Chemical separation of Mo and W from terrestrial and extraterrestrial samples via anion exchange chromatography

Determination of 93Mo in Radioactive Samples of Sulfuric Acid Media from Nuclear Facilities

⁹³Mo is an important radionuclide in view of radioactive waste repository because of its long half-life and high mobility in the environment. ⁹³Mo decays by electron capture without any measurable gamma ray emission. The concentration of ⁹³Mo in most of the radioactive waste is many orders of magnitude lower than the major activation product radionuclides, which makes the accurate determination of ⁹³Mo a big challenge. A new analytical method for the determination of ⁹³Mo in sulfuric acid media from nuclear power reactor was developed. ⁹³Mo was separated from most of the radionuclides by cation exchange chromatography followed by the removal of sulfate by CaSO₄ precipitation. A further purification of ⁹³Mo, especially from anion species of ⁵¹Cr and ¹²⁵Sb, was achieved by anion exchange chromatography and a short alumina column separation. The chemical yield of ⁹³Mo in the entire separation procedure reached about 75%, and the decontamination factors for all potential interfering radionuclides were 1.5 × 10⁶-1.6 × 10⁸. The purified ⁹³Mo was measured by liquid scintillation counting through counting its low-energy Auger electrons. A detection limit of 2 mBq/g for ⁹³Mo in 50 g sample was achieved by this method, which enables the quantitative determination of ⁹³Mo in most of the radioactive samples in the decommissioning waste and coolant water of nuclear power reactors. The developed method has been successfully applied to determine ⁹³Mo in coolant water of nuclear power reactors, providing a robust analytical approach of ⁹³Mo for the radiological characterization of radioactive wastes.

Tungsten-182 evidence for an ancient kimberlite source

Globally distributed kimberlites with broadly chondritic initial ¹⁴³Nd-¹⁷⁶Hf isotopic systematics may be derived from a chemically homogenous, relatively primitive mantle source that remained isolated from the convecting mantle for much of the Earth's history. To assess whether this putative reservoir may have preserved remnants of an early Earth process, we report ¹⁸²W/¹⁸⁴W and ¹⁴²Nd/¹⁴⁴Nd data for "primitive" kimberlites from 10 localities worldwide, ranging in age from 1,153 to 89 Ma. Most are characterized by homogeneous μ¹⁸²W and μ¹⁴²Nd values averaging -5.9 ± 3.6 ppm (2SD, n = 13) and +2.7 ± 2.9 ppm (2SD, n = 6), respectively. The remarkably uniform yet modestly negative μ¹⁸²W values, coupled with chondritic to slightly suprachondritic initial ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios over a span of nearly 1,000 Mya, provides permissive evidence that these kimberlites were derived from one or more long-lived, early formed mantle reservoirs. Possible causes for negative μ¹⁸²W values among these kimberlites include the transfer of W with low μ¹⁸²W from the core to the mantle source reservoir(s), creation of the source reservoir(s) as a result of early silicate fractionation, or an overabundance of late-accreted materials in the source reservoir(s). By contrast, two younger kimberlites emplaced at 72 and 52 Ma and characterized by distinctly subchondritic initial ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁴³Nd/¹⁴⁴Nd have μ¹⁸²W values consistent with the modern upper mantle. These isotopic compositions may reflect contamination of the ancient kimberlite source by recycled crustal components with μ¹⁸²W ≥ 0.

A Chromatographic Method for Separation of Tungsten (W) from Silicate Samples for High-Precision Isotope Analysis Using Negative Thermal Ionization Mass Spectrometry

A new chromatographic method for isolation of W from large masses of silicate samples (>1 g) for ultrahigh precision isotopic analysis was developed. The purification of W was achieved through two stages of rapid chromatographic separations. In the first step, Ti, Zr, Hf, and W were separated collectively from the sample matrix through an AG1-X8 (100-200 mesh) column with a 10 mL resin volume. Subsequently, W was rapidly separated from Ti and Zr-Hf with high purity by a two-step extraction chromatographic method using 0.6 and 0.3 mL TODGA resin columns (50-100 μm particle size), respectively. The total yield of W, including the anion exchange and the TODGA chromatographic separation steps, is greater than 90%. The procedure was employed to isolate W from rock reference materials GSJ JB-3 and USGS BHVO-2; the separated W was analyzed by TRITON Plus TIMS, yielding a ¹⁸²W/¹⁸⁴W of 0.864898 ± 0.000005 (n = 8, 2 SD) for JB-3 and ¹⁸²W/¹⁸⁴W of 0.864896 ± 0.000006 (n = 5, 2 SD) for BHVO-2, which are in agreement with previously reported values within analytical errors.

Precise analysis of the concentrations and isotopic compositions of molybdenum and tungsten in geochemical reference materials

Molybdenum (Mo) is a redox-sensitive element and its concentrations and stable isotope compositions are widely used as a redox proxy in paleoceanography. Tungsten (W) is an emerging new isotope proxy, which has potential as a tracer for hydrothermal and early diagenetic processes. We present a new method for the precise and accurate analysis of Mo and W concentrations and isotope compositions from one single sample aliquot, thus saving mass of a sample and making the results directly comparable without concerns related to analytical or natural sample heterogeneity. After acid digestion, Mo and W are separated from the sample matrix using chelating resin NOBIAS Chelate-PA1 and anion exchange resin AG1 X8. Matrix removal is highly efficient: the remaining percentage is 10^-2 to 10^-5% with respect to the initial weight. Subsequently, samples are measured for Mo and W concentrations and isotope compositions using multi-collector inductivity coupled plasma mass spectrometry (MC-ICP-MS). For mass bias correction and determination of concentrations, we use standard-sample bracketing and in addition an external correction method employing ruthenium (Ru) for Mo and rhenium (Re) for W. This double correction approach results in an external reproducibility of or below 0.10‰ (2SD) for δ⁹⁸Mo and 0.05‰ for δ¹⁸⁶W based on ICP standard solutions (NIST SRM 3134 lot No. 130418 for Mo and NIST SRM 3163 lot No. 080331 for W). We present data for Mo and W in 12 geological reference materials including igneous rocks, sedimentary rocks, marine sediments, and manganese nodules. For Mo our method reproduces published values for the geological standard materials within analytical error of published values. For W, although published data do not always agree for a given geological standard material, our data agree within error with more recent data. We interpret a cause of the deviations is due to unknown effects of a desolvating nebulizer for MC-ICP-MS.

Isolation and quantification of a 93Mo isotope solution from proton irradiated niobium

⁹³Mo (4000 y half-life) formed through the ⁹³Nb(p,n)⁹³Mo reaction was isolated from a niobium target foil previously used in a low energy medical cyclotron. ⁹³Mo has identical characteristic x-ray emission and mass as the isomer ^93mNb and stable Nb present in the target foil at much higher concentrations. This makes distinction between ⁹³Mo, ^93mNb and stable Nb difficult using radiometric or mass spectrometric methods. An anion exchange method in combination with x-ray spectrometry and ICP-MS/OES enabled quantitative isolation of about 0.4 μg ⁹³Mo (14 kBq) from ^93mNb with a separation factor >10⁴ on a single column. An extraction chromatography column (TEVA) was used to reach a^93mNb/⁹³Mo activity ratio of <10^-6 and an atom ratio ⁹³Nb/⁹³Mo <1% making the ⁹³Mo suitable for both radiometric and mass spectrometric testing. ⁹³Mo is the only radioisotope of molybdenum with a long enough half-life suitable for this purpose. Calibration of the ⁹³Mo isotope solution was done through x-ray spectrometry using a characterized BEGe-detector in combination with a^99mTc solution. This is the first reported isolation of a⁹³Mo solution in the literature and the first time a LSC-spectrum of ⁹³Mo is shown.

High-precision molybdenum isotope analysis by negative thermal ionization mass spectrometry

Procedures for the separation, purification, and high-precision analysis of mass-independent isotopic variations in molybdenum (Mo) using negative thermal ionization mass spectrometry are reported. Separation and purification of Mo from silicate and metal matrices are achieved using a two-stage anion exchange chromatographic procedure. Molybdenum is ionized as the MoO₃ ^- species using a double filament assembly. The MoO₃ ^- ion beams are collected using Faraday cup detectors equipped with a mixed array of amplifiers utilizing 10¹¹ and 10¹² Ω resistors, which allows for in situ measurement and correction of oxygen isobars. The long-term external reproducibility of ⁹⁷Mo/⁹⁶Mo, the most precisely measured Mo isotope ratio, is ±5.4 ppm (2SD), based on the repeated analyses of the Alfa Aesar Specpure ^® Mo plasma standard and using ⁹⁸Mo/⁹⁶Mo for fractionation correction. The long-term external reproducibilities of 92Mo/⁹⁶Mo, ⁹⁴Mo/⁹⁶Mo, ⁹⁵Mo/⁹⁶Mo, and ¹⁰⁰Mo/⁹⁶Mo are ±107, 37, 23, and 32 ppm (2SD), respectively. With this precision, smaller differences in Mo isotopic compositions can be resolved than have been previously possible.