In order to get as many analyte ions as possible into the vacuum chamber, a water-cooled sampling cone, with a (typically) 1mm hole in its centre, is placed at the point of maximum ionization. Located behind this cone is a second nickel cone(the skimmer cone), which has a smaller 0.70 to 0.75 mm orifice in it. The purpose of the skimmer cone is to skim the beam of ions and neutral gas atoms/molecules sampled from the plasma by the sampling cone, to reduce the gas load entering the vacuum of the mass spectrometer. If this gas load is not reduced, the analyte ions will collide with species in the gas stream and are lost before they can be mass filtered and detected. The first stage of the vacuum system comprises the sampling cone and the skimmer cone, the first stage vacuum (usually around 2 mbar pressure) being maintained between them.
The difference in temperature between the cone and the plasma leads to the presence of a boundary layer. A thin cool region of gas just at the surface of the cone. By choosing the correct shape and diameter for the sample cone orifice the boundary layer can be penetrated by the hot plasma gas.
The reduction in pressure as the plasma gas passes through the sample cone gives rise to a rapid expansion of the gas. Within a space close to the cone the gas molecules reach supersonic speeds. This jet has a number of useful advantages. Molecules in a supersonic jet flow with non turbulent, laminar motion. If the tip of the skimmer cone is placed in this region, the gas sampled will have a high component of motion in the axis of the instrument and will thus be easier for the ion lenses to bring into focus.
Another advantage of sampling from a supersonic jet is that the molecules travelling at supersonic speeds do not mix with those travelling at subsonic speed. So if the tip of the skimmer cone is placed in the supersonic jet, the gas passing the skimmer cone is a representative sample of that created in the plasma, regardless of the path it has taken to reach the vacuum chamber. Good gas sampling relies on good quality sample and skimmer cones.