Sample introduction and pressure measuring system for chemical ionization mass spectrometers

Abstract

One of the difficulties with using chemical ionization mass spectrometry (CIMS) in magnetic sector instruments has been that of electrical dischargers through the gaseous sample between the ion source, which may be at potentials at 10kV and ground. A sample introduction method is presented to offset this. It was first used at the University of Nebraska, Lincoln on a modified Atlas CH-4 mass spectrometer operated at ion source potentials up to 3 kV and pressures up to 3 torr. The same technique has been extended at the University of California, Santa Barbara (UCSB), for use with a VG-Micromass ZAB-2F mass spectrometer operated at source potentials as high as 10 kV and pressures up to 1 torr. The basic features of the design are presented. The sample leak valve which is at ground potential feeds into a glass tube which has been packed with approximately 4 in. of glass wool. The end of this glass tube is in contact with the ion source potential. The authors believe that the glass wool may prevent discharging by reducing the positive ion velocities below that required for the emission of secondary electrons and/or providing a very large surface area which may act as themore » third body in the ion-electron recombination reaction I/sup +/ + e/sup -/ + M ..-->.. I + M. At UCSB this sample introduction system is used for all gaseous and volatile liquid samples in both EI and CI modes. The only disadvantage with the sample introduction system involves the removal of very polar samples from the glass wool. However, overnight pumping has removed all samples we have used to date.« less