422 



RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



inside of the tube was rinsed with small amounts of distilled water which 

 were added to the nickel dish. The sample was dried and then counted 

 with an end-window Geiger tube using a thick absorber of 4.6 g/cm^ and 

 again with a thin absorber of 0.46 g/cm\ From these differential absorp- 

 tion measurements it was possible to calculate the Na^^ and K^s content 

 of each sample. The tubes containing the standard or comparison sam- 

 ples were opened, the contents made to 25 ml, and aliquots taken for titra- 

 tion of carbonates and preparation of counting samples. 



P^2 was measured in all the samples after allowing them to stand for 

 2 weeks so as to let the Na^^ and K^^ decay and using a 25-mg/cm2 

 absorber to eliminate the soft-beta radiation from S^^ The counts were 

 standardized against the P^- in the comparison samples of KH2PO4. 



Some samples were counted again about 3 months after irradiation to 

 determine the amount of S^^ and hence of chloride, present. Com- 

 parison samples of NaCl were irradiated with the nerves for the chloride 

 estimation. 



Some typical counting rates are shown in Table 10-4 which illustrate 

 the differential effects of the absoi^bers. Consideration of the chemical 

 elements present in the tissue indicated that there was little error except 

 that Br«2 may have increased the apparent Na content by about 1 per 

 cent. Quantities of the order of 0.3 /.g Na and 3 Mg K were easily 

 determined with a standard error of about ±2 per cent. 



Table 10-4. Principal Radioisotopes in Irradiated Sepia Axon" 



Na24 



p32 



No absorber 



15,000 



12,000 



1,500 



150 



Beta absorber, 

 0.46 g/cm2 



450 



3000 



15 







Gamma absorber, 

 4.6 g/cm2 



400 



20 











« Counts per minute for a 1-mg sample at 10 hr after removal from pile 

 [From R D. Keynes, and P. R. Lewis, The Sodium and Potassium Content of 

 Cephalopod Nerve Fibers, /. Pkysiol. London, 114: 151-182 (1951).] 



Gold. In addition to radioactivation analysis for elements naturally 

 occurring in tissue, it is possible to introduce a foreign element into the 

 biological system and to follow its fate by this procedure. This is very 

 similar to the conventional tracer methods. Advantages are offered in 

 that elements may be used whose radioisotopes are too short-lived for 

 conventional procedures. Also the study can be carried out without any 

 possibility of radiation effects on the organism. 



Gold has been used in this manner by Tobias and Dunn (21) . A mouse 

 was intravenously injected with 10 ^g of stable Au as a soluble sodium 



