each radioactive compound on the paper. A quantitative 

 determination of the amount of radiocarbon in each com- 

 pound may then be made by placing a Geiger-Miiller tube 

 with a very thin window over the radioactive compound on 

 the paper and counting electronically the emitted ^ particles. 



The next stage in the method of radiochromatographic 

 analysis is the identification of the radioactive compounds. 

 This identification is accomplished in a variety of ways. 

 When a familiar set of chromatographic solvents has been 

 used, the position of an unknown compound compared to 

 the positions of known substances provides a clue to its iden- 

 tity. The next step may be elution or washing of the com- 

 pound off the paper and the determination of such chemical 

 and physical properties (e.g., the distribution coefficient) of 

 the substance as can be measured with a solution of a few 

 micrograms or less of the material. These properties are then 

 compared with those of known compounds. The final check 

 on the identity of the compound is frequently made by plac- 

 ing on the same spot on filter paper the radioactive com- 

 pound and 10 to 100 /xg of the pure nonradioactive substance 

 with which the radioactive compound is thought to be iden- 

 tical. The new chromatogram is then developed. A radio- 

 autograph is prepared to locate the radioactive substance, 

 after which the paper is sprayed with a chemical spray (for 

 example, ninhydrin for amino acids), which produces a color 

 where the carrier compound is located on the paper. Super- 

 position of the paper chromatogram and the radioautograph 

 (x-ray film) will show an exact coincidence between chem- 

 ically developed color on the paper and the black spot on 

 the film, provided the two substances are identical. 



Once the identity of the radioactive compounds formed 

 during a short period of photosynthesis had been established, 

 experiments were performed under a variety of conditions 

 and times of exposure of the algae to radiocarbon. 



The radioautogram from the experiment with Chlorella 

 described above is shown in Figure 3. Even after only 10 

 seconds of exposure to C^^, a dozen or more compounds are 



14 



