936 



ADVENTURES IN RADIOISOTOPE RESEARCH 



expect 100 per cent of the lead present in the sample to have been 

 recovered as well. If only 50 per cent is recovered, for example, we 

 have to multiply the amount of lead recovered by 2 in order to arrive 

 at a correct analytical figure. The indicator method thus allows a cor- 

 rection for the shortcomings of the analysis. Such corrected analytical 

 figures are seen in Table 2. 



Table 2. — Lead Content of Igneous Rocks 



Rock types 



gm lead per gm ] Percentage Ra 

 rock precipitated I D recovered 

 by electrolysis l^y electrolysis 



gm lead per 



gm rock. 



Corrected value 



Gabbros and related types 



(composite of 67 samples) 

 Essexites and related types . . . . 



(composite of 40 samples) 

 Soda-granites and soda-syenites 



(composite of 26 samples) 

 Granite rocks 



(composite of 58 samples) 

 Basalt, Giant Causeway 



10- 



10- 



9- I0--« 



IS- 10- 



4- 10-« 



80 

 80 

 73 

 53 



100 



5- 10-** 



10. io-« 



11. 10-6 

 30. 10-6 



4. 10-6 



Instead of adding radium D to the solution to be analysed, we may 

 add lead labelled by the presence of some radium D, for example 100 

 mgm of lead having an activity of 1000 units. If we subsequently isolate 

 10 mgm of lead from the solution, this lead should show an activity of 

 100 units, under the assumption that the original sample does not con- 

 tain lead. If the activity of the isolated 10 mgm of lead is, for example, 

 found to be 83 only, we have to conclude that the sample contains lead 

 amounting to 20 mgm. 



In recent years, isotopic indicators have found an extended appli- 

 cation in biochemical analyses. Schoenheimer and his colleagues'^-^ 

 determined the leucine content in the protein of the rat by adding to 

 the hydrolysate a known amount of leucine containing heavy nitrogen. 

 This tracer was also used*^^^ in the investigation of the occurrence 

 of the amino acids of the d series in cancer proteins ; while Chargaff, 

 ZiFF and Rittenberg'-^^ used bases containing a known amount of 

 i^N in the analysis of the nitrogenous constituents of tissue phosphatides. 

 Amino acids containing deuterium as indicator were used by Ussing'"^' 

 and the same tracer was applied by Rittenberg and Foster'-*' in 

 their determination of the palmitic acid content of rats' fat. 



