A method of preparing or operating a mass spectrometer vacuum interface comprising a skimmer apparatus having a skimmer aperture and an internal surface of the skimmer apparatus, comprising disposing an adsorbent or getter material on the internal surface. The internal surface has a deposition region where matter from plasma flows may be deposited and the material is disposed on part or all of the deposition region. The disposing step may be performed before a first use and/or intermittently, especially to refresh a previously disposed material. Providing such material serves to trap or collect deposition matter which might anyway be deposited but in such a way that subsequent liberation of that matter is prevented or at least reduced.