Sec. 9.12] MASS SPECTROGRAPHS 297 



vanometer circuit is used as a null indicator. When a mass peak has been 

 maximized on the galvanometer, an accurately linear voltage from a poten- 

 tiometer or decade-battery circuit is applied to the bottom of the input grid 

 resistor, deflecting the galvanometer back to the base line. The peaks are 

 then found in terms of millivolts rather than centimeters of deflection and 

 are as accurately measured as the applied back voltage is linear. A more 

 satisfactory method that is rapidly replacing the simple electrometer- 

 galvanometer circuit is found in the use of a high gain inverse-feedback 

 amplifier coupled directly to the output of the electrometer stage. With a 

 d-c amplifier gain of 5,000 to 10,000 and 100 per cent negative feedback to 

 the input resistor of the electrometer circuit, the over-all voltage gain of the 

 system is very nearly 1, but the output voltage is accurately proportional to 

 the input. In addition to its linearity, this type of amplifying system has 

 high inherent stability, usually not found in the simple balanced bridge elec- 

 trometer circuit, and a response time considerably shorter than most record- 

 ing mechanisms. An example of this type, of circuit, developed by Nier 

 [13], is shown in Fig. 72. 



Other detector circuits have been proposed which utilize a-c amplifiers 

 and a pulsed ion beam to provide the necessary audio-frequency signal. 

 In one system this is done by oscillating the ion-beam radius in the magnetic 

 field at a frequency of approximately 200 cps by oscillating the accelerating 

 voltage [10]. The pulse received by the collecting electrode as the beam 

 sweeps past is amplified and observed on an oscilloscope in which the linear 

 sweep is synchronized with the beam-sweep frequency. Alternatively, after 

 amplification the pulse may be rectified to operate a d-c recording meter. A 

 second scheme accomplishes essentially the same result by pulsing the ion 

 source [18]. 



9.12. Mass-spectrometer Recording Systems. Routine mass spectrom- 

 eter analyses are now usually carried out by automatically scanning and 

 recording the entire mass range or those portions of the spectrum which may 

 be of interest. The scanning may be done by slowly varying either the 

 electric ion accelerating field or the magnetic field. Some advantage is to 

 be found in the latter method in that the magnetic field varies only as w-- . 

 whereas the electric accelerating field varies directly with m~ l and, therefore, 

 often involves an inconveniently large range in voltage. A serious dis- 

 advantage of voltage scanning sometimes arises from voltage discrimination, 

 an effect that is important primarily in isotope analyses where the increased 

 accelerating voltage for smaller mass favors the observed beam currents 

 of the lighter components of the isotopes. Whether or not this effect is 

 present in all types of ion sources is not yet certain, but when it is present it 

 appears to be an involved function of the source geometry, the electric field 

 strength, and the accelerating voltage difference between any two masses. 



