Enhancement of mass resolution in the quadrupole ion trap via resonance ejection

The Finnigan MAT ion trap mass spectrometer (ITMS) has unit mass resolution and a nominal upper mass/charge limit of only 650 when operated in the standard mass-selective instability method for acquiring mass spectra. Mass resolution up to ∼2500 can be achieved with the use of the axial modulation technique. Ions can also be ejected at a qz value other than that corresponding to the mass-selective instability boundary by application of an auxiliary signal to the end-cap electrodes. When the frequency of this signal is resonant with the secular frequency for a particular m/z, such ions can become kinetically excited and ejected from the trap via a process known as resonance ejection. We demonstrate the capability to mass-selectively isolate or eject ions over a wide mass range and to generate mass spectra of electrospray-generated ions by scanning the frequency of the resonance ejection signal applied to the end-cap electrodes. A block diagram of the experimental modifications to the ITMS required for frequency-swept resonance ejection is shown. 2 refs., 5 figs