46 ADVENTUKES IN RADIOISOTOPE RESEARCH 



Comment on papers 1,2,3 



Radioactive tracers aio very often applied in the solution of problems which 

 can only he solved by making use of this device. Often this method is, however, 

 applied not as a necessity but for the sake of convenience, for facilitating the 

 solution of problems which can be solved by other methods as well, though moic 

 tediously. An example of the latter is the determination of the lead content of 

 platinum black described in paper 2. By making use of labelled lead the analysis 

 and the behavioui' of platinum black under the effect of heat, of electrolytic 

 polarization and so on could be carried out without even dissolving the sample 



The determination of the solubility of lead sulphide and chromate in water 

 discussed in papers is a border case. The writer of the well-known text-book on 

 physical measurements, Kohlrausch, succeeded in calculating the solubility of 

 lead carbonate from the electrolytic conductivity data, but he arrived in the 

 case of lead chromate at a rough, estimate of its solubility only. 



In the determination of the lead content of rocks by making use of isotope 

 dilution control an isotope of lead is necessarily to be used. This investigation 

 was the first application of the isotope dilution method. Being interested in the 

 abundance of the elements, we determined various constituents of an artificial 

 air crust sample prepared by my colleague, the well-known mineralogist Schneidei- 

 hohn, who disposed over a very extended collection of rock and mineral samples. 

 We usually applied the method of analysis of X-ray spectroscopy using a secondary 

 X-radiation method worked out in the Freiburg laboratory in which these in- 

 vestigations were carried out. 



Knowing the intensity ratio of two very closely situated X-ray lines, for example 

 that of HfLj andLu/?2' ^Y adding to a pulverized sample to be analysed a known 

 amount of LuOg we can, after taking an X-ray spectrum, calculate from the 

 intensity ratio of the 2 above-mentioned lines the unlinown hafnium content of 

 the sample. When the X-ray spectrum is excited by cathode rays the sample 

 gets hot and sputters easily ; for this reason a continuous X-ray spectrum is 

 first produced on a metallic tungsten surface and the sample irradiated by this 

 continuous rontgen radiation. Heating of the sample and consequent sputtering 

 is now avoided. The application of the method in the above-mentioned case 

 assumes the absence of significant amounts of the rare lutetium in the sample 

 investigated. 



The sensitivity of this method (in the twenties of this century, when we apphed 

 it) did not suffice to determine the very small amounts of lead present in rock 

 samples and correspondingly we embarked on a chemical determination of lead 

 controlled by making use of isotope dilution. This device, discussed further on 

 p. 9G, proved to be a very useful one in chemical analysis. 



