BROAD-BAND EXCITATION IN THE QUADRUPOLE ION-TRAP MASS-SPECTROMETER USING SHAPED PULSES CREATED WITH THE INVERSE FOURIER-TRANSFORM

This paper reports on broad-band excitation of ions in the quadrupole ion trap mass spectrometer (ITMS) using shaped pulses. In place of a single-frequency excitation signal, applied to the end caps of the ITMS, a shaped pulse which excites a broad spectrum of frequencies is used. Shaped pulses are time domain signals created by taking the complex inverse Fourier transform of a frequency domain function whose magnitude represents the desired excitation spectrum. In mass spectrometry these signals are termed SWIFT (stored wave form inverse Fourier transform) pulses. By selection of a frequency spectrum which includes ion secular frequencies, SWIFT pulses can be constructed to excite a wide range of m/z values in the quadrupole ion trap. Using the phase modulation method described by Chen et al., the frequency domain spectrum is converted to a complex function prior to being transformed to the time domain. The time domain signal is then processed and loaded into an arbitrary wave form generator (ARB) connected to the end-cap electrodes and applied in a dipolar fashion. Three basic applications of SWIFT pulses are demonstrated in the quadrupole ion trap: (i) broad-band ejection of desorbed matrix ions by application of SWIFT pulses during ion injection from an external source, (ii) broad-band ejection of trapped ions for selective ion isolation, (iii) broad-band excitation which results in collision-induced dissociation (CID) of selected ions. Applying SWIFT pulses while ions are being injected from a Cs+ desorption source results in ejection of matrix ions, which reduces space charge and greatly improves parent ion intensity and overall sensitivity. SWIFT pulses are effective at ejecting ions which have been stored for ion isolation, and the method shows good mass resolution.