A new EPMA technique for determination of rare earth elements with the use of automated peak-overlap and modelled background corrections

Abstract

Analysis of rare earth elements (REEs) plays an important part in geology and geochemistry for solving fundamental problems of ore formation, e.g., formation of gold and platinum deposits, as well as in ecology, in searching for materials suitable for burial of highly active wastes, where REEs are used in investigations as actinide imitators. However, electron microprobe analysis (EPMA) of REEs faces a series of problems related to the complex character of their X-ray spectra. For solving these problems, we suggest the use of an improved version of the program developed by us for the analysis of platinum group elements (PGEs), in which a computational method of background determination and an automatic correction for interfering lines are employed before applying the standard ZAF correction procedure. The program enables one to solve the problem of background determination, to simplify significantly REE analyses, to reduce random errors, to improve the correctness and speed of microprobe determinations, and to lower the limit of detection (LOD) of the elements analysed. When analysing thermally stable REE-bearing minerals, the LOD for most REEs lies in the concentration range 200-600 ppm. These values are obtained at an accelerating voltage of 20 kV, with a probe current of 100 nA and a counting time of 50 s at the peaks of analytical lines. For elements with multiple interference of analytical lines, such as Gd, Tm, Eu and Lu, the LOD estimated from the standard formula determines virtually the concentration level that might be achieved only in the case of correct measurements of overlap coefficients of all interfering lines. The use of a computational method for background determinations allows the required analysis time to be reduced by nearly a factor of two. We present in this paper the values of overlap coefficients and assess the reproducibility of results of the analysis of monazite, xenotime, apatite, and zirconolite. The basic principles of our software can be used successfully with various types of microprobe analysers including modern instruments.