622 ADOLF F. VOIGT 



measurement of deuterium combine to lessen the importance of 

 tritium as a biological tracer. Though some instruments for measur- 

 ing its 0.018 m.e.v. /S particle have been described, they will not be 

 discussed here. The other two of these isotopes, C^* and S^^ are 

 more easily made and so readily available that considerable thought 

 has been given to efficient ways of measuring them, particularly 

 C (33-37; l,p. 172). 



One solution is their introduction directly into the counting cham- 

 ber as a gas. In the case of carbon, the obvious gas that can be 

 obtained as the end product in any process is carbon dioxide. This 

 is fine for ionization chamber measurements since the chemical and 

 physical properties of carbon dioxide make it easy to handle in a gas 

 manipulating system. One such instrument described is based on a 

 Lauritsen electroscope and is reputedly easy to use and trustworthy 

 (34,37). The current produced across a charged spherical quartz 

 ionization chamber by the radioactive carbon dioxide put into it is 

 transmitted to the working parts of a Lauritsen electroscope where 

 it is measured. Modifications of this idea for H^ and S^'^ measure- 

 ments are also described. There are reports in the literature on a 

 number of other ionization chambers into which 0^*02 samples can 

 be introduced. 



A GM counter will not operate in its normal counting region if 

 appreciable amounts of oxygen, chlorine, carbon dioxide, or any 

 other gas containing a high percentage of an electronegative element 

 are present, so that one cannot introduce a C^*02 sample into such a 

 counter normally. Since a counter has a considerably greater sensi- 

 tivity than an ionization chamber, it has been a goal to obtain a 

 counter into which a C^* sample could be introduced. Although this 

 is possible if the C^'* is present in methyl alcohol or a similar com- 

 pound, the difficulties in the preparation of the latter from the 

 material in which the tracer is obtained from the experiment are too 

 great. It has been found that by adding a small percentage of car- 

 bon disulfide vapor to the carbon dioxide a stable counting region can 

 be obtained with voltages in the neighborhood of 3-4 kv. (35,36). 

 Since the maintenance of such voltages across resistors is a serious 

 problem and the technique of gas handling is not simple in itself, this 

 does not appear on the surface to be a solution to the problem that 

 would appeal to biologists. 



C^^ users are coming more to the opinion that the mica window 

 counter tube offers the best solution to their problems (3, p. 104). 



