1. Field of the Invention
The present invention relates to a mass spectrometer and a method of mass spectrometry.
2. Discussion of the Prior Art
U.S. Pat. No. 5,783,824 discloses a linear ion trap wherein a quadratic DC or electrostatic potential is maintained along the axial length of the ion trap. Ions are ejected from the linear ion trap in an axial direction by resonantly exciting the ions. Ions having a particular mass to charge ratio are resonantly excited by applying of a supplemental axial AC voltage waveform to the ion trap. If the supplemental axial AC voltage waveform which is applied to the ion trap is at the fundamental or harmonic frequency of ions within the ion trap then these ions will then be ejected from the ion trap.
Considering the pseudo-potential well approximation of Dehmelt, ion motion within an ion trap may be approximated as the combination of small amplitude relatively high frequency micro-motion and a lower frequency secular motion at a frequency proportional to the inverse of the mass to charge ratio of the ion. Resonance ejection may be achieved by applying a supplemental AC voltage waveform which matches the secular frequency of an ion having a mass to charge ratio which is desired to be ejected from the ion trap.
As the frequency of the supplemental AC excitation voltage approaches the frequency of the secular motion then the amplitude of ion oscillation will increase. The amplitude of ion oscillation will continue to increase until the amplitude of oscillation is such that the ion exceeds the boundaries of the ion trap and hence is ejected from the ion trap.