Sec. 9.8] MASS SPECTROGRAPHS 289 



a long period of time and probably produce ions with smaller spread in energy. 

 Particular advantage is gained for the analysis of gases and vapors with these 

 sources since very small quantities need be used for complete quantitative 

 analyses. 



a. High-voltage Discharge Source. Ions may be produced in high-voltage 

 sources either by spark discharge in a vacuum chamber or by a steady dis- 

 charge in an atmosphere of gas at low pressure. The first type of discharge 

 is frequently convenient for the direct ionization of metallic substances. 

 It avoids, among other things, the use of a discharge gas which would produce 

 a spectrum of its own. The sample material can be used as the high-voltage 

 electrode, or when this is impracticable, it can be inserted into a hollow 

 electrode in some usable form, i.e., as a stable salt, an alloy, or a mixture. 

 Suitable high-voltage electrode materials include any of the metals that 

 are stable in a vacuum at elevated temperatures. The material used, how- 

 ever, should not produce ions with values of m/e near similar values expected 

 from the sample unless they can be used for a comparison spectrum. The 

 discharge takes place in a small gap between the two electrodes which are 

 connected to a tesla coil or some similar high-voltage spark device. Some of 

 the ions that are formed drift to the edge of the discharge in the gap where 

 they then fall through an accelerating potential maintained between the dis- 

 charge electrodes and a third electrode. This voltage determines the final 

 energy of the ions passing through the analyzing fields. 



The second type of high-voltage discharge takes place between a grounded 

 cathode and a high-voltage anode in an atmosphere of gas at low pressure. 

 Satisfactory operation is usually obtained with a potential of approximately 

 15,000 volts. Unless the gas itself is to be measured, either neon or argon is 

 usually used. Ions formed in the discharge column are accelerated toward 

 the cathode in the electric field of that part of the discharge known as the 

 cathode fall which extends nearly to the anode or to within a distance from 

 it determined by the space-charge sheath thickness. A portion of the ion 

 beam emerges from the discharge tube through a slot in the cathode and is 

 then collimated by an appropriate arrangement of slits, as shown in Fig. 68. 



Solid substances that are to be analyzed may be placed on or near the 

 cathode, or in the case of some metallic elements, the cathode can be made 

 wholly of the sample material. Otherwise it is used in the form of a suitable 

 compound, such as a halide salt if it has a vapor pressure greater than 10 _; 

 mm Hg at several hundred degrees centigrade. In general, the combined 

 effects of heating and sputtering by positive ions striking the cathode will 

 ensure an ample concentration of sample material in the discharge for the 

 production of ions. 



b. Sources with Electron Emission from a Filament. This type of source 

 is the most widely used at the present time because of its ease of control. 



