Inverse Aqua Regia Research Articles

Optimization of a pre-concentration method for the analysis of mercury isotopes in low-concentration foliar samples.

Hg isotope analysis in samples from background regions is constrained by the presence of low Hg concentration and therefore requires a pre-concentration method. Existing Hg pre-concentration methods are constrained by long sample processing time and limited sample loading capacity. Using foliar samples as a test case, an optimized Hg pre-concentration method is presented that involves the microwave-assisted digestion of samples for Hg isotope analysis with the addition of a pre-digestion step. Microwave-digested foliar samples and CRMs were transferred to an impinger, reduced with SnCl2, and collected in a 2.25 mL concentrated inverse aqua regia (3:1 HNO3:HCl, v/v). This resulted in an optimal acid concentration in the solution ideal for analysis on MC-ICP-MS. The time for purging with Hg-free N2 was optimized to 30 min and the efficiency of the pre-concentration method was tested using a combination of approaches. Tests performed on pure reagents and matrix of foliar samples spiked with 197Hg radiotracer showed recoveries averaging 99 ± 1.7% and 100 ± 3.0%, respectively. Mercury at concentrations as low as 1.83 ng g-1 was pre-concentrated by digesting aliquots of foliage samples in individual digestion vessels. Recoveries following their pre-concentration averaged 99 ± 6.0%, whereas recoveries of 95 ± 4.7% and 95 ± 2.5% were achieved for NIST SRM 1575a (pine needle) and reagents spiked with NIST SRM 3133, respectively. Analysis using multicollector-ICP-MS showed low fractionation of δ202Hg during sample pre-concentration with no significant mass-independent fractionation. The proposed method is a relatively simple and robust way to prepare Hg samples for Hg isotopic analysis and is suitable even for complex biological matrices.

Novel Determination of Trace Metals in Geological Materials Employed in Food Products by Microwave Decomposition and Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES)

A new methodology for the determination of trace metals in geological materials employed in the food industry by single-reaction chamber (SRC) microwave digestion followed by inductively coupled plasma – optical emission spectrometry (ICP-OES) analysis was developed. The presence of toxic elements for human beings, such as As, Cd, Cr, Hg, Ni, and Pb, must be controlled in those materials that are in contact with food. Most regulations require the presence of these elements in these materials in low concentrations, mostly less than 1 mg kg−1, which requires analytical methodologies that can reach these quantification limits. In the optimization of the sample preparation by microwave digestion, different mixtures of acids were used, so that As, Cd, and Hg were decomposed in an inverse aqua regia solution, while extraction of solid sample with aqua regia and HF was needed to determine Cr, Ni, and Pb. The determination of the analytes present in these solutions was carried out by ICP-OES, optimizing the conditions to obtain the best signal-to-noise ratio for each analyte. The study was undertaken using certified reference materials, two samples of clay, and a sample of sepiolite. Quantification limits of 1 mg kg−1 were achieved, which meet the legal requirements. The analyzed samples of clays and sepiolite contained values of Cd and Hg that met the requirements although the results for As, Cr, Ni, and Pb were above the limits required for the food industry.

Determination Of Silver In Geological Samples Using Aerosol Dilution ICP-MS After Water-Bath Extraction With Inverse Aqua Regia

A valid method for trace silver (Ag) detection in geological samples was developed in this study using aerosol dilution inductively coupled plasma-mass spectrometry after extraction with inverse aqua regia. This was proposed primarily to reduce the interference from Nb and Zr during mass spectrometric measurements. Almost 93% of Nb and Zr was removed after the extraction. By mixing an appropriate amount of Ar with the sample aerosol using an aerosol dilution system prior to plasma, the residual Nb oxides and Zr oxides or hydroxides could be successfully removed. The relative yields of the interfering oxides and hydroxides were as low as 0.087% (NbO/Nb) and 0.013% (ZrOH/Zr), which were 3–5 times lower than those in the traditional mode without the addition of Ar. Moreover, the signal-to-noise ratio of Ag was five times higher than that in the traditional mode. The proposed method was applied to the determination of Ag in 68 standard reference materials (SRMs) of soil, sediment, and rock. The results for 47 of these geological SRMs were in good agreement with the reference values. The Ag levels in three SRMs (GSP-2 Granodiorite, STM-2, and SGR-1b) are being reported for the first time herein. For these SRMs, 10 separate aliquots of the sample were digested and analyzed over a period of three months, and analysis revealed that the determined values were reasonable. Thus, the proposed method shows significant potential for the accurate determination of trace Ag in various geological samples.

Purified Na2CrO4–H2SO4: A New Low‐Blank Medium for Re‐Os Dating of Black Shale

The H2SO4–CrO3 method has been widely used in Re‐Os isotope dating of organic‐rich sedimentary rocks. However, the high Re blank of this method inhibits the collection of high‐precision Re‐Os data, especially for organic‐rich sedimentary rocks with low Re and Os mass fractions. Here, we report a new H2SO4–Na2CrO4 method in which the Re blank of the Na2CrO4 reagent can be greatly reduced after efficient purification by acetone. A proportion of 0.6 g Na2CrO4/8 ml H2SO4 was selected as the best digestion combination based on tests of black shales with different organic carbon contents in three digestion media (inverse aqua regia, H2SO4–CrO3, and H2SO4–Na2CrO4). The H2SO4–Na2CrO4 method has an ultra‐low total procedural blank of 1–2 pg for Re and 0.6–1 pg for Os, respectively, which is one order of magnitude lower than the H2SO4–CrO3 method and is comparable with to inverse aqua regia method. The new method was applied to reference material SBC‐1 and two series of black shale samples, and the results were consistent with those obtained via the inverse aqua regia method. These findings confirm the accuracy and feasibility of the H2SO4–Na2CrO4 method, and demonstrate its ability to accurately determine the depositional age of organic‐rich sedimentary rocks with low Re and Os mass fractions.

The Implications of HClO4 for Dissolving Large Masses of Low Level Os in Metal Sulfides and Factors that Influence Re-Os Dating

In general, no more than 1 g of metal sulfide can be completely digested in 20 mL of inverse aqua regia using the Carius tube technique. In this study, the sample weight increased after adding HClO4 to inverse aqua regia while the volume of acid concurrently decreased significantly. Three grams of metal sulfide were digested in 12 mL of acid (3 mL of HClO4 and 9 mL of inverse aqua regia) via the HClO4-inverse aqua regia method. The results using molybdenite reference materials JDC and HLP mixed with 3 g of pyrite were consistent with certified values. Compared to the traditional method, the HClO4-inverse aqua regia method could dissolve a larger sample mass (3 g) with a smaller volume of acids (12 mL). We simultaneously found that the oxidation of digestion acids greatly affected the Os signal but had no influence on the equilibrium of isotope exchange between 185Re and 190Os spikes in Re and Os samples. Remarkably, the heating temperature was the most significant factor influencing the equilibrium of isotope exchange, and the Os in a sample was not equilibrated with the spike until the heating temperature reached 190 °C.

Application of addition HClO4 to improve the dissolution process and sensitivity of Os for low-concentration of Os in pyrite

The rhenium (Re) and osmium (Os) isotope systems have long been recognized as effective tools to directly date mineralization and trace the source of metals. In general, 0.4 g of pyrite was dissolved by 11 mL of inverse aqua regia in a 30-mL Carius tube. Larger volumes of acids and a larger Carius tube were required if the sample weight was over 0.4 g. Herein we present an improved dissolution process of the Carius tube digestion method by adding HClO4 to dissolve larger sample weights in smaller volumes of acid. This approach is more suitable for determining low concentrations of Os in metal sulfides. In detail, 0.4 g of pyrite was completely dissolved in a mixture of 1 mL of HClO4 and 3 mL of inverse aqua regia. Notably, addition of 1 mL of HClO4 to 8 mL of inverse aqua regia significantly increases the amount of sample from 0.4 to 0.8 g. Based on our data, while the sample weight was no >0.8 g, addition of 1 mL of HClO4 to inverse aqua regia can not only improve the dissolving capacity significantly, but also avoid Carius tube explosion. Furthermore, the dissolving capacity of HClO4-inverse aqua regia method is much higher than that of H2O2-inverse aqua regia method, which is a typical digestion method for Re-Os dating. This method was certified by reference materials (HLP and JDC). In brief, this new method allows dissolution of larger amounts of sample while consuming less acid. Therefore, this method has more potential for determining low concentrations of Re and Os in metal sulfides.

A Method to Suppress Isobaric and Polyatomic Interferences for Measurements of Highly Siderophile Elements in Desilicified Geological Samples

Sample decomposition using inverse aqua regia at elevated temperatures and pressures (e.g., Carius tube or high‐pressure asher) is the most common method used to extract highly siderophile elements (HSEs: Ru, Rh, Pd, Re, Os, Ir, Pt and Au) from geological samples. Recently, it has been recognised that additional HF desilicification is necessary to better recover HSEs, potentially contained within silicate or oxide minerals in mafic samples, which cannot be dissolved solely by inverse aqua regia. However, the abundance of interfering elements tends to increase in the eluent when conventional ion‐exchange purification procedures are applied to desilicified samples. In this study, we developed an improved purification method to determine HSEs in desilicified samples. This method enables the reduction of the ratios of isobaric and polyatomic interferences, relative to the measured intensities of HSE isotope masses, to less than a few hundred parts per million. Furthermore, the total procedural blanks are either comparable to or lower than conventional methods. Thus, this method allows accurate and precise HSE measurements in mafic and ultramafic geological samples, without the need for interference corrections. Moreover, the problem of increased interfering elements, such as Zr for Pd and Cr for Ru, is circumvented for the desilicified samples.

Rhenium–Osmium Isotope Measurements in Marine Shale Reference Material SBC‐1: Implications for Method Validation and Quality Control

In this study, the USGS black shale reference material SBC‐1 was investigated as a matrix‐matched reference material for both intra‐laboratory calibration and inter‐laboratory comparison of high‐precision Re‐Os dating for organic‐rich sedimentary rocks. This reference material was analysed for Re‐Os isotopic composition by three digestion protocols – inverse aqua regia, CrO3‐H2SO4 and H2O2‐HNO3. The results for SBC‐1 obtained by inverse aqua regia digestion yielded similar Re mass fractions but slightly (~ 5%) higher Os mass fractions and lower 187Os/188Os values than the CrO3‐H2SO4 and H2O2‐HNO3 digestions. The data set of inverse aqua regia digestion exhibited strong correlations in plots of 187Os/188Os vs. 1/192Os and 187Os/188Os vs. 187Re/188Os, which may signify the incorporation of detrital Re and Os into organic matter in the Re‐Os system. Similar correlations were also observed for the CrO3‐H2SO4 digestion data set, but not for that of H2O2‐HNO3. The data indicate that there is an amount of non‐hydrogenous Os in SBC‐1 and that CrO3‐H2SO4 and H2O2‐HNO3 digestions would minimise liberation of the non‐hydrogenous Os component. We propose that SBC‐1 may be a more suitable reference material to monitor the influence of detrital Re and Os on Re‐Os isochron age data, especially for samples with less organic matter and more siliceous detritus.

Microwave-assisted sample preparation of medicines for determination of elemental impurities in compliance with United States Pharmacopeia: How simple can it be?

This work proposed a procedure for microwave-assisted sample preparation of medicines using diluted nitric acid followed by determination of elemental impurities using inductively coupled plasma optical emission spectrometry (ICP OES) and inductively coupled plasma mass spectrometry (ICP-MS) according to the United States Pharmacopeia Chapters 232 and 233. Three solutions, i.e. inverse aqua regia, 7.0 and 2.0 mol L−1 HNO3, were evaluated for microwave-assisted digestion of nine drugs samples. The applicability of each digestion procedure was assessed by comparison of analyte concentrations determined using total (reference procedure) and partial digestions (proposed procedure) as well as by determining dissolved carbon content and evaluating matrix effects. There were none significant differences at a 95% confidence level among the concentrations determined applying reference and proposed procedures. Internal standardization (ICP OES) and aerosol dilution (ICP-MS) were applied for minimization and correction of matrix effects. Addition and recovery experiments were performed according to oral permissible daily exposures values specific for each element and each sample was spiked with element concentrations of 0.5J and 1.5J in order to check accuracies for 24 analytes. Recoveries ranged from 70 to 138% for ICP OES and from 72 to 128% for ICP-MS, for all elements but Os. All analytes were below the respective limits of quantification when applying all sample preparation procedures, except As, Ba, Co, Cu, Cr, Mo, Ni, Pb, Sb, Sn, Tl and V, however the determined concentrations for these elements were lower than the limits proposed by Chapter 232.

Determination of As, Cd, Hg and Pb in continuous use drugs and excipients by plasma-based techniques in compliance with the United States Pharmacopeia requirements

Some inorganic impurities are toxic to human health even when present at low concentrations and therefore must be carefully monitored in products as continuous use drugs. This work aimed the development of a simple microwave-assisted digestion procedure for different types of drugs and excipients and the analytical determination of elemental impurities according to the new regulations of the United States Pharmacopeia (USP) 232 and 233 using inductively coupled plasma optical emission spectrometry (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS). Eight drugs samples and two excipients of different brands were microwave-assisted digested with inverse aqua regia. Addition and recovery experiments were performed according to J values, once permissible daily exposure value is specific for each element and estimated according to the maximum daily dose of drug indicated by the label. Samples were spiked with values of 1.5J in order to check accuracies for As, Cd, Hg, and Pb. Recoveries obtained by ICP-OES ranged from 75 to 148% and for ICP-MS ranged from 74 to 120%. The limits of detection for ICP-OES ranged from 0.4 to 17mgkg−1 and for ICP-MS from 7.4 to 41.6μgkg−1. Both analytical methods were adequate in terms of accuracies and sensitivities. Considering the maximum daily dose, all drugs samples and excipients contained As, Cd, Hg and Pb below the maximum limits stipulated by USP since all of them presented contents below respective limits of detection.

Precise and accurate Re–Os isotope dating of organic-rich sedimentary rocks by thermal ionization mass spectrometry with an improved H2O2-HNO3 digestion procedure

This contribution presents a new method for Re–Os isotope dating organic-rich sedimentary (ORS) rocks by thermal ionization mass spectrometry using an H2O2–HNO3 solution as the digestion medium, rather than CrO3–H2SO4 or inverse aqua regia. The main underlying principle of this method is that H2O2–HNO3 digestion would preferentially liberate hydrogenous Re and Os, and minimize the dissolution of non-hydrogenous detrital Re and Os, thereby providing more accurate and precise ages. A series of tests were performed, and the experimental data demonstrate the fundamental controls on spike–sample equilibrium and that the amount of detrital Re and Os incorporated into the system are subjected to the volumetric ratio of H2O2 to HNO3. The optimum method is a H2O2:HNO3 ratio of 5 to complete spike–sample equilibration, and to minimize the amount of detrital Re and Os in the system. A comparison of our new method with inverse aqua regia and CrO3–H2SO4 showed that the three techniques yield indistinguishable Re–Os results, suggesting complete spike–sample equilibrium was achieved by all of the digestion techniques. Moreover, the data show that our new technique leaches out the least amount of detrital Re and Os isotopes relative to conventional methods Thus, we propose the H2O2–HNO3 method may increase the precision and accuracy of Re–Os depositional ages of organic-rich sedimentary systems.

Phosphorus in recycling fertilizers - analytical challenges

The importance of secondary raw materials for phosphorus (P) fertilizer production is expected to increase in the future due to resource depletion, supply risks, and heavy metal contamination of fossil phosphate resources. Municipal wastewater is a promising source for P recovery. In Germany for instance, it contains almost 50% of the total amount of P that is currently applied as mineral fertilizer. Several procedures have been developed to recover and re-use P resulting in a growing number of recycling fertilizers that are currently not regulated in terms of fertilizer efficiency. We tested various materials and matrices for their total P content, solubility of P in neutral ammonium citrate (Pnac) and water, and performed robustness tests to check if existing analytical methods are suitable for those new materials. Digestion with inverse aqua regia was best suited to determine the total P content. Pnac sample preparation and analyses were feasible for all matrices. However, we found significant time and temperature dependencies, especially for materials containing organic matter. Furthermore, several materials didn’t reach equilibrium during the extractions. Thus, strict compliance of the test conditions is strongly recommended to achieve comparable results.

Determination of cadmium in geological samples by aerosol dilution ICP-MS after inverse aqua regia extraction

A valid method is developed based on aerosol dilution ICP-MS after inverse aqua regia extraction for direct determination of trace levels of Cd in geological samples.

Study and determination of elemental impurities by ICP-MS in active pharmaceutical ingredients using single reaction chamber digestion in compliance with USP requirements

In this work a method for active pharmaceutical ingredients (APIs) digestion using the single reaction chamber (SRC—UltraWaveTM) system was proposed following the new recommendations of United States Pharmacopeia (USP). Levodope (LEVO), primaquine diphosphate (PRIM), propranolol hydrochloride (PROP) and sulfamethoxazole (SULF) were used to evaluate the digestion efficiency of the proposed method. A comparison of digestion efficiency was performed by measuring the carbon content and residual acidity in digests obtained using SRC and in digests obtained using conventional microwave-assisted digestion system (MultiwaveTM). Three digestion solutions (concentrated HNO3, aqua regia or inverse aqua regia) were evaluated for digestion of APIs. The determination of Cd, Ir, Mn, Mo, Ni, Os, Pb, Pd, Pt, Rh, Ru was performed using inductively coupled plasma mass spectrometry (ICP-MS) in standard mode. Dynamic reaction cell (DRC) mode was used for the determination of 51V, 52Cr, 53Cr, 63Cu and 65Cu in order to solve polyatomic ion interferences. Arsenic and Hg were determined using chemical vapor generation coupled to ICP-MS (FI-CVG-ICP-MS). Masses of 500mg of APIs were efficiently digested in a SRC-UltraWaveTM system using only HNO3 and allowing a carbon content lower than 250mgL−1 in final digests. Inverse aqua regia was suitable for digestion of sample masses up to 250mg allowing the determination of Ir, Pd, Pt, Rh and Ru. By using HNO3 or inverse aqua regia, suitable recoveries were obtained (between 91 and 109%) for all analytes (exception for Os). Limits of quantification were in agreement with USP requirements and they ranged from 0.001 to 0.015µgg−1 for all elemental impurities (exception for Os). The proposed method was suitable for elemental impurities determination in APIs and it can be used in routine analysis for quality control in pharmaceutical industries.

Cadmium-isotopic evidence for increasing primary productivity during the Late Permian anoxic event

Earth's most extreme extinction event near the end of the Late Permian decimated more than 90% of all extant marine species. Widespread and intensive oceanic anoxia almost certainly contributed to the catastrophe, though the driving mechanisms that sustained such conditions are still debated. Of particular interest is whether water column anoxia was a consequence of a ‘stagnant ocean’, or if it was controlled by increases in nutrient supply, primary productivity, and subsequent heterotrophic respiration. Testing these competing hypotheses requires deconvolving sedimentary/bottom water redox conditions from changes in surface water productivity in marine sediments. We address this issue by studying marine shales from East Greenland and the mid-Norwegian shelf and combining sedimentary redox proxies with cadmium-isotopic analyses. Sedimentary nitrogen-isotopic data, pyrite framboid analyses, and organic and inorganic shale geochemistry reveal sulfidic conditions with vigorous upwelling, and increasingly anoxic conditions with a strengthening upwelling in the Greenland and Norwegian sections, respectively. Detailed analysis of sedimentary metal budgets illustrates that Cd is primarily associated with organic carbon and records primary geochemical signatures, thus enabling reconstruction of surface water nutrient utilization. Cadmium-isotopic analyses of the authigenic shale fraction released by inverse aqua regia digestion yield an average δCd114/110 of +0.15±0.01‰ (2 SE, n=12; rel. NIST SRM 3108), indicative of incomplete surface water nutrient utilization up-section. The constant degree of nutrient utilization combined with strong upwelling requires increasing primary productivity – and not oceanic stagnation – to balance the larger nutrient fluxes to both study sites during the development of the Late Permian water column anoxia. Overall, our data illustrate that if bottom water redox and upwelling can be adequately constrained, Cd-isotopic analyses of organic-rich sediments can be used to provide valuable information on nutrient utilization and therefore past productivity.

A method for determination of PGE–Re concentrations and Os isotopic compositions in environmental materials

Automobile catalyst emissions not only result in increasing environmental concentrations of Pt, Pd, and Rh, but also Os, Ir, and Ru. Little is known about the behavior of platinum-group elements (PGE) and Re in the environment and their potential risk to humans and, as such, the monitoring of Os, Ir, Ru, and Re concentrations in environmental samples is necessary. We present an improved procedure for determination of PGE-Re concentrations and Os-187/Os-188 ratios of the environmental reference material BCR-723 by isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS) and negative thermal ionization mass spectrometry (N-TIMS), respectively. The sample is digested with inverse aqua regia in a Carius tube. After Os separation by CCl4 solvent extraction, separation and purification of PGE and Re from sample matrix elements is sequentially performed by a two-stage tandem column setup using cation exchange and N-benzoyl-N-phenylhydroxylamine (BPHA) extraction resin. Elution of 0.1 mol L-1 HCl efficiently removes interfering elements such as Zr, Hf, Mo, and Cd in the final purified sample prior to ID-ICP-MS determination of PGE and Re. Analyses of BCR-723 yield reproducible results and well-defined average Pt, Pd, Os, Ir, Ru, and Re concentrations of 79.8 +/- 6.0, 4.6 +/- 0.8, 0.37 +/- 0.04, 0.23 +/- 0.12, 1.1 +/- 0.3, and 6.5 +/- 0.1 ng g(-1), respectively (95% confidence level; n = 10). The Os-187/Os-188 ratio of this standard was determined to be 0.537 +/- 0.022 (95% confidence level; n = 10). These are the first comprehensive PGE and Re concentrations and Os isotopic compositions reported for BCR-723 and provide a well-characterized standard for assessing the data quality of autocatatyst PGE measurements and related research on environmental materials. The method was successfully employed in the determination of PGE and Re concentrations and Os isotopic compositions in urban road dust samples.

Re-evaluating digestion methods for highly siderophile element and 187Os isotope analysis: Evidence from geological reference materials

Here we evaluate measurement procedures currently used for the combined determination of highly siderophile element concentrations (HSEs: Re, Ir, Os, Ru, Pt and Pd) and Os isotope ratios in geological samples by isotope dilution mass spectrometry (ID-MS) applying high-temperature sample digestion using inverse aqua regia in closed glass vessels, such as Carius tubes or a high-pressure asher system (HPA). In particular, we address the question of whether an additional hydrofluoric acid dissolution step is required to release HSEs hosted in the silicate portions of rock samples. To do so, we conducted systematic experiments for basaltic (TDB-1 and BIR-1), ultramafic (UB-N and JP-1) and sedimentary (SCo-1, SDO-1 and JCh-1) reference materials with and without HF desilicification. Through the extensive tests on TDB-1—varying apparatus (microwave, Carius tube, high-pressure asher), conditions (temperature, duration, sample size) and protocol (HF desilicification before or after aqua regia attack)—we find the optimum digestion method is to use inverse aqua regia for digesting 1–2g of powder over long durations, such as Carius tubes heated to 240˚C for 72h, followed by an HF desilicification step after CCl4 solvent extraction of Os. For basaltic reference materials, the method provides significantly improved HSE recoveries—particularly Ru—largely due to the use of HF. Strong linear correlations between Os and Ir–Ru–Pt concentrations and isochronous behavior on a 187Re/188Os vs. 187Os/188Os diagram (1240±59Ma, MSWD=1.4) are observed for repeat dissolutions of TDB-1, reflecting sample heterogeneity due to minor minerals enriched in Os, Ir, Ru and Pt—most likely sulphides. In comparison, excellent reproducibilities were obtained for BIR-1: relative standard deviations (RSDs) for 1–2g aliquots were 6.9% Os, 5.1% Ir, 2.0% Ru, 5.1% Pt, 1.5% Pd and 0.7% Re (n=9). Thus, BIR-1 might be a useful candidate reference material for obtaining certified values with small measurement uncertainties. Unlike the basaltic reference materials, extraction of HSEs from ultramafic and sedimentary reference materials is largely independent of the use of HF. The data obtained by Carius tube for UB-N and JP-1 agree well with the literature data obtained using apparently more aggressive digestion techniques. The precision of our data for 0.5–2g aliquots of the sedimentary reference materials also compares favourably with limited data available in the literature.

Depositional age of the early Paleoproterozoic Klipputs Member, Nelani Formation (Ghaap Group, Transvaal Supergroup, South Africa) and implications for low-level Re–Os geochronology and Paleoproterozoic global correlations

We present new Re–Os isotope data from the early Paleoproterozoic Klipputs Member (Nelani Formation, Ghaap Group, Transvaal Supergroup, South Africa) to further evaluate low-level Re–Os geochronology of organic-rich sedimentary rocks and improve global correlations of Paleoproterozoic sedimentary rocks. The Klipputs Member contains Re (0.3–0.8ppb), total Os (34–177ppt) and common Os (192Os; 10–66ppt) abundances that overlap upper crustal abundances of 0.2–2ppb, 30–50ppt, and 12–21ppt, respectively. Five samples yield a Model 1 Re–Os date of 2479±20Ma (±22Ma if the 187Re decay constant uncertainty is included, 2σ, Mean Square of Weighted Deviates [MSWD]=0.01) using the CrVI-H2SO4 digestion protocol. Dissolution of the same sample powders using inverse aqua regia yields a Model 3 Re–Os date of 2862±190Ma (2σ, MSWD=148). Comparison of Re–Os isotope data between the two methods reveals that inverse aqua regia liberated a component of radiogenic detrital Os whereas the CrVI-H2SO4 medium did not. The Re–Os date of 2479±22Ma for the Klipputs Member is consistent with published geochronological ages from underlying and overlying stratigraphic units, suggesting the CrVI-H2SO4 protocol yields accurate Re–Os depositional ages from ORS containing very low levels of hydrogenous Re and hydrogenous+radiogenic Os. Hence, there is potential for widespread application of low-level Re–Os ORS geochronology, including sedimentary successions deposited before the Great Oxidation Event. The initial 187Os/188Os (0.12±0.01) from the Klipputs Member CrVI-H2SO4 regression indicates that dissolved Os in the contemporaneous local seawater originated from hydrothermal alteration of unradiogenic oceanic crust and/or cosmic dust rather than oxidative weathering of the radiogenic upper crust. In light of the Re–Os depositional age for the Klipputs Member, we propose that the stratigraphic units in the upper Hamersley Group above the Brockman Iron Formation (Western Australia) are younger than the Ghaap Group. Diamictite from the Doradale Formation in the Ghaap Group is constrained to be older than 2479±22Ma and thus may not be correlative with glacial deposits assigned to the oldest Paleoproterozoic glaciation.

The Design of Re‐usable Carius Tubes for the Determination of Rhenium, Osmium and Platinum‐Group Elements in Geological Samples

The traditional Carius tube technique is cumbersome and requires skilful work to seal the Carius tube, which can be used only once. We describe a modification to the technique that does not require the use of a high‐temperature welding torch to melt the Carius tube to seal it. The newly designed Carius tube consists of a main body with a 3 mm‐thick glass wall, a neck and head with walls 4 mm in thickness, and an efficient screw‐thread stopper. These new features allowed the tube to be used repeatedly. We demonstrate relatively low procedural blanks derived for Re and Os, and platinum‐group elements (PGEs), using the redesigned tube. A temperature of 220 °C could be reached for about 5 ml of HNO3 for a 47 ml tube and for 32 ml of inverse aqua regia for a 200 ml tube. This digestion technique can be used for routine analysis of Re and PGEs in geological samples.

Digestion methods for trace element measurements in shales: Paleoredox proxies examined

Trace elements such as U, Mo, Re, V and Ni in marine shales are commonly used to probe paleoenvironmental conditions. Trace elements may have either a detrital or seawater source, or both. The seawater contribution is often modelled from whole-rock sediment analyses. Using whole-rock trace element ratios as paleoredox proxies, however, presents limitations. In this study, we compare three different methods of digestion using Middle Triassic black shales from Svalbard and Svalis Dome, and Late Permian black shales from the mid-Norwegian shelf and East Greenland. Multi-acid total digestion provides whole-rock compositions, including both detrital and seawater contributions. Aqua regia and inverse aqua regia digestions dissolve organic matter, sulfides and carbonates, leaving behind major detrital silicate components. Thus, trace element concentrations by aqua regia digestion provide a direct, though still imperfect approximation of the seawater component.For most trace elements, the concentrations and their ratios derived from aqua regia and inverse aqua regia digestions agree within 10%, roughly the uncertainty of the analytical procedures. However, inverse aqua regia may not dissolve apatite completely, which leads to slightly lower Ca, P, Sc, U and Pb contents than aqua regia digestion. Multi-acid whole-rock total digestion and aqua regia partial digestion yield comparable abundances of P, S, Co, Ni, Cu, Zn, Mn, Mo and Pb, which are dominantly of seawater origin in our samples. However, concentrations of U, V, Sc and Cr derived from aqua regia digestion are up to 80% lower than those from whole-rock digestion, suggesting significant contributions from both detrital and seawater sources.We conclude that: (1) trace metals released by aqua regia or inverse aqua regia predominantly reflect seawater components, and thus have value as paleoredox proxies, (2) if the absolute trace metal concentrations are used to infer the paleoredox conditions, the total amount of organic carbon and sulphur must be taken into account, and (3) threshold values for trace metal ratios based on whole-rock compositions (e.g., Th/U, V/Sc, V/V+Ni) are not globally applicable to infer redox conditions, as these ratios reflect a combination of both detrital and seawater components.