Performance enhancements of mass selective axial ejection from a linear ion trap.

Mass selective axial ejection (MSAE) of ions from a linear ion trap (LIT) takes advantage of the rf fringing fields at the end of the linear quadrupole to convert radial ion excitation into axial ejection. Ions gain radial amplitude via a mass selective resonance excitation process and are ejected axially over an electrostatic DC barrier. The extraction efficiency and resolution are determined by the length and shape of the extraction region in the vicinity of the exit aperture. In the work presented here, axial DC fields, created by auxiliary electrodes, were used to modify the shape of the trapping electrostatic fields along the quadrupole axis and to increase the density of the ions in the extraction region. Better confinement and ion cloud coherence increased the extraction efficiency and the spectral resolution. As a result, extraction efficiency can be increased by up to one order of magnitude at fast scan rates, i.e., 10 and 20 kTh/s, and a factor of 2-3 at the slower scan speed of 1 kTh/s. The length of the extraction region was also modified by application of a portion of the drive rf voltage to the end lens of the LIT. A comparison of the MSAE spectra at different scan rates and rf levels showed that extraction efficiencies increase at scan rates of 10 kTh/s or higher, with associated improvements in mass spectral peak widths.