XVI. STABLE ISOTOPES AS TRACERS 581 



ammonium salt to a hypobromite treatment results in the liberation 

 of nitrogen. Detailed procedures and precautions based on years of 

 experience in isotope analysis have been published by Rittenberg 

 a, p. 31). 



Carbon dioxide is the gas of choice for the mass spectrometric 

 analysis of C^^. The numerous ionized particles produced are listed 

 in Table II. Although the molecular ions have the greatest mass and 

 therefore the least resolution, their peaks are measured because they 

 possess the highest intensity. The partial interference at mass 45 

 owing to 0^^ can be corrected for satisfactorily. Were it not for the 

 very low intensity of the atomic carbon peaks at the 12 and 13 posi- 

 tions, their improved resolution and freedom from interference 

 would recommend them as a basis for analysis. 



The preparation of carbon dioxide by the oxidation of organi(; 

 compounds is quite convenient. Some workers employ persulfate 

 oxidation (22). The wet combustion method of Van Slyke and Folch 

 [21) has been used with success by several workers in the field (1, p. 

 13). A dry combustion process has also been used and a satisfactory 

 apparatus and procedure have been described {1, p. 40). Decarboxy- 

 lation methods of obtaining carbon dioxide from fatty acids and from 

 amino acids {1, p. 41) should prove usefvil in special cases. The latter 

 technique involves the use of ninhydi'in (1,2,3-indanetrione hydrate) 



5. Measurements and Maintenance 



Before attempting measurements, the residual gas pressure in a 

 mass spectrometer should be reduced to about 10~^ mm. of mercury. 

 This may require heating the main evacuated part of the apparatus 

 to perhaps 300°C. in a degassing operation. In fact, such baking 

 may need to be repeated every time atmospheric air is admitted to 

 the system. It is customary to maintain a vacuum continually in the 

 apparatus by allowing the pumps to run without interruption. In 

 that event, trap 7'i of Figure 2 must always be kept cold by the use of 

 solid carbon dioxide or liquid air. During the progress of a measure- 

 ment on a test sample the pressure in the body of the instrument may 

 be approximately 2 X 10"** mm. of mercury, but the gas pressure in 

 the ionizing box will be still higher, perhaps 5 X 10"" mm. of mer- 

 cuiy. 



Some hints on tlie piai-ticai maiutcuance of a mass spectrometer 



