Mass spectrometer

A mass spectrometer including an ion source configured to ionize a component contained in a sample gas, the ion source comprising:

• an ionization chamber having an ion ejection opening and forming a space substantially partitioned from an outside inside the ionization chamber;
• a thermal electron supply unit configured to supply thermal electrons to an inside of the ionization chamber;
• a magnetic field forming unit configured to form a magnetic field inside the ionization chamber such that the thermal electrons move helically;
• and
• a deflection electric field forming unit configured to form a deflection electric field deflecting ions derived from the component generated in the ionization chamber by a direct or indirect action of the thermal electrons in a direction against a force received from the magnetic field when the ions are moving toward the ion ejection opening,
• wherein the deflection electric field forming unit includes:
• an electrode disposed inside the ionization chamber or as a part of an inner wall of the ionization chamber,
• and a voltage generator configured to apply a voltage to the electrode, and
• a control unit configured to control the voltage generator so that the voltage is intermittently applied to the electrode.