Method of operating an ion trap mass spectrometer in a high resolution mode

The method of operating an ion trap mass spectrometer in a high resolution mode comprising the steps of applying an r.f. voltage V at frequency f and DC voltage U to an ion trap to define at trap volume with a three-dimensional substantially quadrupole field for trapping ions within a predetermined range of mass-to-charge ratio, forming or injecting ions within said trap volume such that those within said predetermined mass-to-charge ratio range are trapped within said trap volume, applying a supplementary AC field superimposed on said three-dimensional quadrupole field to form combined fields, scanning said combined fields to resonantly eject ions of consecutive mass-to-charge ratio from said trap volume for detection characterized in that

• said supplementary field has an amplitude just sufficient to eject said ions and
• said combined fields are scanned at a rate so that a length of time corresponding to 200 cycles or more of the supplementary AC field passes per consecutive thomson.

The method of operating an ion trap mass spectrometer in the high resolution mode comprising the steps of defining a trap volume with a three-dimensional substantially quadrupole field for trapping ions within a predetermined range of mass-to-charge ratio, forming or injecting ions within said trap volume such that those within said predetermined mass-to-charge ratio range are trapped, applying a supplementary AC field superimposed on said three-dimensional quadrupole field to form combined fields, scanning said combined fields to resonantly eject ions of consecutive mass-to-charge ratio from said trap volume for detection characterized in that

• said supplementary field has a frequency to establish a beta value on the stability envelope of the ion trap below 0.891 and
• said combined fields are scanned at a rate so that a length of time corresponding to 200 cycles or more of the supplementary AC field occurs per consecutive thomson.