GENERAL PROCEDURES FOR RADIOASSAY I'M 



From Eq. (5-4) it is tluMi calculated that in the original sample there 

 were 2000 counts of S^^ and 1000 counts of P^'-. In practice, these counts 

 would be converted to amounts by calibration standards and self-absorp- 

 tion corrections, as already described. 



Differential Absorption. In a mixture of two beta emitters of different 

 energies, the assay is performed by measurement with and without an 

 absorber. The ideal situation occurs when the beta particles from one of 

 the isotopes can be completely absorbed. This was illustrated in studies 

 reported by Comar et al. (32) on the simultaneous measurement of Ca"*^ 

 and P^". The mixed source was measured with a thin-mica-window 

 counter and then with an absorber of about 55 mg/cm- which essentially 

 stopped all the Ca^^ beta rays but reduced the P^- contribution by a factor 

 of only 1.5. In line with the previous discussion, accurate results were 

 obtained only when the Ca'^^/P^- activity ratio was 1 or higher. Tait and 

 Williams (53) assayed Na^'* and K^- and reported that the optimum condi- 

 tions were obtained with a sample thickness of 10 to 12 mg/cm- and an 

 absorber of about 300 mg/cm'. 



Equations (5-4) and (5-5) also apply for differential absorption, but the 

 symbols have the following meaning: 



Co = count with no absorber 



Ct = count with absorber 



A ^ count of isotope A wdth no absorber 



B = count of isotope B with no absorber 

 Fa = fraction of isotope A counted with absorber 

 Fb = fraction of isotope B counted with absorber 



Different Radiations. An important example of the use of different 

 radiations for double counting is afforded by the work of Peacock et al. 

 (54) and Saylor and Finch (55) with Fe" and Fe^^ The sample was elec- 

 troplated in the usual way, and measurements were made with an argon- 

 filled Geiger counter with a beryllium filter that responded to the X rays 

 from Fe"^ and with a helium-filled counter that responded to the beta rays 

 from Fe^^ Since there was less than 3 per cent cross-counting in either 

 direction, there were no subtractive errors; this represents the ideal situa- 

 tion. The gamma-beta scintillation spectrometer may become very use- 

 ful for analysis of mixtures (56). 



Chemical Separation Methods. With isotopes of different elements, con- 

 ventional chemical separations may be employed. This has been used 

 with Ca"** and P^- (32). Where calcium oxalate is separated as part of 

 the regular procedure, it is a simple matter to determine chemical phos- 

 phorus and P'^' on the supernatant or filtrate. Usually there is no con- 

 tamination of the calcium oxalate with residual P^'-. However, this can 

 be checked with the absorber method as described above, and corrections 



