Method of plasma mass analysis with reduced space charge effects

Abstract

A method of analyzing an analyte contained in a plasma, in inductively coupled plasma mass spectrometry (ICP-MS). A sample of the plasma is drawn through an orifice in a sampler. The sample is then skimmed in a skimmer orifice, and the skimmed sample is directed at supersonic velocity onto a blunt reducer having a small orifice therein, forming a shock wave on the reducer. Gas in the shock wave is sampled through an offset aperture in the reducer into a vacuum chamber containing ion optics and a mass spectrometer. Because the gas sampled through the skimmer and reducer orifices is substantially neutral (ions and free electrons are in close proximity), and also because the reducer orifice is very small, space charge effects are reduced, thus reducing mass bias and also reducing the mass dependency of matrix effects. Separation of ions from free electrons and focusing of ions into the mass spectrometer largely occurs in and downstream of the ion optics in the vacuum chamber. Since the region between the skimmer and the reducer can operate at about 0.1 Torr, which is the same pressure as that produced by the roughing pump which backs the high vacuum pump for the vacuum chamber, a single common pump can be used for both purposes, thus reducing the hardware needed.