300 



ISOTOPIC TRACERS AND NUCLEAR RADIATIONS 



[Chap. 9 



H3 and other combinations of hydrogen and deuterium depend primarily 

 on the pressure, and under the most general conditions the molecules and 

 masses shown in Table 32 may be observed [24]. 



Table 32. Hydrogen Molecule Masses 



When the concentration of deuterium is small, of the order of 1 per cent 

 or less, it can be assumed that all the deuterium is bound as HD and measure- 

 ments of only masses 2 and 3 are necessary. Mass 3 is the sum of HD and 

 H 3 , but since HD ~ P and H 3 ~ P 2 , by plotting the observed values of the 

 ratio 



(HD + H 3 ) aP + PP~ 



H, 



yP 



where a,/3,y = constants of proportionality 



against the pressure, or against H 2 to which the pressure is proportional, a 

 straight line is obtained. By extrapolating the line to zero pressure (H2 = 0) 

 the intercept on the ordinate gives directly the ratio a/y = HD/H 2 . When 

 the concentrations of hydrogen and deuterium are comparable, a more 

 complete analysis is indicated. 



9.15. Carbon. Carbon is usually introduced into the mass spectrometer 

 as carbon dioxide. Its preparation from the sample substance is straight- 

 forward. Carbon dioxide driven off by combustion of the substance is 

 absorbed in sodium hydroxide and the solution neutralized by addition of 

 ammonium nitrate or ammonium chloride in excess. Barium chloride is 

 then added in excess to precipitate the carbonate which is filtered or cen- 

 trifuged, washed with water, and dried in an oven at 1 10 to 135°C. If the ini- 

 tial alkali solution is weak, i.e., < IN, barium chloride can be added 

 directly. A few milligrams of dried barium carbonate are transferred to 

 a single-ended quartz microcombustion tube which is connected to the 

 sampling system. After preliminary heating under vacuum to drive off 



