The equations describing the pressure, density, and temperature characteristics of isentropic flow and of the formation of a shock structure due to the sudden termination of the directed motion of a flowing plasma are reviewed. The results are applied to describe the flow characteristics of a novel ICP-MS vacuum interface which consists of three apertures: a conventional sampler and skimmer and a third aperture contained in a blunt support which is normal to (or nearly normal to) the axis of the primary expansion through the sampler and skimmer. The flow through the interface apertures is characterized as continuum, effusive, or transitional, and the impact of these forms of expansion on the ion dynamics (kinetic energies and plasma neutrality) is examined. A shock wave may form in front of the third aperture. The effect of this flow disturbance on the gas and ion dynamics in the vicinity of the aperture is discussed. Experimental neutral and ion flow results are compared to the theoretical predictions. It is concluded that the plasma retains its charge neutrality as it flows through the sampler and skimmer and, under the conditions studied, also through the shock and subsequent expansion through the third aperture. The gas behind the shock flows across the surface of the blunt tip of the third aperture, and the aperture itself may be offset from the axis of the original expansion to eliminate clogging of the aperture by unvaporized particles and condensed salts from the plasma and to prevent source plasma photons from contributing to the background signal continuum. The reduction in the ion current introduced into the ion optics region of the mass spectrometer reduces the magnitude of the space charge field and results in a gain in ion transmission efficiency which offsets the reduction of the ion flow.
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