Characteristics of a volcano field ion quadrupole mass spectrometer

Abstract

A field ionization source shaped in the form of a microscopic volcano is coupled with a quadrupole mass spectrometer, and the performance characteristics of the system are measured. The volcano source structure, consisting of a 20-μm diameter rim where field ionization occurs and a 10-μm diameter throat for the passage of sample material, permits the use of high pressure sample gases and yields high efficiency ionization with excellent ion beam optics. Both water and toluene are used as sample gases at source pressures ranging up to the room temperature vapor pressures of these materials (20 torr and 30 torr for water and toluene respectively), and 1-butene has been used up to pressures of 140 torr. The characteristic large background signal produced at low masses when the field ion source is positioned at the entrance of the quadrupole is shown to be due to high energy electrons produced by positive ions colliding with the field ion source structure. This background is effectively eliminated, without offsetting the field ion source from the quadrupole center line, by the use of magnetic and ion optical techniques. Energy analyses are performed on H 3O +, C 4H 5 + (1-butene) and C 6H 5CH 3 + (toluene) ion species by the retarding potential method. The energy full-width at half-height of these species is measured at about 1.9, 1.8 and 1.9 eV respectively.