Abstract
An ideal ion source should create stable ion beams of sufficient intensity with an ion energy spread as low as possible. Most electron-impact ion sources used in mass spectrometers have been based on the design by Nier. Calculation of the potential distribution in the ion source revealed that the ion extraction efficiency could be improved by applying a positive repeller potential to the ionization chamber, which is normally at ground potential in a Nier-type ion source. Ion kinetic-energy distributions were measured in a Nier-type ion source and in a modified ion source by applying a retarding pole bias potential to the quadrupole at various emission currents and chamber pressures. As the emission current or the chamber pressure was increased, the most probable energy was shifted to a higher energy in both ion sources. However, the ion kinetic-energy spread was smaller and varied less in the modified ion source while it was larger in the conventional Nier-type ion source.
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