A technique for improving the efficiency of injecting ions into the
electrode unit of a funnel structure having high
ion-transport efficiency is provided to improve the overall
ion-transport efficiency. From an
ionization chamber 1 for ionizing a sample under
atmospheric pressure, ions are injected through a straight capillary
pipe 3 into the inner space of the
electrode unit 10 of a funnel structure composed of ring electrodes in a first intermediate
vacuum chamber 4. The space for setting the capillary
pipe 3 is formed by replacing one or more ring electrodes with C-shaped electrodes whose circumference portion is partially removed. Each C-shaped
electrode is arranged so that the ions will be injected perpendicularly to the
ion-transport direction. The injected ions lose energy due to collision cooling, become converged onto the ion-beam axis C due to the ion-confining effect of a radio-frequency
electric field, and efficiently move toward the exit aperture along a
potential gradient created by a direct-
current electric field. The gas
stream carrying the ions passes through the gaps of the ring electrodes, without increasing the
gas pressure at the exit of the ring-electrode inner space and thereby deteriorating the degree of vacuum in the next stage.