A mass spectrometer is disclosed wherein ions having a particular desired charge state are selected by operating an ion mobility spectrometer in combination with a quadrupole mass filter. The outer pillar panel includes an outer upper end, an outer lower end and an outer bulkhead attachment section located between the outer lower end and the outer upper end and at or above a mid-point of the outer pillar panel. In the provisionally-fitted condition, each terminal, which is inserted into a tubular insertion path of the outer housing, and is received in the chamber, is retained by the flexible retainer, and a front portion of the inner housing forwardly projects a predetermined distance from the front side of the outer housing.