PLASMA SPECTROMETRY IN THE EARTH-SCIENCES - TECHNIQUES, APPLICATIONS AND FUTURE-TRENDS

Plasma spectrometry is one of the most popular and versatile techniques for the analysis of geological and environmental samples, including rocks and minerals, waters, dust, vegetation, soils, sewage sludges and sediments. Inductively coupled or direct current argon plasmas are used as emission sources in ICP- and DCP-atomic emission spectrometry (ICP-AES, DCP-AES); an ICP provides an ion sources in ICP-mass spectrometry (ICP-MS). Reviews of the two plasma sources, sample introduction systems, and the instrumental and analytical performances of emission and mass spectrometers, demonstrates the superiority of higher-temperature, ICP-based systems. ICP-AES and ICP-MS are characterised by wide linear responses of more than five orders of magnitude. They are rapid and highly cost-effective multi-element techniques which can theoretically determine over 70 elements in < 2 ml of sample solution in < 2 min. In practise such performance is rarely possible because detections limits, and particularly in the case of ICP-AES, spectral interferences, limit the range of elements which may be quantified. Plasma spectrometry is primarily a solution-based technique, and the dissolution step ultimately controls both the range of elements quantifiable and the limits of determination which may be achieved. Limits of quantitative analysis for solid samples are typically in the order of a few mu-g g-1 for ICP-AES and a few hundred ng g-1 for ICP-MS. However, chemical separation and preconcentration procedures are described for the rare-earth elements, precious metals and several other elemental groups, which enable determinations to be made at sub-ng g-1 levels. The better precision of ICP-AES and the greater sensitivity, near-complete freedom from interferences, and isotopic capabilities of ICP-MS, mean that ICP-AES is best used for major- and minor-element determinations, while ICP-MS is reserved for trace- and ultra-trace-element work. Comparisons with atomic absorption, X-ray fluorescence and instrumental neutron activation demonstrate that plasma-based techniques compete well with more established instrumental methods. Further developments in sample preparation and presentation procedures, particularly in the area of solid sample analysis, will increase further the potential applications of plasma spectrometry in the earth sciences.