SELECTIVE INJECTION AND ISOLATION OF IONS IN QUADRUPOLE ION-TRAP MASS-SPECTROMETRY USING NOTCHED WAVE-FORMS CREATED USING THE INVERSE FOURIER-TRANSFORM

Broad-band excitation of ions is accomplished in the quadrupole ion trap mass spectrometer using notched waveforms created by the SWIFT (stored waveform inverse Fourier transform) technique. A series of notched SWIFT pulses are applied during the period of ion injection from an external Cs+ source to resonantly eject all ions whose resonance frequencies fall within the frequency range of the pulse while injecting only those analyte ions whose resonance frequencies fall within the limits of the notch. This allows selective injection and accumulation of the ions of interest and continuous ejection of the unwanted ions. This is shown to result in significant improvement in S/N ratio, resolution, and sensitivity for the analyte ions of interest. Selective ion injection is demonstrated by injecting the protonated molecules of peptides VSV and gramicidin S and the intact cation of 1-carnitine hydrochloride, using singly notched SWIFT pulses. Multiply notched SWIFT pulses are used to simultaneously inject ions of different m/z values of 1-carnitine hydrochloride into the ion trap. A new coarse/fine ion isolation procedure, which employs a doubly notched SWIFT pulse, is demonstrated for isolating ions of a single m/z value of 4-bromobiphenyl from a population of trapped ions. In this procedure, coarse isolation of a group of adjacent ions is achieved in the first step and is followed by a fine isolation of ions of a single m/z value from the coarsely isolated group.