Multi-collector Inductively Coupled Plasma Mass Spectrometry Research Articles

Determination of the Isotopic Composition of Gadolinium Using Multicollector Inductively Coupled Plasma Mass Spectrometry

In this study, we report independent measurements of all stable isotope ratios of gadolinium. Our study employs multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) with National Research Council Canada (NRC) HALF-1 isotopic hafnium standard as a primary calibrator and surveys four commercial gadolinium materials, including a NRC candidate isotopic reference material, GADS-1. The isotopic composition of gadolinium is determined using the regression model without reliance on conventional normalizing isotope ratios or mass-dependent isotope ratio correction models. In this work, all gadolinium isotope ratios were obtained from 160Gd/158Gd which, in turn, was measured from hafnium 178Hf/177Hf either directly or indirectly through 167Er/166Er. The latter approach was used for the final determination of gadolinium isotopic composition, as it provides smaller combined uncertainty. We report high-precision measurements of the isotopic composition of gadolinium, which support a revised standard atomic weight. Isotope amount ratios of R152/158 = 0.008 20(2)k=1, R154/158 = 0.087 98(12)k=1, R155/158 = 0.596 81(63)k=1, R156/158 = 0.825 08(57)k=1, R157/158 = 0.630 60(22)k=1, and R160/158 = 0.879 10(60)k=1, and the atomic weight of Ar(Gd) = 157.2502(6)k=1 were obtained for gadolinium in GADS-1.

Micro‐invasive method for studying lead isotopes in paintings*

A micro‐invasive technique is presented that enables pigment sampling from individual layers of a painting cross‐section by obtaining a furrow 10–50 μm wide of chosen length. Combined with increased sensitivity of lead (Pb) isotope analysis using multicollector inductively coupled plasma mass spectrometry (MC‐ICPMS) equipped with a 1013 Ω resistor or a Pb double‐spike technique, the amount of Pb needed for isotopic analysis is drastically reduced, while maintaining a relative 2 SD precision for 206Pb/204Pb of < 0.02%. The methodology proved able to characterize Pb isotope differences within paint layers.

Application of laser in-situ U-Pb dating to reconstruct the reservoir porosity evolution in the Cambrian Xiaoerbulake Formation, Tarim Basin 激光铀铅同位素定年技术在塔里木盆地肖尔布拉克组储层孔隙演化研究中的应用

The Laser Ablation (LA)Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) technique, co-developed by the CNPC Key Laboratory of Carbonate Reservoir and School of Earth and Environmental Sciences, The University of Queensland, has huge potential to overcome the difficulty in sampling and dating ultra-low concentration U-Pb minerals in ancient marine carbonates that are relatively smaller in diameter in diagenetic fabric, which was untenable by the conventional isotope dilution (ID) methods. The in-situ analysis by LA combined with MC-ICP-MS has been applied to the study on the diagenesis and porosity evolution of the Cambrian Xiaoerbulake reservoir, Tarim Basin.By dating of dolomitic and calcitic cements from filled vugs, matrix pores and fractures in different stages, we suggested that the burial diagenetic history of dolomite reservoir in the Cambrian Xiaoerblake Formation mainly features the filling of primary pores and vugs generated under karstification. The filling of vugs mainly occurred during the early Caledonian, while the filling of matrix pores mainly occurred in the middle Caledonian and Indosinian. Fractures activated during the Caledonian and Indosinian served as the channels for hydrothermal migration, providing the materials to cement the vugs and pores; while the residual vugs, pores and fractures with no filling of dolomite, calcite and siliceous cements were the main reservoir space. The diagenesis and porosity evolution history of dolomite reservoir in the Cambrian Xiaoerbulake Formation, Tarim Basin, was hence established. The understandings mentioned above are highly consistent with the tectonic-burial history and basin thermal history in the study area, which may be effective tools to date the ancient marine carbonate reservoir cements, to reconstruct diagenesis and porosity evolution history, and to evaluate effective pores before hydrocarbon migration in the ancient marine carbonates.

Isobaric Spike Method for Absolute Isotopic Ratio Determination by MC-ICP-MS.

Absolute isotopic ratios are required for isobaric interference corrections, spike calibrations, and isotopic analysis by external normalization methods. However, high-precision natural isotopic abundance data are lacking for many elements, particularly those with less than four isotopes or having isobaric isotopes with other elements. In this study, we developed a method for absolute isotope ratio analysis, which integrates the concept of the double-spike method with isotopic analysis of element pairs that have isobaric isotopes. Using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS), the isotopic composition of a sample can be derived by measuring a series of mixtures of the sample and a spike element that has an isobaric isotope with the element being analyzed. We applied this method to five pairs of elements (Ca-Ti, V-Ti, Cr-Ti, Ni-Zn, and In-Sn) and obtained the absolute isotopic ratios for Ca, V, Cr, Ni, and In, as well as the relative Ca isotopic composition. By simultaneous measurement of Ti and Ca isotopes, a quantitative relationship between the instrumental mass fractionation factors and element masses was developed. After correcting for the difference in instrumental mass fractionation factors, the obtained absolute ratios agree well with the literature data and have per mil level accuracy. This method has considerable potential in measuring the absolute isotopic ratios of elements that have isobaric isotope with other elements. Such precisely determined absolute isotopic ratios and the relationship between the instrumental mass fractionation factors and elemental masses will improve isobaric interference corrections, particularly when chemical purification is imperfect or during laser ablation analysis.

87 Sr/86 Sr isotope ratios in rocks determined using inductively coupled plasma tandem mass spectrometry in O2 mode without prior Sr purification.

An inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) instrument can be developed to determine 87 Sr/86 Sr ratios with an external precision better than 0.05% relative standard deviation (RSD) in "mass shift" mode without prior Sr purification. Previous studies suggested using CH3 F, N2 O, and SF6 as reaction gases for this method because a better reaction rate can be achieved with Sr+ than with O2 in the reaction cell. However, these gases are not commonly used in general chemistry laboratories, and processes using these gases are difficult to implement quickly due to regulations. We aim to develop a rapid method that can be applied to many samples for the accurate determination of 87 Sr/86 Sr isotope ratios with precision below 0.1% RSD (or approximately to the fourth decimal place). We evaluated the accuracy and precision of 87 Sr/86 Sr ratios in certified reference materials and different rock types determined using ICP-MS/MS with O2 as the reaction gas in comparison with those determined using the multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) method. This study showed that by using the ICP-MS/MS method, the 87 Sr/86 Sr ratios of BCR-2 and BHVO-2 do not vary significantly with and without prior Sr purification; when the Sr concentration of the measured solution is within the range of 60-350 ng/mL, there is no significant effect on the measured 87 Sr/86 Sr ratios. The results also showed that the 87 Sr/86 Sr ratios of 23 different rock types measured by ICP-MS/MS and MC-ICP-MS methods agree very well. The precision of the 87 Sr/86 Sr ratio measured using ICP-MS/MS varies between 0.0001 and 0.0019 (2SD). This precision is less than that of the MC-ICP-MS method but is sufficient for certain applications, such as identifying 87 Sr/86 Sr ratios in different rock types. These results suggest that the developed ICP-MS/MS method has the potential for future studies involving the identification of Sr sources.

Nanosecond Laser Ablation-Multicollector Inductively Coupled Plasma-Mass Spectrometry for in Situ Fe Isotopic Analysis of Micrometeorites: Application to Micrometer-Sized Glassy Cosmic Spherules.

This work evaluates the use of nanosecond laser ablation-multicollector inductively coupled plasma-mass spectrometry (ns-LA-MC-ICP-MS) for Fe isotopic analysis of glassy cosmic spherules. Several protocols for data acquisition from the transient signals were compared, with the integration method, i.e., isotope ratios obtained by dividing the corresponding signal intensities integrated over the selected signal segment, providing the best precision. The bias caused by instrumental mass discrimination was corrected for by a combination of internal correction using Ni as an internal standard (coming from a conebulized standard solution) and external correction using a matrix-matched standard. Laser spot size and repetition rate were adapted to match the signal intensities for sample and standard within ±10%. For in situ isotopic analysis, the precision of the δ56Fe values ranged between 0.02 and 0.11‰ (1 SD, based on 4 measurement sessions, each based on ablation along 5 lines for 30 s each) and 0.03-0.17‰ (SD, based on 3 measurement sessions) for glass reference materials and micrometeorites, respectively. Despite this excellent reproducibility, the variation of the isotope ratios along a single ablation line indicated isotopic inhomogeneity exceeding 1‰ in some micrometeorites. Isotopic analysis via pneumatic nebulization MC-ICP-MS, after sample digestion and chromatographic Fe isolation, was performed to validate the results obtained by in situ isotopic analysis, and good agreement was achieved between the δ-values obtained via both approaches and with those reported in literature for MPI-DING and USGS glass reference materials. Also for the glassy cosmic spherules, overall, there was a good match between the ns-LA-MC-ICP-MS and solution MC-ICP-MS results.

Magnesium isotope analysis of olivine and pyroxene by SIMS: Evaluation of matrix effects

The performance of multi-collector secondary ion mass spectrometry (MC-SIMS) for Mg isotope ratio analysis was evaluated using 17 olivine and 5 pyroxene reference materials (RMs). The Mg isotope composition of these RMs was accurately and precisely determined by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), and these measured isotope ratios were used to evaluate SIMS instrumental mass bias as a function of the forsterite (Fo) content of olivine. The magnitude of the Mg isotope matrix effects were ~3‰ in δ25Mg, and are a complex function of olivine Fo content, that ranged from Fo59.3 to Fo100. In addition to these Mg isotope matrix effects, Si+ ion yields and Mg+/Si+ ion ratios varied as a complex function of the Fo content of the olivine RMs. For example, Si+ ion yields varied by ~33%. Based on the observations, we propose instrumental bias correction procedures for SIMS Mg isotope analysis of olivine using a combination of Mg+/Si+ ratios and Fo content of olivine. Using this correction method, the accuracy of δ25Mg analyses is 0.3‰, except for analysis of olivine with Fo86–88 where instrumental biases and Mg+/Si+ ratios change dramatically with Fo content, making it more difficult to assess the accuracy of Mg isotope ratio measurements by SIMS over this narrow range of Fo content.Five pyroxene RMs (3 orthopyroxenes and 2 clinopyroxenes) show smaller ranges of instrumental bias (~1.4‰ in δ25Mg) as compared to the olivine RMs. The instrumental bias for the 3 orthopyroxene RMs do not define a linear relationship with respect to enstatite (En) content, that ranged from En85.5–96.3. The clinopyroxene RMs have similar En and wollastonite (Wo) contents but have δ25Mg values that differ by 0.5‰ relative to their δ25Mg values determined by MC-ICP-MS. These results indicate that additional factors (e.g., minor element abundances) likely contribute to SIMS instrumental mass fractionation. In order to better correct for these SIMS matrix effects, additional pyroxene RMs with various chemical compositions and known Mg isotope ratios are needed.

Quantifying the impact of recovery during chromatographic purification on the accuracy of lithium isotopic determination by multi-collector inductively coupled plasma mass spectrometry.

Lithium (Li) isotopes have increasingly been applied as tracers in Earth and planetary sciences and their effectiveness relies upon accurate and precise Li isotopic data. Nowadays, multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) combined with chromatographic purification is the most common strategy for obtaining Li isotopic ratios in natural samples, with a long-term internal precision better than 0.3‰ in most laboratories. However, there is a large discrepancy in the Li isotopic compositions of the same reference materials determined by MC-ICP-MS among international laboratories (e.g. ca 3.5‰ difference for measurements of homogeneous seawater), which has been attributed to insufficient recovery of Li during chromatographic purification. Despite this recognition, the exact impact of Li recovery during purification on Li isotopic determinations by MC-ICP-MS has never been quantified. We employed a normal distribution function to model Li elution curves and quantified the Li isotopic fractionation resulting from Li recovery during chromatographic purification. Furthermore, we compared the calculated and measured relative recovery (R) with the Li isotopic ratios determined by ICP-MS to validate our theoretical calculation. The theoretical calculations showed that R should be higher than 99.8% in order to avoid observable Li isotopic fractionation during chromatographic purification at IEECAS. This idea is further supported by the better long-term external precisions for data with R≥ 99.8% compared with previous values of 99.5% ≤R< 99.8%. Our results indicated that the large differences in the reported Li isotopic ratios for homogeneous seawater among international laboratories are probably attributable to Li isotopic fractionation occurring during ion exchange chromatography. Our theoretical calculation via R is the first quantitative and convenient approach for monitoring Li isotopic fractionation during sample purification, ensuring that R≥ 99.8% can avoid observable Li isotopic fractionation during purification, which will improve the accuracy of Li isotopic measurements by MC-ICP-MS and the comparability among laboratories.

Determination of isotope fractionation of Cr(iii) during oxidation by LC/low-resolution MC-ICPMS

A robust and sensitive method for the precise isotopic analysis of 53Cr/50Cr during Cr(iii) oxidation by coupling ion pair reversed-phase chromatography and Multicollector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) is presented.

Calibration of boron isotope ratio measurements by MC-ICP-MS using normalisation to admixed internal standards

Online normalisation to isotope ratios of admixed internal standards is an important technique in isotopic analysis by multicollector ICP-MS (MC-ICP-MS).

Decoupling feldspar dissolution and precipitation rates at near-equilibrium with Si isotope tracers: Implications for modeling silicate weathering

Here we show that the isotope tracer experimental method for kinetic studies, aided by the recent advance and accessibility of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) analysis for non-traditional stable isotopes, can provide unidirectional dissolution rates at near-equilibrium conditions. For a long time, the only rates available at near-equilibrium were net reaction rates—dissolution rates minus precipitation rates. This is because the conventional experimental method of kinetic studies is based on element concentrations and can only provide net rates. The availability of unidirectional rates allows us to re-examine some fundamental concepts and practices of modeling weathering in geochemistry.In this study, we used the 29Si isotope tracer to conduct albite and K-feldspar dissolution experiments at near-equilibrium conditions in near-neutral pH solutions at 50 °C. Results show that the saturation indices (SI) of solutions approached zero with respect to albite and K-feldspar after ∼240–360 h (h), but 29Si/28Si ratios of the experimental solutions indicated continual dissolution for another 720–1440 h. The rates of total Si precipitation were much smaller than the rates of Si dissolution. The experimental solutions were supersaturated with respect to amorphous Al(OH)3, gibbsite, quartz, allophane, imogolite, and kaolinite. The SI of the solutions remained constant with respect to these phases while Al concentrations slightly decreased and Si concentrations slightly increased, indicating the coupled feldspar dissolution and precipitation of secondary phases, such as albite → amorphous Al(OH)3 + quartz or albite → solution + Al-Si phase(s), instead of significant albite and K-feldspar precipitation (the reverse reaction) at 50 °C. Reaction path modeling of the temporal evolution of Si, Al, Na, and pH revealed that albite dissolution (without significant backward reaction) coupled with the precipitation of a secondary phase with a Si:Al ratio of ∼2:1 can successfully match the experimental data.Given the negligible feldspar precipitation reactions in low-temperature systems (e.g., T < 100 °C), we recommend modeling feldspar weathering using unidirectional forward rates together with secondary phase precipitation rates in near-equilibrium, feldspar-undersaturated systems. This can be accomplished with minor modifications in geochemical modeling software or input files. The coupled feldspar dissolution with secondary phase precipitation arrests the system in a near-equilibrium steady state. Using affinity-based rate equations such the classical linear Transition State Theory rate law or the Burch empirical relation together with far-from-equilibrium rate data will predict significant feldspar precipitation in solutions undersaturated but close to equilibrium with respect to feldspars, which is unlikely at near ambient temperatures.

Determination of the isotopic composition of lutetium using MC-ICPMS.

In this study, we report the first independent measurements of lutetium isotopic composition using multi-collector ICPMS in four commercial lutetium materials, including a new NRC candidate lutetium isotopic reference material, LUIS-1. The regression model was used to correct for the instrumental isotopic fractionation (mass bias) using NIST SRM 989 isotopic rhenium as the primary calibrator. The regression model is based on a short 15-min measurement sessions at varying ICP plasma power. Isotope ratio of R176/175 = 0.026553(11)k = 1, corresponding isotopic abundances of x176 = 0.025866(11)k = 1, x175 = 0.974134(11)k = 1, and an atomic weight of Ar(Lu) = 174.966693(13)k = 1 were obtained for lutetium in LUIS-1. Uncertainty estimation was performed using a mixture of modeling approaches in accordance with the JCGM 100:2008 "Guide to the Expression of Uncertainty in Measurement" and its Supplement 1. The relative contribution of the rhenium primary standard to the combined uncertainty of 176Lu/175Lu isotope ratio was 15%.

High-precision isotopic analysis of serum and whole blood Cu, Fe and Zn to assess possible homeostasis alterations due to bariatric surgery.

Bariatric surgery is an effective procedure to achieve weight loss in obese patients. However, homeostasis of essential metals may be disrupted as the main absorption site is bypassed. In this study, we determined Cu, Fe and Zn isotopic compositions in paired serum and whole blood samples of patients who underwent Roux-en-Y gastric bypass (RYGB) surgery for evaluation of longitudinal changes and their potential relation to mineral element concentrations and relevant clinical parameters used for monitoring the patient's condition. Samples from eight patients were collected pre-surgery and at 3, 6 and 12 months post-surgery. Multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) was used for high-precision isotope ratio measurements. Alterations in metal homeostasis related to bariatric surgery were reflected in the serum and whole blood Cu, Fe and Zn isotopic compositions. The serum and whole blood Cu became isotopically lighter (lower δ65Cu values) after bariatric surgery, reaching statistical significance at 6 months post-surgery (p < 0.05). The difference between the serum and the whole blood Zn isotopic composition increased after surgery, reaching significance from 6 months post-surgery onwards (p < 0.05). Those changes in Cu, Fe and Zn isotopic compositions were not accompanied by similar changes in their respective concentrations, making isotopic analysis more sensitive to physiological changes than elemental content. Furthermore, the Zn isotopic composition correlates with blood glycaemic and lipid parameters, while the Fe isotopic composition correlates with glycaemic parameters. Graphical Abstract.

Laser ablation in situ U-Pb dating and its application to diagenesis-porosity evolution of carbonate reservoirs

Abstract Through the development and calibration of a reference material which is 209.8 Ma old using a newly-developed Laser Ablation (LA) Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) technique, we successfully overcome the difficulty in sampling and dating ultra-low U-Pb ancient marine carbonates, which was previously untenable by isotope dilution (ID) methods. We developed the LA-MC-ICP-MS in situ U-Pb dating technique for ancient marine carbonates for the study of diagenesis-porosity evolution history in Sinian Dengying Formation, Sichuan Basin. By systematically dating of dolomitic cements from vugs, matrix pores and fractures, we found that the burial and diagenetic process of dolomite reservoirs in Sinian Dengying Formation was characterized by progressive filling-up of primary pores and epigenic dissolution vugs. The filling of vugs happened in three stages, early Caledonian, late Hercynian-Indosinian and Yanshanian-Himalayan, while the filling of matrix pores mainly took place in early Caledonian. The unfilled residual vugs, pores and fractures constitute the main reservoir sapce. Based on the above knowledge, we established the diagenesis-porosity evolution history of the dolomite reservoir in Sinian Dengying Formation, Sichuan Basin. These findings are highly consistent with the tectonic-burial and basin thermal histories of the study area. Our study confirmed the reliability of this in situ U-Pb dating technique, which provides an effective way for the investigation of diagenesis-porosity evolution history and evaluation of porosity in ancient marine carbonate reservoirs before hydrocarbon migration.

Multi-element isotope study of natrocarbonatites (1993 lava flows) from Oldoinyo Lengai volcano, Tanzania: Implications for core-mantle interactions

We contribute to the debate of core-mantle interactions by providing tungsten (W) isotope (ε182W) data from natrocarbonatite samples collected from the 1993 lava flows at Oldoinyo Lengai (OL) volcano, Tanzania. The 182Hf-182W system (half-life = 8.9 Ma) is well suited to constraining the timing of core formation and monitoring incursions of the core into the mantle. Most mantle-derived rocks are difficult to analyze because of their ultra-low W contents. We consider natrocarbonatites as suitable proxy for quantifying the core-mantle interactions given their unusually high W concentrations (published values range from ~40 to 115 ppm). The ε182W values of 7 natrocarbonatites measured by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) represent the first dataset obtained for the OL lavas (OLLs). The ε182W values calculated from W isotope ratios range from −0.01 to +0.16 and are indistinguishable within limits of uncertainty, from those of the ambient modern mantle sources (i.e., EM1, HIMU, and HIMU-like). The relatively uniform W isotope compositions associated with the lack of any ε182W anomaly from the natrocarbonatites imply that there may have been no contribution from the core (i.e., ε182W < 0) into the magma source of the OLLs. Alternately, the core signature could have been obscured either by the addition of mantle components that survived since the Earth's early accretion, or by an unknown region formed by an inefficient percolation of metal during core-mantle segregation that left parts of the mantle enriched in W. New C–O isotope data from the same samples (δ13C = −5.9 to −6.9‰; δ18O = 9.5–10.9‰) rule out any involvement of the continental crust into the OL magma and are consistent with other studies that show a mantle signature for most natrocarbonatites. The heavier δ18O values can be attributed to sub-solidus processes. The potential oxygen isotopic shift would not, however, affect the W isotopic compositions of the OLLs.

Experimental observations of marine iodide oxidation using a novel sparge-interface MC-ICP-MS technique

Iodine speciation is used as a redox indicator and water mass tracer. However, the kinetics and mechanisms of oxidation of the reduced iodine species, iodide (I−), in seawater remain largely unresolved. Here, we provide constraints on I− oxidation rates from seawater incubation experiments using a sensitive radiotracer–incubation approach with high-precision mass spectrometry. Samples are inoculated with iodine-129 as I−, incubated under controlled conditions, then oxidized and reduced iodine species are isolated using liquid chromatographic separation. Samples are subsequently aspirated using a novel sparging interface, and the 129I/127I ratio of both iodine species is determined via multi-collector inductively-coupled plasma mass spectrometry. This highly-sensitive analytical approach is optimized for estimating the predicted slow rates of iodine redox transformations in small volume seawater samples at marine-relevant total iodine concentrations. Application of this method to coastal samples from Martha's Vineyard Sound, USA, evidences the heretofore-elusive oxidation of I− under marine-analogue conditions. Our results demonstrate an increase in the 129I/127I of aqueous oxidized iodine species in unamended seawater, most likely due to formation of hypoiodous acid and then dissolved organic iodine, at rates of 118–189 nM/ yr. These rates are comparable to those inferred from previous open ocean mass balance approaches but have the benefit of directly linking iodine speciation changes to in situ I− oxidation. Though our treatment experiments do not definitively isolate the oxidation mechanism, they provide preliminary insight into important roles for both abiotic and biotic factors as well as iodine transformations between dissolved, particulate, and gaseous forms. Ultimately, the derived rates and novel experimental technique have potential to improve marine iodine cycling models and provide constraints on the importance of in situ processes in setting patterns of iodine speciation.

Nickel isotope fractionation during metal-silicate differentiation of planetesimals: Experimental petrology and ab initio calculations

Metal-silicate fractionation of nickel isotopes has been experimentally quantified at 1623 K, with oxygen fugacities varying from 10−8.2 to 10−9.9 atm and for run durations from 0.5 to 1 h. Both kinetic and equilibrium fractionations have been studied. A wire loop set-up was used in which the metal reservoir is a pure nickel wire holding a silicate melt droplet of anorthite-diopside eutectic composition. During the course of the experiment, diffusion of nickel from the wire to the silicate occurred. The timescale to reach chemical equilibrium was fO2 dependent and decreased from 17 to 1 hour, as conditions became more reducing.The isotopic composition of each reservoir was determined by Multicollector-Inductively Coupled Plasma-Mass Spectrometry (MC-ICPMS) after Ni purification. The isotopic composition was found to be constant in the metallic wire, which therefore behaved as an infinite reservoir. On the contrary, strong kinetic fractionation was observed in the silicate melt (δNi down to −0.98‰.amu−1 relative to the standard). Isotopic equilibrium was typically reached after 24 hours. For equilibrated samples at 1623 K, no metal-silicate fractionation was observed within uncertainty (2SD), with ΔNiMetal-Silicate = 0.02 ± 0.04‰.amu−1.Theoretical calculations of metal-silicate isotope fractionation at equilibrium were also performed on different metal-silicate systems. These calculations confirm (1) the absence of fractionation at high temperature and (2) a weak temperature dependence for Ni isotopic fractionation for the metal-olivine and metal-pyroxene pairs with the metal being slightly lighter isotopically.Our experimental data were finally compared with natural samples. Some mesosiderites (stony-iron meteorites) show a ΔNiMetal-Silicate close to experimental values at equilibrium, whereas others exhibit positive metal-silicate fractionation that could reflect kinetic processes. Conversely, pallasites display a strong negative metal-silicate fractionation. This most likely results from kinetic processes with Ni diffusion from the silicate to the metal phase due to a change of Ni partition coefficient during cooling. In this respect we note that in these pallasites, iron isotopes show metal-silicate fractionation that is opposite direction to Ni, supporting the idea of kinetic isotope fractionation, associated with Fe-Ni interdiffusion.

A method for Si isotope tracer kinetics experiments: Using Q-ICP-MS to obtain 29Si/28Si ratios in aqueous solutions

Here we show that silicon isotope tracer kinetics experiments can be economically conducted with measurements of 29Si/28Si ratios in the experimental aqueous solutions using Quadrupole inductively coupled plasma mass spectrometry (Q-ICP-MS) with sufficient precision. Although the isotope tracer method promises orders of magnitude improved detection sensitivity of reactions, its application has been hampered by the limited availability of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for Si isotope analysis. However, in Si isotope tracer kinetics experiments, dissolution rates are often derived from isotope fractional abundance increments by as much as 0.5 unit (equivalent to 50%) and the method essentially relies on the difference in isotope ratios over a period of time, which render uncertainties in < ±0.01 fraction (or ±1%) in Si isotope abundances tolerable. Laboratory-measured silicate dissolution rates typically carry uncertainties of more than ±25% and with even poorer intra- and inter-laboratory replications. These special situations of kinetics experiments circumvent the conventional concerns of using Q-ICP-MS to measure isotope ratios.To test this hypothesis, we conducted two experiments. The first used Q-ICP-MS to measure a set of mixing solutions or reference solutions with known isotope compositions. The results show that the difference is an average of ±0.009 in f29. A second test used both MC-ICP-MS and Q-ICP-MS to measure the 29Si/28Si ratios in experimental solutions of albite dissolution. Ten albite dissolution experiments were conducted at 50°C and pH 8.8. Student t-test showed that the rates calculated from the two analytical data sets have no statistically significant differences at the 95% confidence level. These results demonstrate that the Q-ICP-MS measurements of 29Si/28Si ratios produced tolerable uncertainties for determining silicate dissolution rates. However, the cost is significantly reduced and more significantly, the reduced wait time makes isotope tracer experiments more feasible. Silicate minerals make up 90% of the Earth’s crust. The kinetics of silicate-water reactions is of paramount significance to many society’s mega-environmental enterprises. This method to conduct Si isotope tracer experiments, as well as the possibility to use this approach for other non-traditional stable isotopes, will have significant impact on geochemical kinetics studies.

Tin isotopes indicative of liquid–vapour equilibration and separation in the Moon-forming disk

The depletion in moderately volatile elements in the Moon relative to Earth and comparison of the isotope compositions of the Moon and Earth have placed important constraints on models of lunar formation. A liquid–vapour protolunar disk from a high-energy giant impact has been proposed to explain some of these constraints. Here we present high-precision tin isotope data for lunar rocks, measured by double-spike MC-ICP-MS (multicollector inductively coupled plasma mass spectrometry). The lunar rocks are enriched in light tin isotopes compared to the Earth (Δ124/116SnMoon–Earth = −0.48 ± 0.15‰). On the basis of our data and constraints on tin speciation, we show that this tin isotope fractionation is inconsistent with volatile loss from a lunar magma ocean. Instead, we propose a scenario with vigorous mixing between the protolunar disk and the Earth in high-energy conditions during the impact, followed by liquid–vapour equilibration and phase separation at around 2,500 K while the disk was cooling. This scenario is consistent with the depletion in moderately volatile elements and isotope composition of the Moon. Vigorous mixing between the protolunar disk and Earth followed by processes in the cooling disk may explain the enrichment in light isotopes of tin on the Moon relative to Earth, as found by analysis of lunar rocks and geochemical calculations.

Physiological requirements for iron in women of reproductive age assessed by the stable isotope tracer technique

BackgroundIron nutrition is important for the health of women of reproductive age, and defining the physiologic requirement for iron can help them accurately plan the iron intake. However, research on the physiologic requirement for iron in women is insufficient worldwide. This study aimed to further improve the methodology and get more precise data for the physiological requirements for iron in women of reproductive age on the basis of our previous study.MethodSixty-one women of reproductive age who had not been pregnant before and during the whole study were included from Hebei province, China in 2015. Each subject participated in a 2-week metabolic trial with consuming 50 mg of the stable isotope 58Fe, and were then followed for ~ 800 days. The abundance of 58Fe and the total iron concentration in the circulation were measured using multi-collector inductively-coupled plasma mass spectrometry and atomic absorption spectroscopy. The physiologic requirement for iron in women of reproductive age was then calculated.ResultsThe average iron circulation rate was 80.4%, and the steady period started from about 1 year. The average physiological requirement for iron of 21 subjects obtained by formula calculation was 1.55 mg/d and 23.63 μg.kg− 1.d− 1 after adjustment for body mass, and that of 33 subjects obtained by linear regression was 1.29 mg/d, 20.98 μg.kg− 1.d− 1 after adjustment for body mass. The results by two methods showed no significant difference. The EAR and RNI calculated from this results was 11–13 mg/d and 15–18 mg/d, respectively, both of which were slightly lower than the recommended value in Chinese Dietary Reference Intake (2013).ConclusionThe physiological requirements for iron in women of reproductive age were in accordance with other studies, while the EAR and RNI calculated from which were slightly lower than Chinese present recommended value.Trial registrationChiCTR, ChiCTR-OCH-14004302. Registered 14 February 2014, http://www.chictr.org.cn/enindex.aspx