166 



RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



suspended in 10 to 15 ml of the alcohol-ether and transferred to the 

 plastic-tube-and-cup assembly, as illustrated in Fig. 5-5 and described 

 on page 170. After centrifugation the supernatant is siphoned off care- 

 fully, the cup is removed, and the precipitate is carefully dried for weigh- 

 ing and counting. 



When manometric measurement of evolved CO2 is desired, the proce- 

 dure of Van Slyke, Steele, and Plazin (25) may be followed. Reference 

 (25) may also be consulted for details of combustion and handling of 



Carbon 

 dioxide 



Mercury 



Drying 

 tube 



Pressure 

 regulator 



\ Ionization 

 chamber 



n 



C 



'-r,-,-r'. 



Ionization 



chamber 



I 



Pressure 

 regulator 



J 



K 



To vacuum 

 pump 



Ionization 

 chamber 



)F A sea rite 



Combustion absorber 

 flask 

 (a) (b) 



Fig. 5-2. (a) Assembly for wet combustion and collection of C'^02 in an ion chamber for 

 measurement. [From O. Kenton Neville, Carbon 14 Sample Preparation and Counting 

 Techniques {Gas Counting), in "The Role of Atomic Energy in Agricultural Research," 

 Proceedings of the Fourth Annual Oak Ridge Summer Symposium {Sponsored by the Oak 

 Ridge National Laboratory and Oak Ridge Institute of Nuclear Studies, Aug. 25-30, 

 1952), r/D-5115, pp. 126-150, January, 1953.] (6) Details of pressure regulator 

 employed in combustion assembly. {Courtesy of 0. K. Neville and W. B. Leslie.) 



samples to be measured in the Bernstein and Ballentine (44) counter, 

 which is a gas-phase proportional counter now commercially available. 

 Figure 5-2a, taken from Neville (28), shows a schematic diagram of a 

 wet-combustion apparatus which may be used for larger samples than 

 above and which illustrates the collection of CO2 in an ion chamber. The 

 ion chamber, placed in position I, can be evacuated and flushed with non- 

 radioactive carbon dioxide which is passed from a cylinder through the 

 bubbler B and drying tube. At the beginning of an assay the ion cham- 

 ber is evacuated in position I and moved to position II. The organic 

 sample of 2 to 20 mg (or larger if relatively more solution is used) is 

 weighed into the combustion flask F together with 200 mg potassium 

 iodate. A 15- to 20-ml portion of combustion fluid is released into the 

 flask, and the chamber stopcock is opened. The combustion fluid is 

 heated to boiling and allowed to stand for about 5 min. At the end of 



