A method of operating a mass spectrometer vacuum interface comprising a skimmer apparatus having a skimmer aperture and downstream ion extraction optics. An expanding plasma is skimmed through the skimmer aperture. Within the skimmer apparatus, a portion of the skimmed plasma adjacent the skimmer apparatus is separated from the remainder of the skimmed plasma by providing means to prevent, inhibit or impede, the separated portion from reaching the extraction optics while allowing the remainder to expand towards the extraction optics. This allows removal of ions liberated from deposition matter on the skimmer apparatus surface, thereby discriminating against such ions, and offering reduced memory effects. The remainder of the plasma can expand towards the extraction optics, so interaction and mixing between the boundary layer and the remainder of the plasma can be reduced or minimized.