Inductively coupled plasma mass spectrometry is now widely accepted as a routine analytical tool for the determination of trace and ultra trace elements in igneous rocks. The technique has been applied to the determination of a wide variety of trace elements but finds particular application in the determination of the rare earth elements (REE). A typical analytical procedure involves the dissolution of powdered samples by acid attack either in open or closed vessels. Whilst the acid attack method has the potential advantage of removing the silicon as a volatile tetra-fluoride, thus simplifying the matrix, it does suffer from one major problem. Many of the accessory minerals, particularly in granitic rocks, are resistant to acid attack and yet these minerals are frequently the hosts of high concentrations of trace elements (e.g. zircon, sphene and monazite). Data will be presented to illustrate this problem in reference materials of different compositions. This problem leads to the underestimation of trace element concentrations and possible misinterpretation of the petrogenesis.
One solution to the problem of acid resistant minerals is the use of laser ablation as a means of sample introduction into the ICP-MS. Work in our laboratory has investigated the use of laser ablation for such analyses (Perkins et al., 1993). This work demonstrated the potential of the method but showed that there were some problems with the sample preparation particularly the homogeneity of pressed powder pellets. Samples were prepared in fused glass form using a method identical to that commonly used for XRF analysis. This overcomes the problem of sample homogeneity but introduces high concentrations of flux and results in a dilution of the sample.
More recently we have returned to the analysis of REE by solution methods after dissolving fused glasses which have
previously been analysed by XRF. The lithium borate flux commonly used for XRF analysis is a highly efficient flux capable of dissolving all the accessory minerals and although the procedure introduces high levels of flux the data obtained show
excellent agreement for reference materials with none of the systematic errors often associated with acid decomposition. This work is currently being extended to the analysis of other petrogenetically useful trace and ultra-trace elements and results for these will be presented.
Perkins, W.T., Pearce, N.J.G. & Jeffries, T.E., Geochim. Cosmochim. Acta 57, 475-482 (1993).