An ion source assembly for a static mass spectrometer, comprises: a mounting element for locating the assembly within the static mass spectrometer; an ion source for generating ions to be analyzed in the static mass spectrometer, the ion source being spaced from the mounting element and arranged to be held in use at a first relatively high potential V1 with respect to the mounting element; and a spacer mounted between the mounting element and the ion source, the spacer arranged to be held in use at a second potential V2 with respect to the mounting element, which is less than the first potential V1.