GENERAL PROCEDURES FOR RADIOASSAY 183 



ously, if there were enough activity present, direct measurements could 

 be made even on dried urine, provided the physical nature of the sample 

 were satisfactory. Likewise, satisfactory direct measurements on bone 

 would be contingent upon the preparation of suitable uniform samples. 

 The lower the beta energy, the more critical the physical nature of the 

 sample. When possible, it is always most convenient to make direct 

 measurements on the biologic material, and this possibility should always 



Table 5-4. Ratios of Calcium to Sample Mass for Typical 

 Biological Materials 



Sample Ca/mann 



Bone ash (acid-treatecl) 0.2 



Soft-tissue ash (acid-treated) 0.005 



Rat feces ash (acid-treated) 0.015 



Plant ash (acid-treated) 0.07 



Urine ash (acid-treated) 0. 0006 



Dried urine . 0002 



CaC204-H,0 0.27 



CaC()3 0.4 



[From C. L. Coniar et al., Use of Calciuni-45 in Biological Studies, Nucleonics, 8: 

 19-81 (1951).] 



be considered. For example, direct measurement of C^"* in organic com- 

 pounds with a high carbon/mass ratio has been found satisfactory (43). 

 Whenever direct measurements of low-counting biological material are 

 made, one must consider the possibility that the radiation measured may 

 be due to naturally occurring K^° (see Chap. 6 under Potassium). If the 

 samples are hygroscopic (and this is often the case with biological mate- 

 rial) , the absorption of water may produce serious counting losses. This 

 can be avoided by keeping the sample in a desiccator until use or by heat- 

 ing just before measurement. In the latter case it must be ascertained 

 that volatilization does not occur. 



INSTRUMENTATION AND METHODS 



As indicated in Chap. 3, all discussions of instruments will be limited to 

 those commercially available and will for the most part be restricted to a 

 particular type found adequate for each purpose. The manufacturer's 

 catalogues can be consulted for specific items (12). It is suggested that, 

 whenever possible, all units that are operated together be purchased from 

 the same manufacturer so that there will be no difficulties in making the 

 connections. The choice as to type of equipment will mainly depend 

 upon the radiation characteristics of the isotope, and these considerations 

 will be discussed below. The decision as to the advantages of the more 



