High-precision measurement of trace-level Nb, Sn, Ta, and W in rutile using electron probe microanalysis

Abstract

The composition of Nb, Sn, Ta, and W in rutile can serve as a highly sensitive indicator for the mineralization of these elements. This information could also be utilized to discriminate different stages of ore deposition or distinct types of deposits. Precise and accurate measurement of trace-level concentrations of these elements in rutile is imperative for such investigations, and can be effectively achieved using electron probe microanalysis (EPMA). In this study, we employed a CAMECA SX5 field emission EPMA to develop an optimal approach for precise and accurate analysis of trace-level Nb, Sn, Ta, and W in rutile. The analytical conditions include an accelerating voltage of 25 kV, a beam current of 200 nA, a beam diameter of 1 μm, optimized background positions for Nb Lα, Sn Lα, Ta Lα, and W Lα, and aggregate intensity counting with a peak counting time of 240 s for each element per spectrometer. The detection limits (3σ) for Nb, Sn, Ta, and W ranged from 22 to 53 ppm. The highest achievable spatial resolution was approximately 4.3 μm. Analytical results obtained from potential rutile standards R10 and R632 were consistent with previous studies within the errors. The precision of these results varied from 1% to 10% (1σ), excluding W in R10 and Nb and Ta in R632, which exceeds the precision achieved in previous EPMA studies. We recommend using R10 as a reference standard for analyses of trace-level Nb, Sn, and Ta, and R632 as an appropriate reference standard for Sn and W when quantifying rutile using EPMA.