588 FRED M. UBER 



To secure accurate determinations of even relative ratios, several 

 conditions must be maintained constant during the measurements. 

 One important factor concerns the pressure of the gas in the neigh- 

 borhood of the ion source. Adequate control of this pressure requires 

 the use of a mechanism that permits the small gas sample to leak ex- 

 tremely slowly into the ionizing compartment of the spectrometer, yet 

 without any time variation. To satisfy this requirement, the initial 

 sample of gas must be sufficient to enable an adequate and constant 

 differential pressure to be maintained across the leak mechanism (L 

 in Fig. 2) throughout the course of the ratio determinations. Like- 

 wise the pumps, which are continually evacuating the mass spectrom- 

 eter, must operate uniformly during the measurement period if 

 pressure fluctuations of the test gas within the ion source are to be 

 avoided. 



In order to create a constant supply of ions, it is also necessary to 

 control accurately the emission of electrons from the filament wdth 

 respect to their quantity, speed, and direction. This involves not 

 only the regulation of the filament temperature and the electron- 

 accelerating voltage applied between the filament and the electron 

 collector, but also the strength of the magnetic field, which collimates 

 the stream of electrons. With available electronic regulators, the 

 variations in the magnitude of each of these quantities can and should 

 be maintained within less than 1% during the course of an analysis. 



When the current through the principal magnet is stabilized at 

 some constant value, the emergence from the exit slit of particles 

 having a particular ratio of mass to charge is determined by the mag- 

 nitude of the ion-accelerating voltage. It will be noted in Figure 4 

 that the ion current peaks corresponding to various masses possess 

 flat tops. The intentional presence of these flat tops makes the peak 

 readings less sensitive to slight variations in either the ion-accelerat- 

 ing voltage or the current through the principal magnet. However, 

 a stabilizer will still be necessary if the equipment operates from an 

 alternating voltage supply. 



The effect of transient fluctuations resulting from instability of 

 the ion current amplifier or other electrical circuit components, as 

 well as from the causes just enumerated, can be minimized statisti- 

 cally by taking an extended series of measurements on the same 

 sample, with alternate readings on the heavier and lighter isotope 

 components. Frequent comparisons with the standard sample 

 serve a similar purpose. 



