A rapid method for REE and trace-element analysis using laser sampling ICP-MS on direct fusion whole-rock glasses

Abstract

A new method of producing glass beads from whole-rock powders has been tested for use in laser sampling-inductively coupled plasma-mass spectrometry (LS-ICP-MS). Glass beads are produced in a tungsten strip heater cell, under an Ar atmosphere of 275 kPa to suppress loss of volatile elements such as Li, B, K and Na, and to minimize oxidation. A suite of whole-rock powders comprising six international standard reference materials, SY 2, SY 3, MRG- 1, BIR- 1, G- 2 and RGM- 1, was selected for this test, to provide a wide range of major- and trace-element compositions. Electron microprobe major-element analyses and backscattered electron imaging were used to evaluate homogeneity of the beads. Results for all the international standards were within ± 15% of recommended values, and most elements are within ± 5%. Trace-element analyses were also carried out using nebulizer mode ICP-MS, to help in defining the optimum conditions for producing glasses. Mounts of glass beads were then analyzed on a Perkin Elmer Sciex® Elan 5000 Laser Sampling ICP-MS for 43 major, minor and trace elements, including the rare-earth elements (REE's). The Nd:YAG laser was operated in Q-switched mode, with a 240-ms delay, at 144 mJ, and 1.5-min laser ablation, of which the last minute was counting time on ICP-MS. Analysis of the reference materials as unknowns, using NBS- 612 glass as a standard for calibration and Al as an internal standard yielded 25 elements within ± 20% of recommended values, and precision of between ± 5% and ± 15% (relative standard deviation, RSD). This group of elements includes Ca, Co, Sr, Y, Cs, Ba, all 14 REE's, and Zr, Nb, Hf, Ta, Th and U. The latter group are difficult to analyze using other methods, and may be unstable in solution. Detection limits for the suite range from 0.02 to 0.3 ppm. Ablation efficiencies for the glasses were assessed by measuring the mass of material ablated, and it was found that the REE content, as well as Fe 2+ content, may strongly affect the absorption of Nd:YAG radiation, and thus the ablation efficiency of a material. This study documents a low contamination method of creating acceptably homogeneous glasses for solid state analyses, and may have wider application in the analysis of highly refractory materials as glasses, when produced with minimum quantities of fluxes.