Nicholas S. Lloyd

Tantalum isotope ratio measurements and isotope abundances determined by MC-ICP-MS using amplifiers equipped with 1010, 1012 and 1013 Ohm resistors

Due to analytical difficulties related to the low abundance of 180Ta (about 0.012%), the absolute isotope composition of tantalum is not well known and possible natural variations in 180Ta/181Ta are so far unconstrained. Improved precision is required in order to evaluate the homogeneity of Ta isotope distributions among solar system materials and whether natural Ta stable isotope variations exist on Earth. Using a Neptune™ multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) system and different resistors in the Faraday cup amplifier feedback loops (a 1010 Ω for 181Ta; 1012 or newly developed 1013 Ω resistors for 180Ta and Hf interference monitor isotopes) now allows relative analyses of 180Ta/181Ta with an intermediate precision of ca. ±4e (e refers to one part in 10 000) using 25 to 100 ng Ta and thus even for sample sizes available from meteorites (e.g., 1 g). The 1013 Ω amplifier resistors proved to be of paramount importance for high-precision Ta isotope ratio measurements of low amounts of material. Tailing effects from the large 181Ta beam have previously been underestimated. A thorough assessment of this effect revealed a tailing contribution of ∼2.5% on the currently recommended IUPAC ratio. Potential systematic biases in the mass discrimination correction are assumed being of minor importance compared to an uncertainty of ∼0.4% achieved for the estimate of the “true” 180Ta/181Ta ratio. We propose a new 180Ta/181Ta isotope ratio of 0.00011705(41), equivalent to 181Ta/180Ta = 8543(30), yielding isotope abundances of 0.011704(41) % for 180Ta and 99.988296(41) % for 181Ta, and an absolute atomic weight for tantalum of 180.9478787(38) u (all uncertainties with k = 2).

High precision determination of lithium and magnesium isotopes utilising single column separation and MC-ICPMS

Rationale Li and Mg isotopes are increasingly used as a combined tool within the geosciences. However, established methods require separate sample purification protocols utilising several column separation procedures. This study presents a single‐step cation‐exchange method for quantitative separation of trace levels of Li and Mg from multiple sample matrices. Methods The column method utilises the macro‐porous AGMP‐50 resin and a high‐aspect ratio column, allowing quantitative separation of Li and Mg from natural waters, sediments, rocks and carbonate matrices following the same elution protocol. High‐precision isotope determination was conducted by multi‐collector inductively coupled plasma mass spectrometry (MC‐ICPMS) on the Thermo Scientific™ NEPTUNE Plus™ fitted with 1013 Ω amplifiers which allow accurate and precise measurements at ion beams ≤0.51 V. Results Sub‐nanogram Li samples (0.3–0.5 ng) were regularly separated (yielding Mg masses of 1–70 μg) using the presented column method. The total sample consumption during isotopic analysis is <0.5 ng Li and <115 ng Mg with long‐term external 2σ precisions of ±0.39‰ for δ7Li and ±0.07‰ for δ26Mg. The results for geological reference standards and seawater analysed by our method are in excellent agreement with published values despite the order of magnitude lower sample consumption. Conclusions The possibility of eluting small sample masses and the low analytical sample consumption make this method ideal for samples of limited mass or low Li concentration, such as foraminifera, mineral separates or dilute river waters.

Application of 1013 ohm Faraday cup current amplifiers for boron isotopic analyses by solution mode and laser ablation multicollector inductively coupled plasma mass spectrometry

RATIONALE Boron isotope ratios (δ11 B values) are used as a proxy for seawater paleo-pH, amongst several other applications. The analytical precision can be limited by the detection of low intensity ion beams from limited sample amounts. High-gain amplifiers offer improvements in signal/noise ratio and can be used to increase measurement precision and reduce sample amounts. METHODS 1013 ohm amplifier technology has previously been applied to several radiogenic systems, but has thus far not been applied to non-traditional stable isotopes. Here we apply 1013 ohm amplifier technology for the measurement of boron isotope ratios using solution mode MC-ICP-MS and laser ablation mode (LA-)MC-ICP-MS techniques. Precision is shown for reference materials as well as for low-volume foraminifera samples. RESULTS The baseline uncertainty for a 0.1 pA 10 B+ ion beam is reduced to <0.1 ‰ for a typical measurement period. The external precision is better than 0.2 ‰ (2SD) for δ11 B measurements for solution samples containing as little as 0.8 ng total boron. For in situ microanalyses with LA-MC-ICP-MS, the external precision of 11 B/10 B from an in-house calcite standard was 1 ‰ (2SD) for individual spot analyses, and 0.3 ‰ for the mean of ≥10 replicate spot analyses. CONCLUSIONS 1013 ohm amplifier technology is demonstrated to offer advantages for the determination of δ11 B values by both MC-ICP-MS and LA-MC-ICP-MS for small samples of biogenic carbonates, such as foraminifera shells. 1013 ohm amplifier technology will also be of benefit to other non-traditional stable isotope measurements.

High-precision 41K/39K measurements by MC-ICP-MS indicate terrestrial variability of δ41K

Potassium is a major component in continental crust, the fourth-most abundant cation in seawater, and a key element in biological processes. Until recently, difficulties with existing analytical techniques hindered our ability to identify natural isotopic variability of potassium isotopes in terrestrial materials. However, measurement precision has greatly improved, and a range of K isotopic compositions has now been demonstrated in natural samples. In this study, we present a new technique for high-precision measurement of K isotopic ratios using high-resolution, cold plasma multi-collector mass spectrometry. We apply this technique to demonstrate natural variability in the ratio of 41K to 39K in a diverse group of geological and biological samples, including silicate and evaporite minerals, seawater, and plant and animal tissues. The total range in 41K/39K ratios is ca. 2.6‰, with a long-term external reproducibility of 0.17‰ (2σ, N = 108). Seawater and seawater-derived evaporite minerals are systematically enriched in 41K compared to silicate minerals by ca. 0.6‰, a result consistent with recent findings. Although our average bulk-silicate Earth value (−0.54‰) is indistinguishable from previously published values, we find systematic δ41K variability in some high-temperature sample suites, particularly those with evidence for the presence of fluids. The δ41K values of biological samples span a range of ca. 1.2‰ between terrestrial mammals, plants, and marine organisms. Implications of terrestrial K isotope variability for the atomic weight of K and K-based geochronology are discussed. Our results indicate that high-precision measurements of stable K isotopes, made using commercially available mass spectrometers, can provide unique insights into the chemistry of potassium in geological and biological systems.