Trace Element Analyses of Rocks and Minerals by ICP-MS
Introduction
Inductively coupled plasma-mass spectrometry (ICP-MS) is well established as a rapid and precise method for the determination of the rare earth elements (REEs) and trace elements in geologic samples – Lichte et al., (1987), Jarvis (1988), Longerich et al., (1990). However, complete sample digestion is required for accurate results. Mixed acid open-vial digestions on a hotplate work well for basaltic and most ultramafic samples, but may fail to completely decompose many trace mineral phases found in more silicic samples. These resistant phases, such as zircon, garnet, and tourmaline, may contain a significant percentage of the total trace elements in a given sample. High-pressure bombs are effective at achieving complete digestion, but are cumbersome, slow, and labor intensive. Fusion with a flux may require large dilutions to avoid unacceptably high levels of total dissolved solids. We have developed a combination fusion-dissolution method that effectively decomposes refractory mineral phases and removes the bulk of unwanted matrix elements. The procedure consists of a low-dilution fusion with di-Lithium tetraborate followed by an open-vial mixed acid digestion. This method allows us to analyze 14 REEs and 13 additional trace elements in a wide range of geologic samples without having to make assumptions as to the presence or absence of resistant mineral phases. The dissolution with HF after the Lithium-tetraborate fusion quantitatively removes silica and more than 90% of the flux as gaseous fluorides, leaving clear, stable solutions for analysis on the ICP-MS.
Experimental
The Flux used for the fusion is di-Lithium-tetraborate (Spectromelt® A-10, EM Science, Gibbstown, NJ). Reagents are HNO3 69-70% (Fisher ACS plus grade), HF 48-52% (Baker ACS reagent grade), HClO4 67-71% (Fisher Trace Metal Grade), and H2O2 (Baker ACS Reagent). The HF is further purified before use by sub-boiling distillation in a teflon still. All water used is >18 M deionized water from a Nanopure analytical grade water system (Barnstead/Thermolyne)
Powdered samples are mixed with an equal amount of lithium tetraborate flux (typically 2g), placed in a carbon crucible and fused at 1000° C in a muffle furnace for 30 minutes. After cooling, the resultant fusion bead is briefly ground in a carbon-steel ring mill and a 250 mg portion is weighed into a 30 ml, screw-top Teflon PFA vial for dissolution. The acid dissolution consists of a first evaporation with HNO3 (2ml), HF (6 ml), and HClO4 (2 ml) at 110° C. After evaporating to dryness, the sample is wetted and the sides of the vial are rinsed with a small amount of water before a second evaporation with HClO4 (2 ml) at 160° C. After the second evaporation, samples are brought into solution by adding approximately 10 ml of water, 3 ml HNO3, 5 drops H2O2, 2 drops of HF and warmed on a hot plate until a clear solution is obtained. The sample is then transferred to a clean 60 ml HDPE bottle diluted up to a final weight of 60g with de-ionized water.
Solutions are analyzed on an Agilent model 4500 ICP-MS and are diluted an additional 10X at the time of analysis using Agilent’s Integrated Sample Introduction System (ISIS). This yields a final dilution factor of 1:4800 relative to the amount of sample fused. Instrumental drift is corrected using Ru, In, and Re as internal standards. Internal standardization for the REEs uses a linear interpolation between In and Re after Doherty (1989) to compensate for mass-dependant differences in the rate and degree of instrumental drift. Isobaric interference of light rare earth oxides on the mid- heavy REEs can be a significant source of error in ICP-MS analysis, so tuning is optimized to keep the CeO/Ce ratio below 0.5%. Correction factors used to compensate for the remaining oxide interferences are estimated using two mixed-element solutions. The first contains Ba, Pr, and Nd, and the second Tb, Sm, Eu, and Gd. Standardization is accomplished by processing duplicates of three in-house rock standards interspersed within each batch of 18 unknowns. Concentrations, oxide- and drift corrections are then calculated offline using a spreadsheet.
Results
Long term precision for the method is typically better than 5% (RSD) for the REEs and 10% for the remaining trace elements. Analyses of USGS and international rock standards show good agreement with consensus values. (Table 1).
References
Doherty W. (1989), An internal standardization procedure for the determination of yttrium and the rare earth elements in geological materials by inductively coupled plasma-mass spectrometry. Spectrochimica Acta, 44B, 263-280.
Jarvis, K.E. (1988), Inductively coupled plasma mass spectrometry; a new technique for the rapid or ultra-trace level determination of the rare-earth elements in geological materials. Chemical Geology, 68, 31-39.
Jenner G.A., Longerich H.P., Jackson S.E., and Fryer B.J. (1990), ICP-MS – A powerful tool for high-precision trace-element analysis in Earth sciences: Evidence from analysis of selectd U.S.G.S. reference samples. Chemical Geology, 83, 133-148.
Lichte, F.E., Meier, A.L., and Crock, J.G. (1987), Determination of the rare-earth elements in geological materials by inductively coupled plasma mass spectrometry. Analytical Chemistry, 59, 1150-1157.
Longerich H. P., Jenner G.A., Fryer B.J., and Jackson S.E. (1990), Inductively coupled plasma-mass spectrometric analysis of geological samples: A critical evaluation based on case studies. Chemical Geology, 83, 105-118.