Trace Element Analysis of Solid Samples Using Laser Ablation ICPMS

Marc D. Norman

GEMOC Key Centre, School of Earth Sciences, Macquarie University, North Ryde NSW 2109

Summary

A focused laser beam can be used to ablate small amounts of solid material which are then introduced into an inductively-coupled plasma mass spectrometer (ICPMS) for trace element and/or isotopic analysis. A UV laser ablation system attached to a Perkin-Elmer ELAN 5100 ICPMS has been operational in our lab for over a year. During this time, we have analysed a variety of geological materials for their trace element abundances, with detection limits of 0.1 to 0.01 ppm and a spatial resolution of about 50 microns. Quantitative results for 30 elements are obtained through calibration of relative element sensitivities using an external standard such as one of the NIST glasses, and normalisation of each analysis to an internal standard, usually a major element such as Ca or Mg. A complete analysis, including wash-out, takes about 5 minutes. Replicate analysis of the calibration standard as an unknown demonstrates a long-term precision of ² 5% for all analysed elements.

Instrumentation

The laser ablation system and supporting software was designed by Drs. Simon Jackson and Henry Longerich of Memorial University, and was installed at Macquarie University in December, 1994. The laser is a Continuum Surelite I-20 Q-switched Nd:YAG laser with a fundamental infrared (IR) wavelength of 1064 nm. Two frequency doubling crystals provide second and forth harmonics in the visible (VIS, 532 nm) and ultraviolet (UV, 266 nm), respectively. We have used primarily the 266 nm wavelength because of the better coupling characteristics of UV wavelengths with silicate materials (especially Fe-poor materials such as feldspar and calcite), and because the UV wavelength can be focused to a smaller diameter spot for ablation. The laser is capable of operating continuously at pulse rates of 1-20 Hz (pulses per second) or in single shot mode. A repetition rate of 4 Hz provides an approximately steady state signal and reduces fractionation of the more volatile elements such as Pb and Rb relative to more refractory elements such as Sr, U, and REE during a run.

The beam which emerges from the laser is a mixture of IR, VIS, and UV. To isolate the 266 nm wavelength, the beam is directed onto a pair of dichroic mirrors which reflect the UV and allow the VIS and IR wavelengths to pass through to an beam trap. The laser produces a maximum of about 500 mW of UV power at 20 Hz, or about 25 mJ per pulse. This is far more energy than actually needed for a typical analysis, so the laser beam is attenuated by passing it through a rotatable half-wave plate followed by a Glan calcite prism. By rotating the half-wave plate (hence the angle of polarisation), the amount of energy delivered to the sample can be varied from nil to approximately 160 mW at 20 Hz, or 8 mJ per pulse. Most analyses are done with beam energy in the range of approximately 0.1-3 mJ per pulse. Ablation is monitored in real time using transmitted light and a video camera. If it is necessary to use reflected light to focus the sample, the reflected light prism must be removed from the beam path prior to ablation. The sample is contained within a chamber which is placed on the microscope stage, and through which the ICPMS nebulizer gas flows during ablation.

The outlet from the sample chamber is connected to a Perkin-Elmer Sciex ELAN 5100 ICPMS torch by ~ 1m of 6.4 mm OD x 4.0 mm ID (0.25 x 0.16 inch) tubing. A 30 ml chamber has been installed between the sample chamber and the ICPMS torch to integrate the ablated material and to suppress spikes which occasionally occur in the signals of some elements during initial stages of ablation. Very little difference in instrument operating conditions have been found for solution work compared to laser ablation analyses, including ion lens setting. Typically, somewhat higher nebulizer flow rates and RF power settings are used for laser ablation compared to solution work to increase sensitivity. Typical settings would be 1.0 ml min-1 nebulizer flow for laser ablation vs. 0.85 ml min-1 for solution analyses, and 1040 W of RF power for laser ablation vs. 1000 W for solutions. Operating conditions are set so that 248ThO/232Th is <1%, and other potentially interfering oxides are assumed to be negligible based on the relative ease of Th and REE oxide production. Data are collected in graphics mode, and processed using a Lotus-based software package provided by Memorial University.

Results

Quantitative laser ablation ICPMS analyses of five USGS powdered rock standards (AGV, BIR, DNC, W2, RGM) which were fused on Ir strips under an Ar atmosphere and quenched to a glass show very good agreement with the consensus values for these standards compiled by Govindaraju (1994, Geostandards Newsletter). During these analyses, the external calibration

standard (NIST 612 glass) was also analysed as an unknown. These data demonstrate a 1s precision of 2-4% for all elements, and an accuracy of < 2% relative to the calibration values for the average of these analyses (n = 24). Good agreement between LAM and PIXE, INAA, and solution ICPMS has also been obtained for a variety of materials, including mantle-derived garnets and pyroxenes.

Conclusion

Laser ablation ICPMS analyses can be used for the rapid and precise determination of trace element abundances in a variety of solid materials. The high spatial resolution (~ 50 µm), low detection limits (0.01-0.1 µg/g), and rapid analysis times (5 minutes), together with the variety of elements and targets which can be analysed make laser ablation ICPMS a powerful tool for trace element analysis. Instrumental precision of < 5% has been demonstrated, along with good agreement with results by other methods. Laser ablation ICPMS promises to make significant contributions to geological and materials science in the years ahead.