Filament assembly

A filament assembly for mounting to a source assembly of a mass spectrometer, the filament assembly comprising a body having one of: an aperture 301 to receive a corresponding spigot or plug provided by a source assembly; or a spigot to be received in a corresponding aperture on a source assembly. The assembly may only be mounted in a single orientation due to a notch or keying in the spigot. Also disclosed are separate filament assemblies each comprising one of: two electrical pads on an outer surface; a body with two filament mounting arms which extend outwards and away from one another; a filament element comprised at least in part of a metal and at least partially coated in one of yttrium oxide and thorium dioxide; and a filament element comprising a mesh. The filament element may be comprised of rhenium (Re), platinum (Pt), iridium (Ir), Molybdenum (Mo), tantalum (Ta) or tungsten (W), or rhenium coated in yttrium oxide (Y2O3).