A mass spectrometer (1A; 1B; 1C, 1D) for analyzing ions according to selected mass-to-charge ratios comprising: an ion source (10; 110; 114) for ionizing a sample; an ion storage portion (20; 120) for repeatedly performing a storing operation for storing ions generated by the ion source and an expelling operation for repeatedly expelling the stored ions as pulsed ions, the ion storage portion comprising an ion guide (22; 122) made of a multipole, an entrance electrode (24; 124) and an exit electrode (26; 126) disposed on opposite sides of the ion guide; a quadrupole mass filter (32; 132) for receiving the pulsed ions expelled by the ion storage portion and permitting only a desired ion species to pass having the selected mass-to-charge ratio defined by selected AC and DC voltages supplied to the quadrupole mass filter; a detector (60; 160) for detecting the pulsed ions passed through the quadrupole mass filter and outputting an analog signal responsive to a detection intensity; and a controller arranged to apply two different voltages periodically and alternately to the exit electrode to perform the operation for storing and expelling ions from the ion storage portion, one voltage being higher than an axial voltage across the ion guide and the other voltage being lower than the axial voltage across the ion guide, so that all of the ions stored in the ion storage portion are expelled as one pulse by the two different voltages, wherein the controller is further arranged to control said selected AC and DC voltages supplied to the quadrupole mass filter so as to maintain said selected voltages constant while the pulsed ions are passing through the quadrupole mass filter, and to change the selected voltages while the ions are stored in said ion storage portion and are not being introduced in the quadrupole mass filter. an ion source (10; 110; 114) for ionizing a sample; an ion storage portion (20; 120) for repeatedly performing a storing operation for storing ions generated by the ion source and an expelling operation for repeatedly expelling the stored ions as pulsed ions, the ion storage portion comprising an ion guide (22; 122) made of a multipole, an entrance electrode (24; 124) and an exit electrode (26; 126) disposed on opposite sides of the ion guide; a quadrupole mass filter (32; 132) for receiving the pulsed ions expelled by the ion storage portion and permitting only a desired ion species to pass having the selected mass-to-charge ratio defined by selected AC and DC voltages supplied to the quadrupole mass filter; a detector (60; 160) for detecting the pulsed ions passed through the quadrupole mass filter and outputting an analog signal responsive to a detection intensity; and a controller arranged to apply two different voltages periodically and alternately to the exit electrode to perform the operation for storing and expelling ions from the ion storage portion, one voltage being higher than an axial voltage across the ion guide and the other voltage being lower than the axial voltage across the ion guide, so that all of the ions stored in the ion storage portion are expelled as one pulse by the two different voltages, wherein the controller is further arranged to control said selected AC and DC voltages supplied to the quadrupole mass filter so as to maintain said selected voltages constant while the pulsed ions are passing through the quadrupole mass filter, and to change the selected voltages while the ions are stored in said ion storage portion and are not being introduced in the quadrupole mass filter.