PURPOSE:To allow negative ions of a sample to exist in a stabilized state by making a grid of Rydberg atom generator needle-like and providing a cooling means in an ion source block. CONSTITUTION:In an ion source of a mass spectrometer where sample molecules of gas are made collide against Rydberg atoms to ionize a sample for mass analysis, a grid 6 of a Rydberg atom generator 3 of an ion source block is made needle-like as well as a cooling means 9 is provided on the ion source block. That is, by making the needle-like grid 6, discharge caused by deposition of atoms sputtered from a filament 5 is eliminated and further concentration of thermions in the vicinity of the grid 6 can be raised. In addition a rise in temperature of the ion source block is suppressed by providing the cooling means 9, so that decomposition of the sample and decomposition of generated ions can be suppressed to raise generation efficiency of parent molecule ions of the sample. Thus negative ions of the sample are allowed to exist for a long time in a stable state.
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