X 10^^ fiv, whereas a scintillation 

 counter, which measures mainly the gamma rays, was about half as sensi- 

 tive, requiring 3.6 X 10"* nc to give a count equal to that of the back- 

 ground. The scintillation counter would usually be the instrument of 

 choice despite the lesser sensitivity, because if the gamma rays are meas- 

 ured, there would be no need to digest the tissues, separate out the cobalt, 

 and correct for self-absorption. All these operations reciuire considerable 

 time and labor as well as provide opportunity for losses. As another 

 example, the scintillation counter has enabled the use of Cr^^ for routine 

 blood-volume determinations where otherwise it would not be feasible. 



The scintillation counter is also of advantage for in vivo directional 

 detection, as will be discussed later. The possibilities of Hquid scintilla- 

 tion counters are exciting, because the sample ma}^ be intimately mixed 

 with the scintillation liquid to give 100 per cent geometry and zero absorp- 

 tion losses even with weak radiations. This method should be especially 

 advantageous for carbon 14 and tritium (50 to 52). Although the proce- 

 dures have yet to reach the practical stage, it is anticipated that they will 

 eventually become of considerable importance. 



Assay of Radioisotope Mixtures. As indicated in Chap. 1, there are 

 many studies in which it is of considerable advantage to use two radio- 

 isotopes simultaneously. This poses the problem of the measurement of 

 each when both are present in a single sample. The same problem arises, 

 as mentioned in Chap. 2, when it becomes necessary to eliminate or eval- 

 uate the contribution that may be made by a radioactive impurity. Of 

 the four methods discussed below, the chemical separation procedures are 

 limited to isotopes of different elements, whereas the others may, in addi- 

 tion, be used with isotopes of the same element. 



The decay, absorption, and differential radiation methods are similar 

 in principle, and certain aspects apply to all three. A mathematical 

 treatment has been presented by Tait and Williams (53) which may be 

 consulted especially for its discussion of optimum conditions. A most 

 important consideration is the error that may result on account of sub- 

 tractive procedure^. This is illustrated for the decay method in Table 

 5-5. The term count, as it is used here, refers to counts pef unit time. 

 Assume in ca.se 1 that a mixture of isotopes A and B counts 10,200 

 at zero time and 5000 at time t, during which .4 has decayed one-half and 

 B has decayed to below detection. The amount of A originally present 

 is 2 X (5000 + 71) = 10,000 ± 142, and the amount of B is then the 

 difference l)etween 10,200 ± 100 and 10,000 ± 142 = 200. Obviously 

 the value of B has little significance because of the errors in the two large 

 numbers. In case 2, where B is in excess, values for both A and B are 



