Mass analyzer and mass analyzing method

Abstract

A mass analyzer includes: an ion trap device including an ion trapping space surrounded by a plurality of electrodes; a time-of-flight mass analyzer for determining a mass to charge ratio of ions ejected from the ion trapping space; a trapping voltage generator for generating an ion trapping RF voltage to at least one of the plurality of electrodes; an ejecting voltage generator for generating an ejecting voltage to at least one of the plurality of electrodes to form an ion ejection electric field for ejecting ions trapped in the ion trapping space; and a controller for stopping the ion trapping RF voltage at a timing when ions are trapped in the ion trapping space and the ion trapping RF voltage is at a predetermined phase, and for applying the ion ejecting voltage a predetermined period after the ion trapping RF voltage is stopped. Here the predetermined phase and the predetermined period are predetermined so that, when the ion trapping RF voltage is stopped at the predetermined phase and the predetermined period passes, the voltage of said at least one of the electrodes to which the ion trapping voltage is generated becomes almost a certain fixed value irrespective of the amplitude of the ion trapping RF voltage when it is stopped. Thus, by stopping the ion trapping RF voltage and applying the ion ejecting voltage at such a timing, the initial kinetic energy of the ejected ions does not vary with the amplitude of the ion trapping voltage before it is stopped, and a precise determination of the mass to charge ratio of the ions becomes possible.