Patent ID: 8173961

Claim:
An ion trap mass spectrometer having an ion source for supplying pulsed ions and an ion trap for capturing the ions by an electric field formed in a space surrounded by a plurality of electrodes, where ions supplied from the ion source are injected into the ion trap to be captured there and then mass analyzed by the ion trap or mass analyzed after the ions are ejected from the ion trap, the ion trap mass spectrometer comprising: a) a voltage applier for applying a square wave voltage for capturing the ion in the ion trap to at least one of the plurality of electrodes which compose the ion trap; and b) a controller for controlling a timing of supplying pulsed ions from the ion source, in synchronization with a phase or a level change of the square wave voltage, with the square wave voltage applied to the electrode or electrodes by the voltage applier, in order that in addition to existing the ions already captured in the ion trap, ions supplied from the ion source are injected into the ion trap, wherein the controller controls, in a case where a cation is to be mass analyzed, the timing of supplying the pulsed ions from the ion source in such a manner that the pulsed ions enter the ion trap in a low level period of the square wave voltage, and in a case where an anion is to be mass analyzed, the timing of supplying the pulsed ions from the ion source in such a manner that the pulsed ions enter the ion trap in a high level period of the square wave voltage, in order that the pulsed ions enter the ion trap at a timing when the ions in a captured state in the ion trap move toward a center from an expanded state in a periphery of a capture region.