Standard-sample Bracketing Research Articles

The performance of single and multi-collector ICP-MS instruments for fast and reliable 34S/32S isotope ratio measurements.

The performance and validation characteristics of different single collector inductively coupled plasma mass spectrometers based on different technical principles (ICP-SFMS, ICP-QMS in reaction and collision modes, and ICP-MS/MS) were evaluated in comparison to the performance of MC ICP-MS for fast and reliable S isotope ratio measurements. The validation included the determination of LOD, BEC, measurement repeatability, within-lab reproducibility and deviation from certified values as well as a study on instrumental isotopic fractionation (IIF) and the calculation of the combined standard measurement uncertainty. Different approaches of correction for IIF applying external intra-elemental IIF correction (aka standard-sample bracketing) using certified S reference materials and internal inter-elemental IIF (aka internal standardization) correction using Si isotope ratios in MC ICP-MS are explained and compared. The resulting combined standard uncertainties of examined ICP-QMS systems were not better than 0.3–0.5% (uc,rel), which is in general insufficient to differentiate natural S isotope variations. Although the performance of the single collector ICP-SFMS is better (single measurement uc,rel = 0.08%), the measurement reproducibility (>0.2%) is the major limit of this system and leaves room for improvement. MC ICP-MS operated in the edge mass resolution mode, applying bracketing for correction of IIF, provided isotope ratio values with the highest quality (relative combined measurement uncertainty: 0.02%; deviation from the certified value: <0.002%).

An in-depth evaluation of accuracy and precision in Hg isotopic analysis via pneumatic nebulization and cold vapor generation multi-collector ICP-mass spectrometry

Mercury (Hg) isotopic analysis via multi-collector inductively coupled plasma (ICP)-mass spectrometry (MC-ICP-MS) can provide relevant biogeochemical information by revealing sources, pathways, and sinks of this highly toxic metal. In this work, the capabilities and limitations of two different sample introduction systems, based on pneumatic nebulization (PN) and cold vapor generation (CVG), respectively, were evaluated in the context of Hg isotopic analysis via MC-ICP-MS. The effect of (i) instrument settings and acquisition parameters, (ii) concentration of analyte element (Hg), and internal standard (Tl)-used for mass discrimination correction purposes-and (iii) different mass bias correction approaches on the accuracy and precision of Hg isotope ratio results was evaluated. The extent and stability of mass bias were assessed in a long-term study (18 months, n = 250), demonstrating a precision ≤0.006% relative standard deviation (RSD). CVG-MC-ICP-MS showed an approximately 20-fold enhancement in Hg signal intensity compared with PN-MC-ICP-MS. For CVG-MC-ICP-MS, the mass bias induced by instrumental mass discrimination was accurately corrected for by using either external correction in a sample-standard bracketing approach (SSB) or double correction, consisting of the use of Tl as internal standard in a revised version of the Russell law (Baxter approach), followed by SSB. Concomitant matrix elements did not affect CVG-ICP-MS results. Neither with PN, nor with CVG, any evidence for mass-independent discrimination effects in the instrument was observed within the experimental precision obtained. CVG-MC-ICP-MS was finally used for Hg isotopic analysis of reference materials (RMs) of relevant environmental origin. The isotopic composition of Hg in RMs of marine biological origin testified of mass-independent fractionation that affected the odd-numbered Hg isotopes. While older RMs were used for validation purposes, novel Hg isotopic data are provided for the latest generations of some biological RMs.

Evaluation strategies and uncertainty calculation of isotope amount ratios measured by MC ICP-MS on the example of Sr.

This paper critically reviews the state-of-the-art of isotope amount ratio measurements by solution-based multi-collector inductively coupled plasma mass spectrometry (MC ICP-MS) and presents guidelines for corresponding data reduction strategies and uncertainty assessments based on the example of n(87Sr)/n(86Sr) isotope ratios. This ratio shows variation attributable to natural radiogenic processes and mass-dependent fractionation. The applied calibration strategies can display these differences. In addition, a proper statement of uncertainty of measurement, including all relevant influence quantities, is a metrological prerequisite. A detailed instructive procedure for the calculation of combined uncertainties is presented for Sr isotope amount ratios using three different strategies of correction for instrumental isotopic fractionation (IIF): traditional internal correction, standard-sample bracketing, and a combination of both, using Zr as internal standard. Uncertainties are quantified by means of a Kragten spreadsheet approach, including the consideration of correlations between individual input parameters to the model equation. The resulting uncertainties are compared with uncertainties obtained from the partial derivatives approach and Monte Carlo propagation of distributions. We obtain relative expanded uncertainties (Urel; k = 2) of n(87Sr)/n(86Sr) of < 0.03 %, when normalization values are not propagated. A comprehensive propagation, including certified values and the internal normalization ratio in nature, increases relative expanded uncertainties by about factor two and the correction for IIF becomes the major contributor.Electronic supplementary materialThe online version of this article (doi:10.1007/s00216-015-9003-9) contains supplementary material, which is available to authorized users.

Sulfate and sulfide sulfur isotopes (δ34S and δ33S) measured by solution and laser ablation MC-ICP-MS: An enhanced approach using external correction

Isotope ratio measurements using a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) commonly use standard-sample bracketing with a single isotope standard for mass bias correction for elements with narrow-range isotope systems measured by MC-ICP-MS, e.g. Cu, Fe, Zn, and Hg. However, sulfur (S) isotopic composition (δ34S) in nature can range from at least −40 to +40‰, potentially exceeding the ability of standard-sample bracketing using a single sulfur isotope standard to accurately correct for mass bias. Isotopic fractionation via solution and laser ablation introduction was determined during sulfate sulfur (Ssulfate) isotope measurements. An external isotope calibration curve was constructed using in-house and National Institute of Standards and Technology (NIST) Ssulfate isotope reference materials (RM) in an attempt to correct for the difference. The ability of external isotope correction for Ssulfate isotope measurements was evaluated by analyzing NIST and United States Geological Survey (USGS) Ssulfate isotope reference materials as unknowns. Differences in δ34Ssulfate between standard-sample bracketing and standard-sample bracketing with external isotope correction for sulfate samples ranged from 0.72‰ to 2.35‰ over a δ34S range of 1.40‰ to 21.17‰. No isotopic differences were observed when analyzing Ssulfide reference materials over a δ34Ssulfide range of −32.1‰ to 17.3‰ and a δ33S range of −16.5‰ to 8.9‰ via laser ablation (LA)-MC-ICP-MS. Here, we identify a possible plasma induced fractionation for Ssulfate and describe a new method using external isotope calibration corrections using solution and LA-MC-ICP-MS.

Bromine isotope ratio measurements in seawater by multi-collector inductively coupled plasma-mass spectrometry with a conventional sample introduction system.

A simple and accurate methodology for Br isotope ratio measurements in seawater by multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) with pneumatic nebulization for sample introduction was developed. The Br(+) signals could be measured interference-free at high mass resolution. Memory effects for Br were counteracted using 5 mmol L(-1) of NH4OH in sample, standard, and wash solutions. The major cation load of seawater was removed via cation exchange chromatography using Dowex 50WX8 resin. Subsequent Br preconcentration was accomplished via evaporation of the sample solution at 90 °C, which did not induce Br losses or isotope fractionation. Mass discrimination was corrected for by external correction using a Cl-matched standard measured in a sample-standard bracketing approach, although Sr, Ge, and Se were also tested as potential internal standards for internal correction for mass discrimination. The δ(81)Br (versus standard mean ocean bromide (SMOB)) values thus obtained for the NaBr isotopic reference material NIST SRM 977 and for IRMM BCR-403 seawater certified reference material are in agreement with literature values. For NIST SRM 977, the (81)Br/(79)Br ratio (0.97291) was determined with a precision ≤0.08‰ relative standard deviation (RSD).

Model selection during sample-standard-bracketing using reversible jump Markov chain Monte Carlo

We present a technique to tackle ambiguity in model select for instrument calibration by sample-standard bracketing.

Direct determination of Si isotope ratios in natural waters and commercial Si standards by ion exclusion chromatography multicollector inductively coupled plasma mass spectrometry.

Silicon isotope ratios in natural waters and several commercial Si standards were determined by online ion exclusion chromatography (IEC) multicollector inductively couple plasma mass spectrometry (MC-ICPMS). As recent studies have shown that mass-independent fractionation (MIF) also exists in MC-ICPMS, e.g., Nd, Ce, W, Sr, Hf, Ge, Hg, and Pb isotopes, the nature of mass bias for Si isotopes was thus investigated. MIF was observed for Si isotopes on both Neptune and Neptune plus MC-ICPMS instruments in this study. Therefore, a standard-sample bracketing (SSB) mass bias correction model, capable of correcting both mass-dependent and mass-independent bias, was employed to obtain accurate Si isotope ratio results in all samples by using NBS28 Si standard as the bracketing standard. Medium resolution was used for all measurements in order to resolve polyatomic interferences on Si isotopes. NBS28 Si standard solutions prepared in nutrient-free seawater and 0.1% NaOH matrix, respectively, were used for the method validation and subjected to the online IEC MC-ICPMS determination of Si isotope ratios. Values of -0.01 ± 0.06 and 0.00 ± 0.06 ‰ (1 SD, n = 10) and -0.01 ± 0.03 and 0.01 ± 0.06 ‰ (1 SD, n = 10) for δ(29/28)Si and δ(30/28)Si, respectively, were obtained, confirming accurate results can be obtained using the reported method for natural waters. Significant variations in Si isotope ratios from -0.72 ± 0.09 to -0.24 ± 0.03 ‰ (1 SD, n = 10) and -1.36 ± 0.11 to -0.46 ± 0.04 ‰ (1 SD, n = 10) for δ(29/28)Si and δ(30/28)Si, respectively, were found among commercial Si standards of NIST SRM3150, SCP Si, and Sigma-Aldrich Si. Values of -0.06 ± 0.07 and -0.20 ± 0.11 ‰ (1 SD, n = 10) for δ(29/28)Si and δ(30/28)Si, respectively, were obtained for the MOOS-3 seawater whereas 0.59 ± 0.11 and 1.19 ± 0.15 ‰ (1SD, n = 10) for δ(29/28)Si and δ(30/28)Si, respectively, were obtained for the SLRS-5 river water. To the best of our knowledge this is the first report of an application of online IEC MC-ICPMS for the high accuracy and precision determination of Si isotope ratios in natural waters. The reported method provides for a relatively rapid (10 min per run) and simple online technique that requires no sample pretreatment for the Si isotope ratio measurements.

Ultra-precise titanium stable isotope measurements by double-spike high resolution MC-ICP-MS

In this contribution, we present a new technique for the ultra-precise determination of titanium stable isotope composition (expressed as δ49Ti or deviation of the 49Ti/47Ti ratio to the reference standard) of geological samples by multi-collection plasma source mass spectrometry (MC-ICPMS) using the double spike method to correct for instrumental mass bias. Tails of polyatomic spectral interferences on 46Ti are accounted for by using sample-standard bracketing in high-resolution mode. Choice of ideal double and triple spike composition is investigated and results show that analytical error for a single measurement is optimised for a 47Ti–49Ti double spike composed of ca. 50% of each spike and mixed with ca. 52% of sample. Measurements of pure Ti solution show that internal error on single measurements of ca. 0.010‰ (95% c.i.) is attainable on δ49Ti, in agreement with the error model. Due to the lack of a widely available reference isotopic standard for titanium, all results are expressed as deviations relative to newly created reference material (OL-Ti standing for Origins Laboratory – titanium) prepared from an ultra-pure titanium metal rod. A range of analytical tests demonstrates the robustness of our method. An external reproducibility of ca. 0.020‰ (2sd) is routinely achievable for Ti stable isotopes. Data for a range of basaltic rock standards as well as a subduction zone basalt reference suite is presented and show that the Ti stable isotope compositions of terrestrial basalt show resolvable variations but are overall very close to the OL-Ti reference standard. The average Ti isotopic composition of the basalts studied here is the present best estimate of the upper mantle composition; δ49Ti = +0.004 ± 0.062‰ (2sd).

A rapid and high‐precision method for sulfur isotope δ34S determination with a multiple‐collector inductively coupled plasma mass spectrometer: matrix effect correction and applications for water samples without chemical purification

Previous studies have indicated that prior chemical purification of samples, although complex and time-consuming, is essential in obtaining precise and accurate results for sulfur isotope ratios using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). In this study, we introduce a new, rapid and precise MC-ICP-MS method for sulfur isotope determination from water samples without chemical purification. The analytical work was performed on an MC-ICP-MS instrument with medium mass resolution (m/Δm ~ 3000). Standard-sample bracketing (SSB) was used to correct samples throughout the analytical sessions. Reference materials included an Alfa-S (ammonium sulfate) standard solution, ammonium sulfate provided by the lab of the authors and fresh seawater from the South China Sea. A range of matrix-matched Alfa-S standard solutions and ammonium sulfate solutions was used to investigate the matrix (salinity) effect (matrix was added in the form of NaCl). A seawater sample was used to confirm the reliability of the method. Using matrix-matched (salinity-matched) Alfa-S as the working standard, the measured δ(34)S value of AS (-6.73 ± 0.09‰) was consistent with the reference value (-6.78 ± 0.07‰) within the uncertainty, suggesting that this method could be recommended for the measurement of water samples without prior chemical purification. The δ(34)S value determination for the unpurified seawater also yielded excellent results (21.03 ± 0.18‰) that are consistent with the reference value (20.99‰), thus confirming the feasibility of the technique. The data and the results indicate that it is feasible to use MC-ICP-MS and matrix-matched working standards to measure the sulfur isotopic compositions of water samples directly without chemical purification. In comparison with the existing MC-ICP-MS techniques, the new method is better for directly measuring δ(34)S values in water samples with complex matrices; therefore, it can significantly accelerate analytical turnover.

Resonance ionisation mass spectrometry of krypton and its applications in planetary science

A new resonance ionisation time-of-flight mass spectrometer for determining krypton isotope ratios in extraterrestrial samples is presented. Laser heating is used to extract gas from mg-size samples. A cryogenic sample concentrator is employed. Atoms continuously condense on a 75 K stainless steel substrate at the back plate of a Wiley-McLaren laser ion source from where they are desorbed by a pulsed 1064 nm laser and resonantly ionized in the plume. A three-colour (116.5 nm, 558.1 nm and 1064 nm) excitation scheme is used. Tuneable coherent Vacuum Ultraviolet (vuv) radiation near 116.5 nm is generated by four-wave sum frequency mixing of 252.5 nm and 1507 nm pulsed dye laser beams in a binary mixture of negatively and positively dispersive gases (Xe and Ar). Isotope effects have been observed that reduce the reproducibility of isotope ratio measurements between odd-mass, non-zero nuclear spin isotopes and even-mass, zero nuclear spin isotopes. This can be minimised and stabilised by controlling the laser fluences, experimental geometry, and the population of the magnetic sub-levels of the excited atomic states used in the ionisation process. Once stability is achieved, sample-standard bracketing (during which the known isotope ratios of a standard are determined before and after the measurements of the sample under the same conditions) allows precision and reproducibility of \(\sim \)1 % for the major isotope ratios to be achieved in samples \(\sim 10^{6}\) krypton atoms. Detection limits of \(<1000\) atoms/isotope have been demonstrated, ratios of \(^{81}\)Kr in meteorites have been made with \(\sim \)5–10 % precision. Applications of the instrument in various areas of planetary science are also discussed.

Precise determination of seawater calcium using isotope dilution inductively coupled plasma mass spectrometry

We describe a method for rapid, precise and accurate determination of calcium ion (Ca(2+)) concentration in seawater using isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). A 10 μL aliquot of seawater was spiked with an appropriate (43)Ca enriched solution for (44)Ca/(43)Ca ID-ICP-MS analyses, using an Element XR (Thermo Fisher Scientific), operated at low resolution in E-scan acquisition mode. A standard-sample bracketing technique was applied to correct for potential mass discrimination and ratio drift at every 5 samples. A precision of better than 0.05% for within-run and 0.10% for duplicate measurements of the IAPSO seawater standard was achieved using 10 μL solutions with a measuring time less than 3 minutes. Depth profiles of seawater samples collected from the Arctic Ocean basin were processed and compared with results obtained by the classic ethylene glycol tetra-acetic acid (EGTA) titration. Our new ID-ICP-MS data agreed closely with the conventional EGTA data, with the latter consistently displaying 1.5% excess Ca(2+) values, possibly due to a contribution of interference from Mg(2+) and Sr(2+) in the EGTA titration. The newly obtained Sr/Ca profiles reveal sensitive water mass mixing in the upper oceanic column to reflect ice melting in the Arctic region. This novel technique provides a tool for seawater Ca(2+) determination with small sample size, high throughput, excellent internal precision and external reproducibility.

Analysis of stable isotope ratios of Ba by double-spike standard-sample bracketing using multiple-collector inductively coupled plasma mass spectrometry

A precise analytical method for determining stable Ba isotope ratios was developed by applying a 130Ba-135Ba double-spike corrected standard-sample bracketing method with multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). Data were expressed as per mil deviation from a Ba standard in the δ137/134Ba notation. Careful examinations of the temporal instabilities of the instrument and of the 130Xe and 134Xe interference permitted accurate analysis of δ137/134Ba. The isotopic ratios of the 130Ba-135Ba double-spike used in this study were 134Ba/130Ba = 0.076528, 135Ba/130Ba = 1.060129, and 137Ba/130Ba = 0.209145. These were determined iteratively by measurements of δ137/134Ba in the IAEA-CO-9 standard with different spike-sample mixing ratios in order to achieve an invariant δ137/134Ba value of IAEA-CO-9. The reproducibility of δ137/134Ba of IAEA-CO-9 was ±0.032‰ (2SD, n = 42), which was about 5 times better than that reported by a previous study. The δ137/134Ba values of JB-2, JA-2, and BHVO-2 igneous rock standard reference materials were 0.086 ± 0.016‰ (2SD, n = 6), 0.016 ± 0.034‰ (2SD, n = 7), and 0.058 ± 0.019‰ (2SD, n = 5), respectively. A significant difference was observed between the JB-2 and JA-2 results, whereas no significant difference was observed for BHVO-2. These results show that the improved precision opens up a possibility to use δ137/134Ba as a tracer in igneous processes.

U–Pb LA–ICPMS dating using accessory mineral standards with variable common Pb

Precise and accurate U–Pb LA–ICPMS dating of many U-bearing accessory minerals (e.g. apatite, allanite, titanite and rutile) is often compromised by common Pb. LA–ICPMS dating of these U-bearing accessory phases typically requires a matrix-matched standard, and data reduction is often complicated by variable incorporation of common Pb not only into the unknowns but also particularly into the reference material. We present here a general approach to common Pb correction in U–Pb LA–ICP–MS dating using a modified version of the VizualAge U–Pb data reduction package for Iolite (VizualAge_UcomPbine). The key feature of the method is that it can correct for variable amounts of common Pb in any U–Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems following common Pb correction. Common Pb correction of the age standard can be undertaken using either the 204Pb, 207Pb or 208Pb(no Th) methods, and the approach can be applied to raw data files from all widely used modern multi-collector and single-collector ICPMS instruments.VizualAge_UcomPbine first applies a common Pb correction to the user-selected age standard integrations and then fits session-wide “model” U–Pb fractionation curves to the time-resolved U–Pb standard data. This downhole fractionation model is applied to the unknowns and sample-standard bracketing (using a user-specified interpolation method) is used to calculate final isotopic ratios and ages. 204Pb- and 208Pb(no Th)-corrected concordia diagrams and 204Pb-, 207Pb- and 208Pb(no Th)-corrected age channels can be calculated for user-specified initial Pb ratio(s). All other conventional common Pb correction methods (e.g. intercept or isochron methods on co-genetic analyses) can be performed offline.The approach was tested on apatite and titanite age standards (for which there are independent constraints on the U–Pb crystallization age) using a Thermo Scientific iCAP-Qc (Q–ICP–MS) coupled to a Photon Machines Analyte Excite 193nm ArF Excimer laser. Madagascar apatite, OLT1 titanite and R10 rutile were used as primary standards and were corrected for variable common Pb using the new VizualAge_UcomPbine DRS. The secondary Durango (31.44±0.18Ma) apatite standard yielded a U–Pb TW concordia intercept age of 31.97±0.59Ma (MSWD=1.09; primary standard corrected by the 207Pb-method) and a U–Pb concordia age of 31.82±0.40Ma (MSWD=1.4; primary standard corrected by the 204Pb-method). McClure Mountain (523.51±1.47Ma) yielded a U–Pb TW concordia intercept age of 524.5±3.7Ma (MSWD=0.72) while the Fish Canyon Tuff (28.201±0.046Ma) and Khan (522.2±2.2Ma) titanite standards yielded U–Pb TW concordia intercept ages of 28.78±0.41Ma (MSWD=1.4) and 520.9±3.9Ma (MSWD=4.2) respectively. The suitability of the 208Pb(no Th)-correction is demonstrated by the agreement between a U–Pb TW concordia intercept age of 452.6±4.7Ma (MSWD=0.89) and a 208Pb(no Th)-corrected TW concordia age of 448.6±4.5Ma (MSWD=1.4) on a c. 450Ma rutile which exhibits variable incorporation of common Pb.A range of LA–ICPMS U–Pb dating applications are presented and include U–Pb dating of apatite from >3.8Ga gneisses from Akilia, SW Greenland. These apatites host 13C-depleted graphite inclusions that are interpreted as biogenic in origin and representing the oldest indications of life on Earth. The U–Pb age profiles on single apatite grains presented here are characteristic of Pb loss by volume diffusion with core–rim age differences of up to 300Ma. These data explain the scatter and poor precision of earlier U–Pb apatite age determinations on Akilia apatite. Other LA–ICPMS dating applications include U–Pb apatite dating as a rapid method for determining the age of mafic intrusions, U–Pb titanite and apatite dating of ash fall tuffs, determining temperature–time histories using multiple U–Pb thermochronometers and improving concordance in LA–ICPMS primary zircon standard datasets by analysing young, common Pb-bearing primary zircon standards that have not accumulated significant radiation damage.

Common analyte internal standardization as a tool for correction for mass discrimination in multi-collector inductively coupled plasma-mass spectrometry

It is well known that to achieve accurate isotope ratio data using multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS), mass discrimination needs to be adequately corrected for. In this work, the capabilities and limitations of common analyte internal standardization (CAIS) as a method for correction for mass discrimination were assessed for two target elements, one in the low mass region (boron) and another in the medium mass region (antimony). CAIS has already been used in the context of element determination and, more recently, also in isotope ratio measurements with quadrupole-based ICP-MS, but it has, to the best of the authors' knowledge, never been applied to isotope ratio determination via MC-ICP-MS so far. Results obtained relying on CAIS were compared with those obtained via more established correction models, i.e. external correction in a standard-sample bracketing approach (SSB) and internal correction based on both the Russell's law (original and empirical) and the revised Russell's law. To the best of the authors' knowledge, such comparison has not been done before, especially not for low mass elements. Also the robustness of these approaches with respect to concomitant matrix elements was assessed using synthetic solutions, containing various concentrations of the matrix elements Be, Cs, Be+Cs or Fe. Whereas for B, the isotope ratio result did not seem to be significantly affected by the matrix, thus suggesting that complete separation of B from a matrix might not be necessary, at least in the cases studied, the mass discrimination observed for Sb was influenced by the presence of Cs. The experiments carried out demonstrate that the CAIS technique can be successfully applied for mass bias correction in MC-ICP-MS, providing data of the same quality as the revised Russell's law, while being more transparent and accessible. While for B, all mass bias correction approaches tested provided similar data quality (external correction with SSB gives slightly better precision), for Sb, CAIS and the revised Russell's law provide better precision and accuracy.

MC-ICP-MSを使用した、ダブルスパイク-サンプルブラケット法による高精度Ba安定同位体分析法

MC-ICP-MSを使用した、ダブルスパイク-サンプルブラケット法による高精度Ba安定同位体分析法

Solution and laser ablationMC-ICP-MS lead isotope analysis of gold

A new approach is presented for the analysis of lead isotopes in gold using solution and laser ablation multi-collector inductively-coupled plasma mass-spectrometry (MC-ICP-MS). Repeat analyses of two gold reference materials (RAuGP3: 34.1 ± 0.5 ppm Pb and RAuGP5: 129 ± 4 ppm Pb, from SPEX CertiPrep Ltd) yield an external reproducibility of 20. Instrumental mass bias is corrected by standard sample bracketing, using NIST SRM981 for solution analyses and NIST SRM610 for laser ablation analyses. Both techniques agree within error, and they have been further demonstrated through the analyses of both geological and archaeological samples: multiple grains from two Irish alluvial gold deposits and samples of two Early Bronze Age Irish gold lunulae. The applicability of these techniques to studies concerning the sources and age of gold mineralisation, and the identification of gold sources exploited in antiquity, is highlighted.

Isotopic analysis of antimony using multi-collector ICP-mass spectrometry for provenance determination of Roman glass

Based on promising results obtained in earlier work on antimony ores, we have investigated the use of natural variation in the isotopic composition of Sb as determined using multi-collector ICP-mass spectrometry (MC-ICP-MS) for provenancing Roman glasses. In antiquity, Sb was used either as a decolourizer or as an opacifier and thus, colourless and opaque coloured glasses from different regions and periods were selected for investigation. Both, the sample digestion and isolation of Sb preceeding isotopic analysis via MC-ICP-MS have been optimized and validated for this type of materials. For this purpose, NIST SRM 610 glass reference material and an in-house standard were employed. Isotope ratio determinations were carried out by MC-ICP-MS, relying on In as an internal standard for correction for instrumental mass discrimination. The merits of external calibration in a sample-standard bracketing approach and the so-called “revised Russell's law” for mass bias correction were compared, with the latter providing higher precision and better reproducibility. The 123Sb/121Sb isotope ratio in glass could be determined with a reproducibility (or external precision) of ca. 0.4ε, while in Roman colourless and Hellenistic opaque glass samples, a variation of 3 to 4ε units was found. The results seem to suggest the use of similar Sb sources for both types of glasses, and the existence of at least two different Sb sources for glass production in the Roman era.

Germanium chemistry and MC-ICPMS isotopic measurements of Fe–Ni, Zn alloys and silicate matrices: Insights into deep Earth processes

Chemical separation and isotopic measurement of germanium using hexapole-collision cell-MC-ICPMS were developed in various Fe–Ni, ZnS and silicates matrices in order to investigate the potentiality of Ge as an isotopic tracer of planetary differentiation and rock-forming processes. Analytical procedures are described for the critical step of silicate dissolution in HF+HNO3 medium, as well as for Ge chemical purification using a single cationic-exchange resin step for Fe–Ni and ZnS matrices, and two anionic and cationic resin steps for silicate matrices. Germanium isotopic measurements using MC-ICPMS were performed with appropriate Ar+H fluxes in the collision cell to eliminate argide interferences on Ge masses. Three methods of mass bias correction, including sample standard bracketing, external Ga mass bias correction using the exponential law, and the empirical “regression method”, give similar results and demonstrate the use of Ga as an appropriate element for mass bias correction of Ge. Results are presented as delta values with respect to JMC Ge standard, and NIST3120a Ge standard for comparison. We show a long-term 2SD reproducibility of less than 0.24‰ on the δ74Ge.These analytical methods have been applied to Fe-meteorites, sphalerite (ZnS) deposits, and geostandard silicates ranging from ultramafic to basaltic to granitic compositions, and to an iron formation composition. Fe-meteorites and terrestrial silicate samples display small variations of δ74GeJMC=+1.77±0.22‰ and +0.89±0.16‰ (2SD reproducibility), respectively. This contrasts with the large variations seen in low-temperature rocks, such as the ZnS ores (δ74GeJMC=−0.37 to −1.69‰), and banded iron formations (IF-G Isua, δ74GeJMC=+1.38‰). A slight δ74GeJMC–NBO/T negative tendency in silicate samples indicates that polymerisation of silicate melt would control the small Ge isotope fractionation among mantle silicates.A comparison of δ74Ge values of iron meteorites and Earth silicate mantle opens new perspectives in deep Earth processes. On the basis of theoretical metal-silicate isotopic equilibrium processes, the low δ74Ge of silicate Earth cannot reconcile one-stage process of core–mantle segregation. It is proposed that the δ74Ge(JMC) value of silicate earth samples of +0.89±0.16‰ (or δ74Ge(NIST3120a)=+0.53‰) represents the composition of the accessible Earth modern mantle. The δ74Ge of the silicate mantle in equilibrium with the core at time of core formation would be distinct to that of the present mantle in result of distinct thermodynamic parameters, e.g. fO2, pressure, inducing changes in coordination and valence state of Ge in the silicate crystallographic structure. In addition, the light isotopic composition of the Earth's mantle could result from reverse diffusive processes induced by an increase in oxidation state at the end of core formation. This would have some implications on core formation modelling and the use of Ge isotopes for tracing the origin of deep mantle plumes.

The measurement of Pb isotope ratios in sub-ng quantities by fast scanning single collector sector field-ICP-MS

This work describes an analytical protocol for the measurement of isotope ratios in samples containing small amounts of lead (i.e. ≤0.2ng total Pb) using the Nu Attom single collector-ICP-MS. The method was evaluated using the NIST SRM 981 isotopic standard and standard-sample bracketing for correction of the instrumental mass fractionation. The long-term precision (n=32, ±2SD) was 0.11% for the 208Pb/206Pb and 207Pb/206Pb ratios and 0.21–0.23% for the 20xPb/204Pb ratios (where x=208, 207, 206). The precision of the measured Pb isotope ratios reported here (for equivalent total Pb quantities) is comparable to multi-collector (MC) methodologies, using either an array of ion counters (Cocherie and Robert, 2007, Chem. Geol. 243, 90–104) or conventional Faraday collectors coupled with a high efficiency sample cone (Makishima and Nakamura, 2010, J. Anal. At. Spectrom. 25, 1712–1716). The accuracy of the measured Pb isotope ratios and the effect of the sample matrix were evaluated for a series of USGS rock standard digests and doped solutions of the NIST SRM 981 standard. For total cation concentrations <10μgmL−1 and concomitant low concentrations (<1.5μgmL−1) of easily ionisable elements (i.e. Na and K), matrix effects were not resolvable at the level of precision reported. However, significant matrix effects were associated with sample compositions typical of natural waters, and removal of (at least) the major cations is required for accurate isotope ratio measurements.

Ultra-precise stable Fe isotope measurements by high resolution multiple-collector inductively coupled plasma mass spectrometry with a 57Fe–58Fe double spike

We have developed a method for high precision measurement of stable Fe isotopes by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), utilizing a 57Fe–58Fe double spike to correct for instrumental mass bias. Polyatomic interferences of 40Ar14N+, 40Ar16O+ and 40Ar16O1H+ on Fe isotopes are minimized by the use of a desolvating nebulizer and pseudo-high resolution mass spectrometry with typical mass resolutions of 3300, 3100 and 2700 on 54Fe+, 56Fe+ and 57Fe+, respectively. Sample-standard bracketing with double spiked IRMM-014 Fe isotope standards accounts for residual tails from these interferences beneath the Fe ion peaks.A range of analytical tests demonstrates that it is possible to attain internal precisions (2 se) of ±0.02 to 0.04‰ on a single 15min analysis. Long-term reproducibility (2 sd) of ±0.02‰ was obtained by replicate (n=51) analyses of IRMM-14 over a 6-month period. Replicate digestions and analyses of the basalt standard BHVO-2 (δ56Fe=+0.128±0.019‰) demonstrate that this external reproducibility is also obtainable on natural samples.Utilizing this new technique, we present high precision stable Fe isotope data for a range of international reference materials (BHVO-2, BCR-2, JB2, BIR-1, AGV-2, JA1, GSP-1, JG3a, JR1, RGM-1 and JF2). Data for this range of igneous standards (basalt, andesite, granodiorite, rhyolite) are in good agreement with previously published values obtained by high precision sample-standard bracketing techniques, and highlight that BIR-1 is the best calibrated standard available with a reference δ56Fe value of +0.052±0.009‰ (95% c.i.). Other reference values for BHVO-2, BCR-2, JB2, BIR-1, AGV-2 are also presented. Stable Fe isotope data for the JF2 alkali feldspar from a pegmatite are markedly heavy (δ56Fe=+0.816±0.025‰), perhaps due to the incorporation of isotopically heavy magmatic Fe3+ into the feldspar mineral structure during crystallization.