For generation and delivery of ions from an ionization chamber through an ion entrance orifice to a mass analyzer operating at high vacuum, high pass ion filtration is effected within the ionization chamber by application of electrical potentials to an electrode associated with the ion entrance orifice and to an electrode between the ionization region and the ion entrance orifice to create a retarding electric field upstream from the ion entrance orifice. The retarding electric field hinders the movement to the ion entrance orifice of ions having drift velocities below a lower limit, and as the retarding voltage gradient is made steeper, the lower limit increases.