Method for digitally generating an additional alternating voltage for the resonance excitation of ions in ion traps

摘要

The invention relates to a method and a device for mass-selective excitation of the secular oscillations of selected types of ions in an RF quadrupole ion trap, also suitable for the simultaneous excitation of a plurality of ion types. The mass-selective excitation can be used for the elimination of unwanted ion types from the ion trap, for increasing the kinetic energy of ion types for the fragmentation by collisions with a braking gas, or for generating oscillations for FT ion traps. The secular oscillation frequency is in this case excited by resonances with specific frequencies of the frequency bands of the additional alternating fields, the frequency bands being digitally generated in a fundamental clock OMEGA . In this case, there result, however, for each frequency omega within the frequency bands disturbing frequency sidebands, OMEGA - omega , OMEGA + omega , 2 OMEGA - omega 2 OMEGA + omega , etc. In an unwanted way, these sidebands can excite and remove other ions which are not to be eliminated. The invention consists in the fact that the sidebands are to be designed in such a manner that even they excite only the selected ions. This is done by making the fundamental clock for the digital generation of the frequency bands for the additional alternating voltage equal to the frequency of the drive voltage of the ion trap or an integral multiple thereof and by coupling the fundamental clock in a phase-locked manner to said frequency. <IMAGE>