Mass scanning method using an ion trap mass spectrometer

Abstract

An improved method of using an ion trap mass spectrometer is disclosed. According to the method an asymmetrical trapping field is applied to the trap. Preferably, the asymmetrical trapping field comprises a quadrupole field and a dipole field having the same frequency. In addition, higher order trapping field components, such as hexapole or octopole fields, may also be included, and the electrodes of the ion trap can be shaped to introduce such higher order field components. The effect of the asymmetrical trapping field of the present invention is to cause the center of the trapping field to be displaced from the mechanical center of the ion trap. A supplemental quadrupole field is then applied to the ion trap, the center of the supplemental quadrupole field being located at the mechanical center of the trap, i.e., it is displaced from the center of the trapping field. The supplement quadrupole field and the trapping field may be viewed as forming one combined field which acts upon the ions in the trap. The combined field is then scanned to cause ions of differing masses to be resonantly ejected from the ion trap in sequential mass order. Preferably, the combined field is scanned by scanning the voltage of the trapping field. Preferably, the supplemental field is set to have a frequency which is two-thirds of the trapping field frequency and is phase locked with the trapping field frequency.