Mass spectrometer

A mass spectrometer, comprising:

• a linear ion trap unit including:
• a plurality of rod electrodes arranged so as to surround a central axis;
• an auxiliary electrode surrounding the central axis, or having the central axis in between, and provided outside an ion-ejection end of a plurality of rod electrodes or protruding from the ion-ejection end;
• and an extracting electrode located further outside the auxiliary electrode;
• an RF voltage generator configured to apply an RF voltage to the plurality of rod electrodes and the auxiliary electrode in order to create an RF electric field within an ion-capturing space surrounded by the plurality of rod electrodes and the auxiliary electrode;
• an extracting voltage generator configured to apply a DC voltage to the extracting electrode so that a DC electric field for ion extraction reaches the ion-capturing space;
• and
• a controller configured to control the RF voltage generator and the extracting voltage generator so as to eject ions from the ion-capturing space in a direction along the central axis according to the mass-to-charge ratios of the ions by changing at least the RF voltage or the DC voltage when the ions are confined within the ion-capturing space.