The development and application of inductively coupled plasma mass spectrometry for geochemical analysis

Abstract

The applicability of the technique of inductively coupled plasma mass spectrometry to the analysis of geological samples was investigated using a variety of sample introduction techniques including: solution nebulisation; slurry nebulisation; flow injection; electrothermal vaporisation; and laser ablation, Solution sample introduction is limited by the amount of time required to prepare the sample, and the relative intolerance of the technique to high concentrations of sample matrix. The maximum level of dissolved solids for a refractory matrix such as a digested igneous rock was found to be 0.2% w/v. Good accuracy and precision are achievable. Acceptable results can be obtained using slurry nebulisation. However, standardisation is a problem due to the difference in response for aqueous and slurried analytes. Calibration against aqueous standards and the use of an internal standard is therefore precluded. In addition, the preparation of stable slurries is a highly skilled and time consuming task. Flow injection analysis offers the most benefit to the geochemical analyst. Flow injection was found to increase sample throughput and, more important, to improve matrix tolerance by a factor of l0x, thus allowing the direct determination of the platinum group elements and gold in geological samples without pretreatment. Small samples, such as fluid inclusion leachates can also be analysed without dilution and the matrix effects experienced when analysing samples containing high salt concentrations can be reduced by careful control of dispersion. The potential for increased detection limits by electrothermal vaporisation was not proved for geological materials due to the same matrix tolerance problems which limit detection limits in solution work. Laser ablation sampling allows direct analysis of the solid but quantitation requires matrix matched standards or independent variable internal standardisation, limiting the applicability of the technique for bulk screening. The use of laser ablation to analyse trace element concentrations in individual mineral grains has been investigated and partition coefficients for trace elements, including the rare earths in a large zoned pyroxene crystal, were determined.