694 ADYEA^TURES IX RADIOISOTOPE RESEARCH 



t 



action of fast neutrons on carbon disulphide but in this case the neutrons 

 had been obtained by means of a high voltage apparatus, usuahy at 

 the Institute of Theoretical Physics of the University of Copenhagen 

 but occasionally at the Research Laboratory of Philips Gloeilampfen- 

 fabrik in Eindhoven. 



The samples produced with the help of the radium sources tiid not 

 contain any chemically detectable amount of phosphorus. They were 

 obtained by shaking the 10 1. of carbon disulphide used for irradiation 

 with dilute nitric acid. The active phosphate remained in the residue 

 after evaporating the nitric acid solution; it was taken up in water and 

 the solution was filtered through a glass filter. This process was repeated 

 a few times. The activity was finally dissolved in physiological saline 

 solution and then injected into the experimental animals; 0.1 — 1.0 cm^ 

 was injected into each rat. The activity of the injected solution amounted 

 to about 0.1 ^c. 



Measurement of the Radioactivity of the Nucleic Acid Phosphorus 



The rat was killed 2 hr after injection and the nucleic acid of the 

 sarcoma was isolated. The Klein and Beck^ method of isolation was 

 used for this purpose. Besides the weakly active nucleic acid, very strongly 

 active acid-soluble compounds and strongly active phosphatides are 

 also formed in the sarcoma in the course of the experiment; contamina- 

 tion of the nucleic acid with the slightest amount of acid-soluble phos- 

 phorus or with phosphatide phosphorus can therefore interfere with 

 the results of measurement of the nucleic acid activity. For this reason, 

 the samples of nucleic acid were purified still more thoroughly than 

 in the above-mentioned experiments of Klein and Beck. The 

 purified nucleic acid was wet-ashed with sulphuric acid and 30% HgOa. 

 One-fifth of the solution obtained was used for the colorimetric deter- 

 mination of phosphorus; four-fifths of the solution were treated with 

 80 mgm of sodium phosphate and the phosphorus content was isolated 

 as magnesium ammonium phosphate. By this means the active phos- 

 phorus content of the sample was present in a mixture with about 70 mgm 

 of inactive magnesium ammonium phosphate. All our preparations 

 were treated in this way and we were thus able, in determining the activity, 

 always to compare samples of nearly the same weights and volumes. 

 Any contingent correction for different absorption of ^^-radiation in 

 the samples being compared, was thus avoided. 



The activity of the magnesium ammonium phosphate or of a known 

 fraction of the sample was measured with a Geiger— Miiller counter. 

 The activity of 1 mgm of phosphorus was calculated from the measured 



2 O. Klein and Beck, Z. Krebsforsch. 42, 172 (1935). 



