Mass analyzer and mass analyzing method

#The present invention relates to an ion trap device in which ions are trapped with a three-dimensional quadrupole electric field, and an ion trapping method in the ion trap device. Such an ion trap device, which may also be called simply an “ion trap”, is used in mass spectrometers, for the ion source of time-of-flight mass spectrometers (TOF-MS), and for other ion analyzers.

#In a TOF-MS, accelerated ions are injected into a flight space where no electric field or no magnetic field is present, and the ions are separated by their mass to charge ratios with the flight time of the ions in the flight space. For the ion source of a TOF-MS, an ion trap device is used in many cases.

#As shown in FIG. 4, a typical ion trap device 1 is composed of a ring electrode 11, and a pair of end cap electrodes 12, 13 placed opposing each other with the ring electrode 11 between them. Usually, an RF (radio frequency) voltage is applied to the ring electrode 11 to form a quadrupole electric field in the ion trapping space 14 surrounded by the electrodes 11, 12, 13, whereby ions are trapped within the ion trapping space 14. In one case, ions are generated outside of the ion trap device 1 and introduced into it, and in another case they are generated within the ion trap device 1. The theory of such an ion trapping method is described in detail in, for example, “Quadrupole Storage Mass Spectrometry” by R. E. March and R. J. Hughes, John Wiley & Sons, 1989, pp. 31–110.

#FIG. 1 schematically shows a mass analyzer of the invention.

#FIG. 2 shows an example waveform of the ring voltage in a conventional method.

#FIG. 3 shows an example waveform of the ring voltage set at a certain phase of the RF voltage.

#FIG. 1 schematically shows a mass analyzer embodying the present invention, where the same or similar elements as those in FIG. 4 are assigned the same numerals. The ion trap device 1 is composed of a ring electrode 11 and a pair of end cap electrodes 12, 13 opposing each other with the ring electrode 11 therebetween. An RF voltage is applied to the ring electrode 11, which forms a quadrupole electric field in the ion trapping space 14 surrounded by the electrodes 11, 12, 13. Ions are trapped in the ion trapping space 14 by the quadrupole electric field. End cap voltage generators 15, 16 are connected respectively to the end cap electrodes 12, 13 to apply appropriate voltages to them at every analyzing stage.

#When ions generated in a Matrix-Assisted Laser Desorption/Ionization (MALDI) ion source 2 are introduced in the ion trap device 1, for example, voltages are applied to the end cap electrodes 12, 13 to decrease the kinetic energy of the ions. When a mass analysis is to be conducted in a TOF-MS 3, other voltages are applied to the end cap electrodes 12, 13 to accelerate the ions being ejected from the ion trapping space 14. When ions are selected or dissociated in the ion trapping space 14, still other voltages are applied to superimpose a selection electric field or a dissociation electric field, in addition to the ion trapping quadrupole electric field formed by the ion trapping RF voltage.

#A coil 42 is connected to the ring electrode 11 as a part of a ring voltage generator 4 for applying an RF voltage to the ring electrode 11. The coil 42, the ring electrode 11 and the capacitance formed between the ring electrode 11 and the end cap electrodes 12, 13 constitute an LC resonant circuit. To be precise, in addition to the capacitance between the ring electrode 11 and the end cap electrodes 12, 13, the capacitance formed by a monitor circuit (not shown) for monitoring the RF voltage, a tuning circuit 43, high voltage switches 46, 47 and the wires connecting them, and the inductance of the coil 42 determines the resonance frequency.