MEASUREMENT OF KINETIC ENERGIES OF IONS EJECTED FROM A QUADRUPOLE ION TRAP

Kinetic energies of ions of different mass-to-charge ratios exiting a Paul-type ion trap, operated in the mass-selective instability mode, are measured by a retarding potential method. Average kinetic energies increase with mass, a typical value being 2900 eV with a distribution of 900 eV (FWHM) for m/z 502, C9F20N+. The maximum value remains in the same range but the width of the distribution increases when the trap is operated at lower He bath gas pressure. When ion ejection is performed at reduced values of the Mathieu parameter beta-z, the most probable ion kinetic energy decreases, following the amplitude of the main r.f. voltage. For example, m/z 502 when resonantly ejected at beta-z 0.48 shows a maximum kinetic energy of only 644 eV. Simulations of ion motion employing stepwise integration yield values of the maximum kinetic energies which are in good agreement with the experimental data. The implications of these results for the mass resolution of the ion trap and for other performance data are discussed.