Method and apparatus for ejecting unwanted ions in an ion trap mass spectrometer.

摘要

The mass spectrometer includes an ion trap with multiple electrodes, and establishes trapping fields within the trap to trap ions over a preset mass range. Ion excitation circuitry resonantly ejects trapped ing ions from the trap by determining spaced discrete frequencies over the range of frequencies of the characteristic motion of unwanted ions and processes the discrete frequencies to generate time dependent voltage amplitude values which vary throughout the time domain so that (i) the frequency component of the time dependent voltage amplitude values is relatively uniform over the entire time domain, and (ii) the magnitude associated with the discrete frequencies is relatively uniform over the frequency domain. The time dependent voltage amplitude values are applied to the ion trap electrodes to resonantly eject the ions. The frequencies may be equally or unequally spaced. Pref. the spaced discrete frequencies have phases so that the frequency components are not in phase at any one point e.g. the phases are varied non-linearly with frequency. Pref. one or more of the determined discrete frequencies are removed from the ion characteristic motion frequency range, so that ions with characteristic motion at these frequencies are not resonantly ejected.