A capacitor assembly (1) for measuring the level of radio frequency voltage in a mass spectrometer. The assembly (1) includes an RF sensing capacitor (2) with first and second capacitor plates (3, 4), a rectifying circuit (5) and a vacuum housing feedthrough (6), all of which are mounted within a vacuum enclosure of the mass spectrometer. The first capacitor plate (3) is adapted for connection to a voltage source and mounted within the enclosure by first insulating spacers (31). The second capacitor plate (4) is nested within the first insulating spacers (31) and mounted within the enclosure by second insulating spacers (41). The rectifying circuit (5) is electrically connected to the second capacitor plate (4) and to the vacuum housing feedthrough (6).