Abstract: A mass spectrometer including an ion source, a detector, a first end cap electrode arranged proximate to the ion source, a second end cap electrode arranged proximate the detector, and a ring electrode arranged between the first and the second end cap electrodes. The ring electrode can be either connected to a radio-frequency voltage source or to a constant voltage source. When the ring electrode is connected to the radio-frequency voltage the first end cap, the second end cap and the ring electrode form an ion trap and the mass spectrometer operates as an ion trap mass spectrometer. When the ring electrode is connected to a constant voltage the mass spectrometer operates as a time-of-flight mass spectrometer.