Mass spectrometer

A mass spectrometer that performs mass spectrometry by activating a plasma to ionize a sample, the mass spectrometer comprising:

• a roughing vacuum pump;
• a turbomolecular pump;
• a first chamber from which a gas is discharged by the roughing vacuum pump;
• a second chamber into which a hydrogen gas is introduced, the second chamber being located at a stage subsequent to the first chamber;
• a third chamber provided with a mass spectrometry unit, the third chamber being located at a stage subsequent to the second chamber;
• a first flow path that forms a gas-discharge flow from the first chamber to the roughing vacuum pump;
• and
• a second flow path that forms a gas-discharge flow from each of the second chamber and the third chamber to the first flow path by the turbomolecular pump, wherein
• the mass spectrometer introduces, into the second flow path, an additional gas having a molecular weight larger than a molecular weight of the hydrogen gas,
• the mass spectrometer further comprising a valve that adjusts a flow rate of the additional gas to be introduced into the second flow path, and
• the valve is brought into a closed state when the plasma is activated, and is brought into an opened state when the plasma is deactivated.