CHAPTER 5 



GENERAL PROCEDURES FOR RADIOASSAY 



Pretreatment of Samples — Collection; Weighing and Drying; Ashing: Pseudo wet- 

 ashing; Wet-ashing; Dry-ashing; Oxidation for C^'^ assay; Preparation of Sample for 

 Counting: Direct evaporation; Filtration; Settling and centrifugation; Electroplating; 

 Briquet formation. Some Considerations in Counting Procedures — Statistical 

 Evaluation of Counting Errors; Comparison of Counter Sensitivities; Instrumental 

 Counting Losses; Calibration Curves and Standards; Self-absorption of Beta Parti- 

 cles: Thin samples; Samples of standard thickness; Empirical relationships; Infinitely 

 thick samples. Instrumentation and Methods — The Scaling Unit; Soft-beta 

 Counting; Hard-beta Counting; Gamma Counting; Assay of Radioisotope Mixtures: 

 Differential decay; Differential absorption; Different radiations; Chemical separation 

 methods; In Vivo Measurements. Expression of Data. 



The methods siiital)le for a specific analysis will be dependent upon the 

 element involved, the nature of the sample, and the degree of accuracy 

 and precision required. The present discussion will be concerned with 

 some of the basic techniques that serve as the keystone for the radioassay 

 of biological material, and the apphcation to individual radioisotopes will 

 be illustrated in Chap. 6. 



Whenever it is necessary to measure only radioactivity, the problem 

 is often simplified and becomes a matter of presenting a uniform sample 

 to the radiation-detecting device. However, in many experiments it is 

 essential to have chemical values in addition to radioassays so that turn- 

 over rates can be calculated. In any event, recourse should be made to 

 the extensive literature on chemical analysis, as indicated in general 

 references (1 to 5). The methods to be used should be planned so as to 

 give both chemical and radioactivity values with the minimum number 

 of steps. Usually the standard chemical procedures can be employed, 

 and somewhere in the operation an aliquot taken for radioassay. In some 

 cases it may be necessary to use a different sample or aliquot for the chem- 

 ical and radioactivity determinations because of the necessity of adding 

 carrier to improve the recovery of the radioisotope. 



It may be obvious, but there is one good way of testing an over-all 

 method: the addition of a known amount of radioactivity to a typical 

 biological sample and the determination of recovery after processing. 



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