An electron impact ion source includes an ionization chamber in which a first rf multipole field can be generated and an ion guide positioned downstream from the ionization chamber in which a second rf multipole field can be generated wherein electrons are injected into the ionization chamber along the axis (on-axis) to ionize an analyte sample provided to the ionization chamber.
CROSS-REFERENCE TO RELATED APPLICATIONS
The present application is a continuation of U.S. patent application Ser. No. 10/992,191, filed on Nov. 17, 2004 now U.S. Pat. No. 6,998,622.
