693 ADVENTURES IN RADIOISOTOPE RESEARCH 



parison. In our later experiments we have also compared the activity of 

 the nucleic acicl with that of the free phosphorus in the sarcoma. In the 

 latter procedure a portion of the sarcoma must be sacrificed in order 

 to isolate the free phosphate; in spite of this the procedure is definitely 

 preferred to the one first described and at first applied in our experi- 

 ments . 



Plasma and liver samples were treated with 10 and 25% trichloro- 

 acetic acid, respectively; 80 mgm of sodium phosphate was added to four- 

 fifths of the solution, the free phosphate in the solution was precipitated 

 as magnesium ammonium phosphate and the radioactivity of the sample 

 was measured, as described above, with the Geiger— Miiller counter. 

 One-fifth of the solution w^as used for colorimetric determination of the 

 free phosphorus. The magnesium ammonium phosphate samples were 

 placed in aluminium trays, 1.2 cm in diameter and 2 mm deep, before 

 the activity was measured; the trays were pushed beneath the window 

 of the counter. Whereas the liver fractions measured yielded more than 

 1000 pulses/min in the counting tube, the activity clue to the nucleic 

 acid of the sarcoma usually amounted to only a few pulses per minute; 

 in isolated cases, indeed, it was necessary to measure activities amount- 

 ing to only a few-tenths of a pulse per minute. Such measurements were 

 performed by alternating measurements of the activity of the sample 

 and of inactive magnesium ammonium phosphate for periods of 24 hr 

 each. The background amounted to about 4 pulses/min and could be 

 reproduced with an accuracy of 2 per cent. Temperature variations of 

 the counting apparatus in excess of about 1° C must be avoided if 

 such accuracy is to be attained. 



Determination of the Percentage of Injected ^^P Found in the Nucleic 

 Acid of the Sarcoma 



In determining the percentage of the injected radioactively labelled 

 phosphorus incorporated in the nucleic acid of the sarcoma, the procedure 

 is as follows: 80 mgm of sodium phosphate are added to a known small 

 fraction, e. g. one-threehundredth of the solution used for injection; 

 the P content of the solution is then precipitated as magnesium ammo- 

 nium phosphate. The activity of this sample is compared with the acti- 

 vity of the nucleic acid phosphorus, isolated from the sarcoma, which 

 is also in the form of magnesium ammonium phosphate and of the same 

 weight as the above sample. If the activity of one-threehundredth of 

 the injected solution is equal to 100 pulses/min, then 30,000 pulses 

 were injected into the rat. If the activity of 1 mgm of nucleic acid phos- 

 phorus is found to be 10, then 1 mgm of nucleic acicl phosphorus contains 

 0.033 per cent of the injected ^^p. The values recorded in Tables 1 to 



