224 CARBON 



increased by using up to 3 mc and a 6-day exposure period (C-13). 

 Labeled cotton cellulose was prepared by introducing 12.5 ^c glucose- 

 1-C^^ through the sliced stem just below the 21-day boll (C-14). Plant 

 pathogenic fungi have been grown satisfactorily on a medium containing 

 30 Mc/ml of C^''-labeled sucrose (C-15) ; fungal spores, tagged by a 5- to 

 7-day growth period on this medium, showed no radiation effects. Levels 

 up to 100 Mc/ml were used, and although growth was retarded, there was 

 no loss of pathogenicity in the surviving organisms. Callus cultures of 

 Sequoia sempervirens were grown for 2, 16, 40, and 85 days on a medium 

 containing 1 to 1.5 /ic/ml of C^Mabeled sucrose (C-16). 



Preparation of Sample. Reference (C-17) may be consulted as a 

 standard text for many of the procedural details in connection with C^^ 

 analysis. Perhaps the most general method involves the wet combustion 

 of the biological materials, followed by collection of BaCOs, details of 

 which have been described in Chap. 5. However, many workers obtain 

 adequate measurements by direct plating of the samples, and following 

 are a few examples of some techniques employed: It was shown that 

 C'^-nicotinic acid in urine, plasma, or laked erythrocytes could be directly 

 plated over a given area to give results reproducible within 2 per cent 

 (C-18). It was emphasized that calibration curves must be prepared 

 under the same conditions as for sample measurement using the partic- 

 ular biological material. A procedure is described for direct mounting of 

 C^ ''-labeled fatty acids which was less tedious and gave more sensitive 

 measurements than conversion to BaCOs on account of a more favorable 

 C^^/sample mass ratio (C-19). An aluminum disk, 1.75 in. in diameter, 

 was lined with a piece of lens paper, weighed, and placed about 6 in. below 

 an infrared lamp. An aliquot of the fatty acid solution (usually 1 ml) 

 was added dropwise to the warmed lens paper so as to keep the surface 

 constantly wet but without permitting the fat solution to creep over the 

 edge. The disk and contents were reweighed for mass self-absorption 

 corrections. Up to 40 mg was safely mounted in this way. If an ether 

 solution and evaporation by an air stream were employed, it was practica- 

 ble to deposit up to 150 mg of fatty acids. After administration of 

 C'Mabeled adrenalin to rats, the tissues were ground with four portions 

 of 1 per cent trichloroacetic acid with the aid of sand, and the combined 

 extracts were centrifuged, washed, evaporated on aluminum plates, and 

 counted (C-20). Combustion of macerated tissues indicated that more 

 than 90 per cent of the radioactivity was consistently extracted by this 

 procedure. Urine, plasma, and blood cells were plated directly. 



Counting Techniques. The choice as to method of counting lies pri- 

 marily among the thin-mica-window tube, the internal counter, and the 

 ionization chamber plus the vibrating reed. The decision will be based 

 in large part upon the required sensitivity, the self-absorption character- 



