GENERAL PROCEDURi:S FOR RADIOASSAY 1 ()7 



this time the stopcock connectiiifi; the nonradioactive carbon dioxide 

 supply with the combustion flask is opened, and the gas is aUowed to 

 sweep through the sohition for several minutes. The chaml)er is moved 

 to position I, where cari)on dioxide is admitted rapidly to bring the gas 

 to atmospheric pressure. The use of the pressure regulator // between 

 the combustion flask and the ion chamber obviates liquid-nitrogen traps 

 and permits the oxidation to be carried out at atmospheric pressure. At 

 the begiiHiing of the detei-mination, when the evacuated chamber is 

 opened to the regulator, the mercury seal prevents the entrance of gas 

 through the disk. When carbon dioxide is evolved with conse(|Uent 

 increased pressure on the right arm of the mercury, the mercury column 

 is depressed, and gas is allowed to flow through the sintered disk until the 

 pressure on the right side is again ecjual to atmospheric pressure. Since 

 the rate of gas evolution is relatively low, all that is observed is a slight 

 flutter of the mercury surface. Figure 5-26 shows the details of the pres- 

 sure regulator. 



With this very simple apparatus, which can be mounted on one ring 

 stand, samples may be combusted in 10 to 15 min with very good preci- 

 sion. Whenever materials containing halogen or nitrogen are burned, a 

 U tube containing 5 to 10 g hydrated stainious chloride is inserted between 

 the water condenser and the pressure regulator. Small amounts of these 

 gases interfere with the operation of the regulator but do not affect the 

 ion-chamber performance. 



Preparation of Sample for Counting. The form of the sample required 

 will be determined primarily by the energy of radiation emitted, and this 

 matter will be discussed in detail in later sections. In general, samples 

 must be geometrically and physically uniform, these recjuirements being 

 more rigorous for the lower-energy beta emitters. If litiuid counting is 

 feasible, the ash solutions or aliquots thereof may be used directly. A 

 combination of wet-ashing and liquid counting makes a very convenient 

 and simple procedure. 



Direct Evaporation. A radioisotope in solution may be converted to a 

 solid sample by direct evaporation. However, most biological samples 

 W'ill require a preliminary separation of unwanted soluble matter because 

 of crystallization and creeping, which may result in nonuniform deposits. 

 Nevertheless direct evaporation is widely used, especially for the prepara- 

 tion of calibration standards, since these solutions usually have verj^ little 

 soluble material. The texts of Cook and Duncan (8) and Calvin et al. 

 (6) have described some of the various procedures for the preparation of 

 samples in this manner. 



The sample is mounted on the same material of the same geometric 

 shape as will be used for all later samples that are to be compared with it. 

 The mount may be a flat metal or plastic disk, a planchet, a glass surface. 



