Laser Microprobe ICPMS: A Robust and Cost Effective Microbeam Technique for Quantitative Trace Element Analysis of Solids

Norman M.D., Pearson N.J., Sharma A., and Griffin W.L., GEMOC, Macquarie

Laser ablation ICPMS is a relatively new microbeam technique that can provide rapid, precise determinations of trace element abundances at sub-ppm detection limits in a variety of solid targets. A laser microprobe system was installed at Macquarie University in December 1994 and has been used mainly for the analysis of geological materials. The laser is a Q-switched, frequency-quadrupled Nd:YAG laser typically operated at 4 Hz and 1-3 mJ per pulse. Laser repetition rates of 4 Hz have been found to produce a nearly steady state signal for several minutes with much less inter-element fractionation per unit time compared to higher pulse rates (e.g., 10-20 Hz; Norman et al., 1996, Geostandards Newsletter 20, 247-261). Spot diameters are 50 microns or less, and drill rates are about 1 micron per sec.

A typical spot analysis takes about 5 minutes, including backgrounds and wash-out, making the laser microprobe a highly cost-effective analytical tool. Up to 30 masses per analysis are determined, with relative element sensitivities calibrated against NIST glasses. Concentration values for these glasses have been established by calibration against other natural and synthetic material, including a variety of minerals and fused rock standards. Comparisons of laser microprobe analyses of garnets and pyroxenes, with data obtained by proton microprobe, solution ICPMS, INAA, and XRF show no matrix effects.

A typical analysis consists of 100 sweeps of the mass range at a dwell time of 50-100 msec per mass. For each sample, 30 readings are counted on the dry carrier gas to establish the background, followed by 70 readings during ablation. Peak and background regions are selected graphically from the time-resolved spectra of each sample to determine the net count rate for each mass. Each analysis is normalized to a major element such as Ca as an internal standard. Detection limits range from < 2 ppm for Ni to ² 50 ppb for a diverse group of elements, including several of the REE, Th, and U. Replicate analyses of the standards as unknowns indicate an analytical precision of 2-5% at the ppm level. Error analysis shows that counting statistics and the external precision on the internal standard concentration are the most significant sources of analytical uncertainty. The method has been used to understand large scale geological events in the mantle and to determine the compositions of indicator minerals for the mining and minerals industry. Laser microprobe ICPMS is a flexible microanalytical approach applicable to a wide variety of geological problems.

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