High-precision determination of lithium and magnesium isotopes utilising single column separation and multi-collector inductively coupled plasma mass spectrometry

Experimental Investigation of Oxide Leaching Methods for Li Isotopes

To examine the applicability of different leaching methods used to extract secondary oxides from silicate solids for lithium isotope (δ⁷Li) measurement, this study has conducted leaching experiments on five different types of silicate solids, including a fresh basalt, two weathered basalts, a Yellow River sediment (loess-dominated) and a shale. Four factors were assessed in the experiments: the concentration of the leaching reagent hydroxylamine hydrochloride (HH), the leaching temperature (20 °C vs 95 °C), the leaching time and the reagent/solid ratio. Based on elemental concentrations and Li isotopes, 0.04 mol l^-1 hydroxylamine hydrochloride (HH) in 25% v/v acetic acid at room temperature for 1 h with 40 ml g^-1 reagent/solid ratio is recommended. At high temperatures, low δ⁷Li and high magnesium/iron ratios indicate that minerals other than secondary oxides are dissolved. With increased leaching time, there is no evidence for Li isotopic fractionation at room temperature. However, longer leaching time or increased reagent/solid ratios may increase the risk of leaching from non-oxide phases. Meanwhile, results suggest that low concentrations of HH are not sufficient to target the secondary oxides evenly, while high concentrations of HH can leach out more non-oxides. We also examined the optimal oxide leaching method within a full sequential leaching procedure (i.e., exchangeable, carbonate, oxide, clay and residual phases). Elemental concentrations show that no elements exist exclusively in oxides, so it is essential to analyse multi-elemental concentrations to verify that the leaching has accessed this phase in a given sample. Comparing secondary oxides with their corresponding solutions, we estimate the isotopic fractionation (Δ⁷Li_{oxide-solution}) is -16.8‰ to -27.7‰.

Determination of magnesium isotopic ratios of biological reference materials via multi-collector inductively coupled plasma mass spectrometry

RATIONALE

Despite a wide range of potential applications, magnesium (Mg) isotope composition has been so far sparsely measured in reference materials with a biological matrix, which is important for the quality control of the results. We describe a method enabling the chemical separation of Mg in geological and biological materials and the determination of its stable isotope composition.

METHODS

Different geological (BHVO-1, BHVO-2, BCR-1, and IAPSO) and biological (SRM-1577c, BCR-383, BCR380R, ERM-CE464, DORM-2, DORM-4, TORT-3, and FBS) reference materials were used to test the performance of a new sample preparation procedure for Mg isotopic analysis. The procedure consisted of a simple three-stage elution method to separate Mg from the matrix. Mg isotopic analyses were performed in two different laboratories and with three different multi-collector inductively coupled plasma mass spectrometry instruments.

RESULTS

The biological reference materials show a wide range of δ²⁶ Mg values (relative to DSM3 standard), spanning over 2‰, from 0.52 ± 0.29‰ (2SD, n = 7) in bovine liver (SRM-1577c) to -1.45 ± 0.20‰ (2SD, n = 5) in tuna fish (ERM-CE464), with an external precision of 0.03‰ (2SD, n = 85).

CONCLUSIONS

This study indicates that isotopic measurements of Mg in biological reference materials show good performance, with the results being within the accepted range. We confirmed that δ²⁶ Mg values in liver are the most positive of all biological materials reported so far.

Strategies to overcome interferences in elemental and isotopic geochemical analysis by quadrupole inductively coupled plasma mass spectrometry: A critical evaluation of the recent developments

Quadrupole Inductively Coupled Plasma Mass Spectrometry (ICP-MS) instruments were introduced into geochemical and mineral exploration laboratories nearly four decades ago, providing a technique that could meet their longstanding requirement for the precise and accurate determination of several groups of trace elements and isotopes in geological materials such as rocks, minerals, ores, soils, sediments, and natural water samples. Despite its popularity among geochemists, the technique suffered from spectral and non-spectral interferences some of which seriously affected the quality of the data generated. These interferences have also had a significant impact on the ability of ICP-MS systems to achieve low detection limits. Over the last three decades, technical advances such as the development of high-resolution (HR)-ICP-MS, cool plasma, collision/reaction cell technology (CCT), dynamic reaction cell (DRC) technology, collision reaction interface (CRI), kinetic energy discrimination (KED), tandem mass spectrometry (ICP-MS/MS)/triple quadrupole ICP-MS, and multi-quadrupole ICP-MS have been introduced to eliminate/minimize many of these interferences, with each technique having its strengths and limitations. These technologies have extended the range of elements that can be measured accurately not only in geological materials, but also in several other matrices, with lower detection limits than before. In addition, other methods such as internal standardization, isotope-dilution, standard addition and matrix-matching calibrations have contributed to improving the quality of the data. This paper provides a review of these new developments from the geochemical analysis point of view.

A Rapid and Simple Method for Lithium Purification and Isotopic Analysis of Geological Reference Materials by MC-ICP-MS

A simple method has been developed to purify lithium (Li) from matrix elements in geological reference materials, using a single-column packed with AGMP-50 cation exchange resin, followed by high-precision Li isotope measurements by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). A series of tests, such as different types of resin, loading amount of Li, loading volumes, and various eluents, were conducted to ascertain the optimal conditions for Li purification and the effects of intensity, acidity, and presence of potential matrix elements on Li isotope measurements were also evaluated. In our experiment, Al and high-field-strength elements (HFSEs), such as Ti, Zr, and Hf, were eluted by 0.2 M HCl + 0.3 M HF, and 0.73 M HCl was used to separate Li from other matrix elements, such as Na. This method is suitable for processing large amount of Li (60–270 ng) and enabling a Li recovery of close to 100%, with effective removal of matrix elements such as Na and Ca. Besides, our method achieves low matrix interferences (e.g., Na/Li << 1 and Ca/Li << 1 for rock and seawater via a single-column procedure; Ca/Li < 2 for carbonate via a two-column procedure) and also uses small volume of eluents and is rapid (~5 h), enabling a total separation to be completed in ~0.5 d. Using this method, we report Li isotopic compositions of various geological reference materials, including igneous rocks, seawater, and carbonate. The Li isotopic compositions are consistent with the data published previously for the analyzed reference materials. As such, the reported method is ideally suited for Li separation from multiple types of geological samples prior to isotopic analysis.

In situ Mg isotope measurements of biogenic carbonates using laser ablation multi-collector inductively coupled plasma mass spectrometry: A new tool to understand biomineralisation

RATIONALE

Magnesium is one of the most abundant elements in the earth's crust and in seawater. Fractionation of its stable isotopes has been shown to be a useful indicator of many geological, chemical, and biological processes. For example, biogenic carbonates display an ~5‰ range of δ²⁶ Mg values, which is attributed to variable degrees of biological control on Mg ions during biomineralisation. Understanding this biological control is essential for developing proxies based on biogenic carbonates.

METHODS

In this work, we present a new approach of measuring Mg isotopes in biogenic carbonates using Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS).

RESULTS

Our results show that this microanalytical approach provides relatively fast, high spatial resolution (<0.2 μm) measurements with high precision and accuracy down to 0.2‰ (2SE). To achieve high levels of precision and accuracy, baseline interferences need to be monitored and a carbonate standard with a relatively low trace metal composition similar to biogenic carbonates should be used. We also demonstrate that the matrix effect on Mg isotopes in carbonates with low Fe and Mn is limited to less than 0.2‰ fractionation under different laser parameters and low oxide condition (<0.3% ThO/Th).

CONCLUSIONS

Our newly developed LA-MC-ICPMS method and its applications to biogenic carbonates show significant advantages provided by the microanalytical approach in understanding complex processes of biomineralisation in marine calcifiers.

A coherent method for combined stable magnesium and radiogenic strontium isotope analyses in carbonates (with application to geological reference materials SARM 40, SARM 43, SRM 88A, SRM 1B)

We undertook ⁸⁷Sr/⁸⁶Sr analyses for a range of carbonate bearing geological reference materials, and combined these with δ²⁶Mg for a subset of samples. Following chemical purification in a series of chromatographic extractions, isotope ratios were measured by Multi-Collector-ICP-MS using a Plasma II (Nu instruments, Wrexham, UK). To validate efficient sample digestion procedures of carbonate fractions, total samples were treated with either 3 mol l^-1 HNO₃ and 0.5 mol l^-1 HCl, respectively. Results of both leaching procedures are identical within reproducibility. Reference values for SRM 88A (formerly NBS 88A), SRM 1B (formerly NBS 1B), SARM 40, SARM 43, JDo-1, JLs-1, and San Carlos olivine range from 0.70292 to 0.73724 in ⁸⁷Sr/⁸⁶Sr and from -2.80 to -0.41 ‰ for δ²⁶Mg, respectively. This set of geological reference materials can be used for sedimentary rock material with different carbonate mineral and matrix composition as quality control measurements of combined stable Mg and radiogenic Sr isotope analyses.•We present a protocol that facilitates the chemical separation of Mg and Sr in carbonate bearing geological reference materials including ⁸⁷Sr/⁸⁶Sr and δ²⁶Mg of certified reference materials.