routine for incompatible trace elements in rocks samples using. ICP-MS. The international reference materials we analysed were 4 basaltic rocks (JB-2 and JB-3 ...
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Precise determination of trace elements in geological standard rocks using inductively coupled plasma mass spectrometry (ICP-MS) Chang Qing1, Tomoyuki Shibata2, Kazunori Shinotsuka1, Masako Yoshikawa2 and Yoshiyuki Tatsumi1 1 2
Center for Data and Sample Analyses, Institute for Frontier Research on Earth Evolution (IFREE) Institute of Geothermal Science, Kyoto University
unstable in simple HNO3 solutions. For other trace elements, sample introduction was through a conventional nebulizer and spray chamber (Meinhard and Scott-type, respectively). Oxide formation level was routinely maintained at less than 0.5% of CeO relative to Ce. Instrument calibration was undertaken using a synthetic mutil-element solutions prepared from original solution supplied by Spex® Industry. TAMAPURE-AA grade 38% HF, 70% HClO4 and 68% HNO 3 (Tama Chemicals Co., LTD., Japan) were used as received, without additional purification. Water was de-ionized to a resistivity of >18.3 MΩ cm-1 using a Milli-Q system (Millipore, Japan). 23-ml Teflon® PFA vials with screw caps (Savillex®, USA) were used for sample decomposition. Polyethylene bottles were used to store the sample solutions after decomposition and dilution, prior to ICP-MS analysis. To avoid contamination and to achieve low blanks, new polyethylene bottles were washed before using with TMSC®, HNO3 and Milli-Q water using an ultrasonic bath. PFA vials were rinsed carefully following a sequence of TMSC® (ultrasonic bath), 6N HNO3 (hot plate), 10% HF (ultrasonic bath) and Milli-Q water (hot plate).
Introduction Trace element concentrations in volcanic rocks, although negligible in mass, are an essential requirement for studying crust/mantle evolution processes. Increasingly, developments in analytical techniques have been made in order to obtain precise and accurate trace element abundances at ng g-1 levels in rocks using inductively coupled plasma mass spectrometry (ICP-MS). The application of ICP-MS for rapid and precise measurement of a wide range of elements has been successful previously (Eggins et al., 1997; Makishima and Nakamura, 1997). However, high field strength elements and elements present at ultralow levels in rocks are still not easily measured by ICP-MS for routine analysis because of difficulties in sample preparation, complicated matrix/ interference effects, and sensitivity limits of the instrument. Recent advances in ICP-MS technology, such as the development of a chicane lens system and micro sample introduction methods have significantly improved its performance in trace element quantification. We report on the establishment of a quantitative analytical routine for incompatible trace elements in rocks samples using ICP-MS. The international reference materials we analysed were 4 basaltic rocks (JB-2 and JB-3 from the GSJ, BHVO-1 and BCR-2 from the USGS), JG-3 (granite from the GSJ) and JP-1 (peridotite from the GSJ). The trace elements analyzed include high field strength elements (HFSE: Zr, Nb, Hf and Ta), rare earth elements (REE, Y) and large ion lithosphere elements (LILE: Rb, Sr, Ba, U, Th and Pb).
Sample preparation The sample decomposition procedure for REE and LILE analyses was slightly modified from a conventional HClO4/HF acid digestion procedure (Yokoyama et al., 1999; Sharma et al., 2000). Typically ~100 mg of rock powder was weighed in a 23-ml PFA Teflon vial. After adding concentrated HClO4/HF (v/v: 25/75), the vial was capped tightly and placed on a hotplate at 130-140ºC for 3 days. HClO4 instead of HNO3 was used in this step, as it produces a more effective attack against refractory minerals by improving the efficiency of the HF. At this stage, most silica bonds were broken. The sample was then evaporated to incipient dryness to remove volatile SiF4. Concentrated HClO4 was added again, and the vial was closed and placed on a hotplate at 160ºC for one day, then opened to dry the sample at a gradually increasingly temperature of up to 190ºC, to drive out excess HF and convert fluorides into chlorides. The residue was refluxed with 2ml 6N HNO3 and moderately heated for 2 hours, then dried down at a temperature of 120ºC to incipient dryness. The final sample residue was taken into in 5 ml 2% HNO3 and kept as a ‘mother sample solution’. At this stage, except for JP-1, no visible undissolved residue or floccule remained in the solution for the basaltic reference rocks (JB-2, JB-3, BHVO-1 and BCR-2) or JG-3. The ‘mother sample solution’ can also be utilized for
Analytical methods Instrumentation, reagents and labware The ICP-MS used in this work is a VG Elemental® PQ3 enhanced with a chicane lens system; a new electronic lens technique which crucially improves the signal to noise ratio. The noise count of our ICP-MS is 5-10cps, with a high signal sensitivity of (1-2)×108 cps/ppm for a broad spectrum of masses (m/z), as monitored at masses of 59Co, 115In, 140Ce, 209Bi and 238 U. The sensitivity is about 3-5 times higher than prior to the upgrade with the chicane lens, and significantly higher than those shown in published ICP-MS works (e.g., Eggins et al., 1997; Münker, 1998; Yang and Pin, 2002). Consequently, analytical precision and detection limits were greatly improved. A HF-resistant spray chamber and PFA microflow nebulizer were employed to enable direct introduction of samples in a HNO3HF solution to the plasma when analyzing HFSE, which are 357
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mura was acquired without an oxide correction (polyatomic interference of 135BaO to 151Eu and 137BaO to 153Eu), based on the hypothesis that the oxide contribution was small under high operating power conditions. However such an interference can indeed affect results, and was probably significant in their case, as they used a concentrated sample solution (a total dilution about 100 fold). Pin and Joannon (1997) obtained an Eu concentration of 0.817 µg g-1 in JB-2 after chemical separation, which is closer to the value obtained in this work. The REE concentrations for JP-1 are relatively variable between analysis runs, especially for Sm, Eu, Gd Tb and Lu, which had RSDs of >20% (Table 1). We attribute this poor reproducibility to low signal counts and relatively high procedural blanks (Table 4, in later section). Our JP-1 data were obtained before the upgrade to the chicane lens system, and the analyte signal intensities were then merely several tens time higher than the background noise. The poor reproducibility of JP-1 might also come from the sample preparation. We observed a few undissolved black particles (probably spinel) that remained in the decomposed solution sample. The REE concentrations of JP-1 obtained in this work, especial the light and middle REE, are significantly lower than complied data (Imai et al., 1995), which were mainly obtained by on XRF. However, they are perfectly consistent with the ICP-MS data published by Makishima and Nakamura (1997). The quality of REE data is generally illustrated using chondrite normalized REE patterns, which should yield a smooth line from La to Lu due to their similarity in charge and ionic radii, with the possible exceptions of Eu and Ce, which may have different charges in certain geological environments. Such plots are shown in Fig. 1, for JB-2, BHVO-1 and JP-1. The REE patterns clearly show that the compiled data for JP-1 are unreasonably variable, especially at Ho and Tm, because of the extremely low REE abundances in JP-1.
the measurement of Sr and Nd isotope compositions after appropriate chemical separation (Chang et al., 2000). Further dilution was made with 2% HNO3 and a known weight of internal standard solution was added before ICP-MS analysis. Total dilution factors were compromise for samples: usually 2000-5000 for the analysis of REE, Y, Pb, Th and U, and 20000-40000 for the analysis of Rb, Sr and Ba in basaltic rocks. For HFSE measurement, the sample solution was prepared using a different digestion procedure to ensure the complete decomposition of the rock powder. The starting sample weight for the basaltic rocks was ~100mg. The sample powder was first attacked with the mixture of concentrated HF/HNO3 (v/v: 15/85) in a PFA vial, using an ultrasonic bath for one day. The ultrasonic operation was set for a cycle of 15 minutes on, followed by 45 minutes rest. The vial was then moved to a hotplate at 80ºC, to evaporate the sample to dryness. A mixture of concentrated HClO4/HF (v/v: 20/80) was then added to sample, and the vial closed and placed on a hotplate at 160ºC for 3 days. Because the major silicate phases were almost completely removed, HF can thus effectively attack any residual refractory minerals that are considered to be the main residence for high field strength elements. Following a dry down at 120ºC, and an HClO4 treatment as described earlier, the residues were finally dissolved in 10ml of 2% HNO3-0.1% HF solution. The trace amount of free HF is necessary to prevent the HFSE from coming out of solution (Münker, 1998; Barth et al., 2000). This solution was further diluted and an internal standard was added shortly before ICP-MS analysis.
Results and discussion Determination of REE, Pb, Th and U, and Rb, Sr and Ba With the aim of establishing a quantitative method for the analysis of incompatible trace elements in basaltic rocks, six well-studied international standard reference rocks were selected to encompass a wide concentration range of the trace elements of interest. According to concentration level and ICPMS measurement convenience, the trace elements in these rocks were analyzed in three groups: the first group includes REE, Y, Pb, Th and U; the second group is Rb, Sr and Ba; and the third is the HFSE group. The analysis solutions for the two former groups were diluted from same sample solution. The results obtained are given in Table1 for REE, Y, Pb, Th, U, and Table 2 for Rb, Sr and Ba. Compiled and/or reference values are listed for comparison. The measured concentrations of REE, Pb, Th and U in the basaltic rocks, and Rb, Sr and Ba in both the basaltic and granite reference rocks are in excellent agreement with the reference values, and are all highly reproducible, as indicated by the relative standard deviation (RSD) of the replicated analyses (n=3~5). The RSD of repeated analyses of an identical BHVO-1 solution sample during the course of this study implies that the instrument and method were reasonably stable over the long term (Tables 1 and 2). Eu in JB-2 and BCR-2 were 7% and 21% lower than reference values (0.803 vs 0.86 and 1.58 vs 2.0 µg g-1, respectively). Makishima and Nakamura (1997) reported a similar Eu value for JB-2 to that of the reference. Although there is no detailed methodological information available for the BCR-2 value, the Eu concentration for JB-2 reported by Makishima and Naka-
HFSE analyses: reproducibility and accuracy HFSE data are shown in Table 3 for basaltic reference rocks (JB-2, JB-3, BHVO-1 and BCR-2) analyzed in this study. The reproducibility of the three individual analyses is between 0.1% and 1.3% RSD, with the exception of Nb and Ta in JB-2 (2.3% and 9.3%, respectively). The in-run precision was generally less than 2% relative standard deviation of 5 repeated data acquisitions. The good reproducibility achieved here results from: (1) the presence of suitable amounts of HF (0.1%) in the sample solution, which kept the HFSE elements stable in solution but was low enough to avoid the formation of insoluble fluorides that might incorporate the analytes of interest (significant Ca/Mg precipitation occurred when the HF concentration was increased to 5%, for a 100 mg starting sample powder); (2) using 2% HNO3-0.1% HF as a washing solution efficiently cleared memory effects during ICP-MS analysis; and (3) the complete dissolution of the sample using the procedure described above. The results of Ta for JB-2 show the worst reproducibility and within run precision (5-10%, Table 4). Contamination probably is the first factor that must be considered (this will be discussed later), although JB-2 has an extremely low Ta concentration. The reproducibility of Nb appears to correlate with its concentration (Table 4), also implying a background effect. 358
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sured concentrations for most of the elements are in good agreement with compiled data or published reference values. However, significantly different values for Ta and Nb in JB-2, Nb in JB-3, and some REE in JP-1, were obtained. Excellent reproducibility has been achieved in HFSE analysis for all the analyzed basaltic reference rocks, with the exception of Ta in JB-2. Trace amounts of free HF are necessary to keep the HFSE stable in solution, and to effectively wash out memory effects when performing HFSE analyses with solution ICPMS. The good reproducibility and accuracy of the HFSE determinations in this work indicate that the decomposition procedure used here was sufficient for the complete decomposition of the analyzed basaltic reference rocks. Although detection limits of the instrument in this work are low enough for all analyses of interest, the principal obstacle for the determination of ultra low level elements is the procedural blank. This is particularly true in the case of low level Ta determinations, because of the Ta contamination from Teflon containers.
The accuracy of HFSE analysis was estimated by comparing our data with reference values, and to some extent can also be judged from the Zr/Hf and Nb/Ta ratios. In mantle-derived rocks, these ratios are regarded as constant and indistinguishable from the chondritic values of Zr/Hf=36.3 and Nb/Ta=17.6 (Jochum et al., 1986). The estimation of the accuracy of a method inherently depends on the accuracy of references used. The results for Zr and Hf in JB-2, JB-3 and BHVO-1 are in good agreement with the compiled values (between 0.6 and 4%), and yield nearly chondritic Zr/Hf ratios. By contrast, reference values for Nb and Ta are widely scattered. The Nb data for JB-2 and JB-3 are substantially lower than the compiled values (68% and 16%, respectively), but are comparable to those obtained using an alkali fusion method (Hall et al., 1990). It is noted that our HFSE results are consistently higher than those reported by Makishima et al., (1999), with exception of Ta in BHVO-1 (Table 4). This discrepancy most likely comes from the difference in the decomposition methods. To avoid Ta contamination, Makishima and coworkers used polypropylene bottles instead of PFA Teflon vials for sample decomposition under rather low temperatures (90ºC). Their procedure may not have completely decomposed the sample powder, consequently producing systematically low values for all the HFSE.
References Anders, E., and N. Grevesse, Abundance of the elements: meteoritic and solar, Geochim. Cosmochim. Acta, 53, 197-214, 1989. Barth, M. G., W. F. McDonnough, and R. L. Rudnick, Tracking the budget of Nb and Ta in the continental crust, Chem. Geol., 165, 197-213, 2000. Chang, Q., T. Mishima, S. Yabuki, Y. Takahashi, and H. Shimizu, Sr and Nd isotope ratios and REE abundances of moraines in the mountain areas surrounding the Taklimakan Desert, NW China, Geochemical J., 34, 407-427, 2000. Eggins, S. M., J. D. Woodhead, L. P. J. Kinsley, G. E. Mortimer, P. Sylvester, M. T. McCulloch, J. M. Hergt, and M. R. Handler, A simple method for the precise determination of 40 trace elements in geological samples by ICP-MS using enriched isotope internal standardization, Chem. Geol., 134, 311-326, 1997. Govindaraju, K., 1994 compilation of working values and sample description for 383 geostandards, Geostandards Newsletter, 18, 1158, 1994. Hall, G. E. M., and J-C. Pelchat, Analysis of standard reference materials for Zr, Nb, Hf and Ta by ICP-MS after lithium metaborate fusion and cupferron separation, Geostandards Newsletter, 14, 197-206, 1990. Imai, N., S. Terashima, S. Itoh, and A. Ando, 1994 compilation values for GSJ reference samples, “Igneous rock series”, Geochemical J., 29, 91-95, 1995. Jarvis, K. E., A critical evaluation of two sample preparation techniques for low-level determination of same geologically incompatible elements by inductively coupled plasma- mass spectrometry, Chem. Geol., 93, 89-103, 1990. Jochum, K. P., H. M. Seufert, B. Spettel, and H. Palme, The solar-system abundance of Nb, Ta, and Y and the relative abundance of refractory lithophile elements in differentiated planetary bodies, Geochim. Cosmochim. Acta, 50, 1173-1183, 1986. Makishima, A., and E. Nakamura, Suppression of matrix effects in ICP-MS by high power operation of ICP: application to precise determination of Rb, Sr, Y, Cs, Ba, REE, Pb, Th and U at ng g-1 level in milligram silicate samples, Geostandards Newsletter, 21, 307-319, 1997. Makishima, A., E. Nakamura, and T. Nakano, Determination of zirconium, niobium, hafnium and tantalum at ng g-1 levels in geological materials by direct nebulisation of sample HF solution into FI-ICP-MS, Geostandards Newsletter, 23, 7-20, 1999. Münker, C., Nb/Ta fractionation in a Cambrian arc/back arc system, New Zealand: source constraints and application of refined ICPMS techniques, Chem. Geol., 144, 23-45, 1998. Pin, C., and S. Joannon, Low-level analysis of lanthanides in eleven silicate rock reference materials by ICP-MS after group separation using cation-exchange chromatography, Geostandards
Determination limit and procedural blank The 3σ instrumental detection limits are given in Table 4. These values were calculated from the concentration equivalent of 3 times the standard deviation of 10 analyses of 2% HNO3 for all the trace elements excluding the HFSE, and of 2% HNO3-0.1% HF for the HFSE. The detection limits have been critically improved by the high sensitivity of the ICP-MS enhanced with the chicane lens system, and are about 10-1000 times higher than those reported for previous ICP-MS applications (Jarvis, 1990; Makishima and Nakamura, 1997; Makishima et al., 1999). These detection limits theoretically allow the determination of ng g-1 level elements in rock samples, even using a highly diluted sample solution. Consequently, the procedural blank for a trace element is the main limitation in such high sensitive ICP-MS analyses. Although the blanks in this work are negligible for most of the elements analyzed using a sample size of 100 mg, blanks may seriously affect the precise determination of elements at ng g-1 levels, such as REE in JP-1, and Ta and Nb in JB-2. For example, the Ta blank (446 pg) is about 10% of the Ta content in a 100 mg sample powder. Previous studies have shown that PFA Teflon vials are a significant source of Ta contamination and thus not recommended for accurate analysis of Ta (Makishima et al., 1999; Yang and Pin, 2002).
Conclusions A simple and rapid method was established for the determination of a wide range of incompatible trace elements in basaltic rocks using an ICP-MS enhanced with a novel chicane lens system. REE+Y, LILE (Rb, Sr, Ba, Pb, Th and U) and HFSE (Zr, Nb, Ta, Hf and Ta) were determined using only a small aliquot of sample solution prepared from 100 mg of rock powder (JB-2, JB-3, BHVO-1, BCR-2, JG-3 and JP-1). Mea359
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Newsletter, 21, 43-50, 1997. Sharma, A. L., O. Alard, S. Elhlou, and N. J. Pearson, Evaluation of perchloric versus nitric acid digestion for precise determination of trace and ultra trace elements by ICP-MS, J. Conference Abst., Goldschmidt 2000, 5, 2, 914, 2000. Weyer, S., C. Münker, M. Rehkamper, and K. Mezger, Determination of ultra-low Nb, Ta, Zr and Hf concentrations and the chondritic Zr/Hf and Nb/Ta ratios by isotope dilution analyses with multiple collector ICP-MS, Chem. Geol., 187, 295-313, 2002. Yang, X. J., and C. Pin, Determination of niobium, tantalum, zirconium and hafnium in geological materials by extraction chromatography and inductively coupled plasma mass spectrometry, Analytical Chimica Acta, 458, 375-386, 2002. Yokoyama, T., A. Makishima, and E. Nakamura, Evaluation of the coprecipitation of incompatible trace elements with fluoride during silicate rock dissolution by acid digestion, Chem. Geol., 157, 175187, 1999.
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Figure 1. REE patterns of JB-2, BHVO-1 and JP-1. Error bars are standard deviations in replicated analyses. CI chondrite values are from Anders and Grevesse (1989).
Table 1. Measured REE, Pb, Th and U concentrations (É g g-1) in JB-2, BHVO-1, BCR-2 and JP-1
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Table 2. Rb, Sr and Ba concentrations of JB-2, BHVO-1 and JG-3
Table 4. Measured isotopes, corresponding limits of detection and procedural blanks
Table 3. Measured Zr, Nb, Hf and Ta concentrations in basaltic reference rocks
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