Abstract: A method of detecting ions of a single ion species that have been selectively stored in a quadrupole ion trap mass spectrometer is disclosed. After the selected ion species is isolated the trapping field in rapidly changed to cause ions to leave the ion trap in the axial direction where they are detected using a conventional detector. Preferably, a dipole pulse is applied to the ion trap simultaneously with the reduction of the trapping field, such that all of the ions are caused to leave the trap in a single direction, doubling the ion current over prior art methods. The method of the invention allows ejection of all of the ions in a time period which is nearly twenty times faster than the prior art resonance ejection scanning technique, and without the artifacts in the signal current caused by frequency beating.