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 s ingle-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 magnitu de represents the desired excitation spectrum. In mass spectrometry th ese signals are termed SWIFT (stored wave form inverse Fourier transfo rm) pulses. By selection of a frequency spectrum which includes ion se cular frequencies, SWIFT pulses can be constructed to excite a wide ra nge of m/z values in the quadrupole ion trap. Using the phase modulati on method described by Chen et al., the frequency domain spectrum is c onverted to a complex function prior to being transformed to the time domain. The time domain signal is then processed and loaded into an ar bitrary wave form generator (ARB) connected to the end-cap electrodes and applied in a dipolar fashion. Three basic applications of SWIFT pu lses are demonstrated in the quadrupole ion trap: (i) broad-band eject ion 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 r esults in collision-induced dissociation (CID) of selected ions. Apply ing SWIFT pulses while ions are being injected from a Cs+ desorption s ource results in ejection of matrix ions, which reduces space charge a nd greatly improves parent ion intensity and overall sensitivity. SWIF T pulses are effective at ejecting ions which have been stored for ion isolation, and the method shows good mass resolution.