Patent ID: 8637810

Claim:
An atmospheric pressure ionization mass spectrometer having a multi-stage differential pumping system including one or more intermediate vacuum chambers between an ionization chamber for generating ions under atmospheric pressure and an analysis chamber for mass-separating and detecting the ions under high vacuum, wherein: either a partition wall separating the ionization chamber and a neighboring first-stage intermediate vacuum chamber, or an exit end of an ion introduction part for making these two chambers communicate with each other, is used as a first electrode; either a partition wall separating the first-stage intermediate vacuum chamber and either a second-stage intermediate vacuum chamber or an analysis chamber in a next stage, or an entrance end of an ion transport part for making these two chambers communicate with each other, is used as a second electrode; and an ion transport electrode for creating an electric field for transporting the ions while converging them is provided in the first-stage intermediate vacuum chamber, and the atmospheric pressure ionization mass spectrometer further comprising: a) a first voltage setting section for setting voltages individually applied to the first electrode and the ion transport electrode, to adjust a direct-current potential difference between these two electrodes so that a smaller amount of cluster ions will be formed; and b) a second voltage setting section for setting voltages individually applied to the ion transport electrode and the second electrode, to adjust a direct-current potential difference between these two electrodes according to whether or not it is necessary to create fragment ions.