Inductively coupled plasma-mass spectrometry

Abstract

The inductively coupled plasma-mass spectrometer combines two analytical facilities to produce an instrument with powerful potential in the field of multi-element trace analysis. The inductively coupled argon plasma is used, not as a source for optical emission measurements (as in ICP-atomic emission spectrometry, Chapter 5), but rather as a source of ions. The mass spectrum of this ion source is measured using a quadrupole mass spectrometer. This was made possible by the development of an interface whereby plasma gases can be physically sampled through an orifice, via a differential pumping unit and into the quadruple mass filter (Figure 20.1). The technique, therefore, combines the freedom from matrix interferences characteristic of the inductively coupled argon plasma with the very favourable signal-tobackground ratios obtainable by mass spectrometry. Samples must first be taken up into solution and are then aspirated into the argon plasma in the conventional manner. Detection limits for applications in multi-element silicate rock analysis have not, at the time of writing, been fully characterized. However, they appear, in general, to be significantly lower than for ICP-AES. Nevertheless, it is not yet clear what position this technique will occupy in the hierarchy of analytical instrumentation. ICP-MS has a clear application in the analysis of waters and is likely to have a significant impact on routine trace geochemical analysis. This is so, in view of the potential of this technique to determine simultaneously a wide range of heavier trace elements, often down to ppm whole-rock levels, using a simple sample dissolution technique with no preconcentration stage. Of particular interest in this respect is the application of ICP-MS in the determination of the rare-earth elements and a wide range of trace elements important in economic geology studies including As, Sb, Se, Te, Bi, Hg, Au, Ag, and the platinum group elements.