Abstract: A combined ion source capable of operating either in an electron-impact-ionization mode or in a chemical ionization mode and being quickly shiftable from one mode to the other, consists of a cavity into which either a sample gas can be introduced to be ionized alone or together with a reaction gas introduced into the cavity, the cavity having at least one opening for the introduction of ionizing particles, or radiation, into the cavity and another opening for the discharge of ions, with a flexible band covering the exterior surface of the cavity between the openings, the band having suitable patterns of openings, each of which patterns cooperates with the various openings to provide a specific combination of openings and at the same time adjusting the width of each opening used in each combination.