Mass spectrometer

A mass spectrometer adopting a differential pumping system includes an ionization chamber with substantially atmospheric pressure, two intermediate vacuum chambers, and a mass analyzing chamber with a very low pressure (or a very high vacuum), where the pressures of these chambers are in the order of higher to lower. A hole is provided in every wall between two adjoining chambers for allowing ions to be mass analyzed to pass through. An auxiliary electrode with an aperture is placed near the hole, and an AC voltage is applied to the auxiliary electrode. Owing to an AC electric field generated by the AC voltage around the hole, an ion coming near the hole is exerted with such a force that confines the ion to the ion optical axis C. Under these circumstances, even when the ion collides with a residual gas molecule or atom and is deflected away from the ion optical axis, the confining force exerted by the electric field pulls the ion back toward the ion optical axis. This improves the ion passing efficiency of the hole, and increases the number of ions reaching the mass filter and ion detector, which consequently enhances the sensitivity of the mass analysis.