Patent ID: 7858933

Claim:
A mass spectrometer having an ion source that performs electron ionization, the ion source having a filament for generating thermions by heating and an ionization chamber in which sample molecules are ionized by the thermions, the ionization chamber having an electron injection port through which the thermions are introduced into an inner space thereof, wherein: a ratio Db/Da is 1.7 or larger, where Da is a distance between an inner surface of a wall of the ionization chamber in which the electron injection port is formed and a center of the ionization chamber, and Db is a distance between the filament and the center of the ionization chamber, the ratio indicating that the filament is located at a place so remote from the ionization chamber that the inner space of the ionization chamber is free from an influence of an electric field created in a space between the filament and the electron injection port by a potential difference between the filament and the ionization chamber.