A special set of metal cones and ion-focusing elements are used to extract
the charged atoms from the plasma, which operates at atmospheric pressure, into the mass analyzer,
which operates at a pressure of 1 x 10-7 mBar. The tongue of
the plasma is played across the tip of a water-cooled nickel sampler cone. At the tip of the
sampler cone is a small hole that opens into the mass spectrometer. The space behind the
sampler cone, the expansion chamber, is evacuated to a pressure of around 1 - 1.5 mBar.
The jet of gases passing through the sampler exceeds the speed of sound and creates a
conical shock wave, which terminates at a Mach disk as the gases slow down. The inside of
this shock wave is known as the zone of silence.
The distance from the sampler cone to the Mach disk is proportional to the ratio of the
pressure in the expansion chamber and the atmospheric pressure in the plasma. The expansion
chamber terminates with another cone known as the skimmer cone. It is situated such that its
orifice is located within the zone of silence. Behind the skimmer cone is the mass spectrometer,
which is maintained at a pressure of <2x10-7 mBar.
The ions that successfully pass through the skimmer cone orifice are first
accelerated by a high voltage potential gradient and are then passed through a series of
focusing lenses into the mass analyzer. The cones and expansion chamber are often collectively
referred to as the interface to the mass spectrometer.
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University of Missouri-Columbia
