A Simple Mass Spectrometer

Abstract

A mass spectrometer of unusually simple construction is described. The three electrodes were hand formed from coarse stainless steel mesh. Despite the imprecise shape of the electrodes, the spectrometer has a half-height resolution of 75. The device is a three dimensional quadrupole, or ion trap type of mass spectrometer. It is operated so as to store a single m/e species at a time. After a suitable storage time, the ions are pulsed out of the trap into an electron multiplier for detection. The optimization of operating parameters and the ion buildup characteristics for various gases are presented. The performance of the instrument is compared to that of a magnetic sector mass spectrometer. The cracking patterns are simpler than for the magnetic analyzer. In contrast with the magnetic analyzer, no extraneous peaks due to ion desorption from surfaces are present. The lowest measured partial pressure was about 10−12 Torr, and this was limited only by the minimum total pressure attained. Trace analysis capability of 1 part in 2500 was observed with little effort to optimize this property.