Mass spectrometer vacuum interface method and apparatus

A channel-forming member for engagement with a skimmer apparatus, the skimmer apparatus being for use in a mass spectrometer vacuum interface and having an internal surface and a skimmer aperture for skimming plasma therethrough to provide a skimmed plasma downstream of the skimmer aperture,

• wherein the channel-forming member is electrically conductive and dimensioned to be accommodated within a recess in the internal surface of the skimmer apparatus so as to be in conductive contact with the skimmer apparatus, whereby the channel-forming member is electrically neutral relative to the skimmer apparatus when disposed in the recess, and wherein when disposed in the recess the channel-forming member defines one or more channels between the recess and the channel-forming member for separating within the skimmer apparatus a portion of the skimmed plasma adjacent the internal surface of the skimmer apparatus from the remainder of the skimmed plasma.