582 



FRED M. UBER 



have been furnished by Young {19), who also has analyzed the per- 

 sonnel and time factors involved in its full time operation. 



In order to observe a peak corresponding to a given mass number, 

 the electrical constants of the mass spectrometer must satisfy the 

 following equation : 



4.82 X 10-5 r2(cm.) H"^ (gauss) 



M ^ 



— (mass units) = 



e 



V (volts) 



(2) 



By "mass unit" we mean the ratio of the atomic or molecular mass to 

 the number of unit charges on the ionized particle. For example, 

 M/e for CV^Qieo^'* would be 44 for the singly charged ion, and 22 for 

 the doubly ionized. The radius of curvature of the path around 

 which the ions are deflected (15 cm. in the apparatus of Figure 2) is 



en 



UJ 

 (£ 

 UJ 

 Q. 



s 

 < 



!2 

 1 



2 2 



X 



o 



UJ 



> 



o 



Q. 



Fig. 4. Mass spectrum of carbon 

 dioxide from early data of Nier (^), 

 showing only the most abundant molec- 

 ular ions (see Table II). Ion-acceler- 

 ating potential was approximately 500 

 V. Note the resolution, the fiat-topped 

 peaks, and the background current. 



47 46 45 44 43 



ATOMIC MASS UNITS 



represented by r, the uniform magnetic field by H, and the ion-acceler- 

 ating potential by V. In practice it is usually convenient to maintain 

 a constant magnetic field and to vary V. When observing particles 

 with mass numbers as different as hydrogen and carbon dioxide, for 

 example, it may also be necessary to vary H. To calibrate a mass 

 spectrometer initially, fix either H or V at a reasonable value and then 



