Precise determination of major and trace elements in micrometer-scale ilmenite lamellae in titanomagnetite using LA-ICP-MS technique: application of regression analysis to time-resolved signals.

Laser ablation ICP-MS (LA-ICP-MS) is a powerful microbeam technique capable of rapid and precise determination for a large spectrum of trace elements at ppm or sub-ppm levels. Micrometer-scale minerals and inclusions are very common in geological materials, for which direct measurement is restricted by the spot size using LA-ICP-MS (generally 20-50 µm). In this study, ilmenite lamellae intergrown with magnetite were selected as an example to describe a practical algorithm that applies regression analysis to extract the chemical compositions of binary phases from mixed LA-ICP-MS signals. The method accuracy is confirmed by the agreement between the regressed value for various trace elements in ilmenite exsolutions and their reference values (direct analyses using EPMA and LA-ICP-MS). Results were obtained for most detectable components (Mg, Mn, V, Nb, Ta, Sc, Zr, Hf, Sn, et c.) and their relative deviations are within ±10%, even for those <10 ppm (such as Hf and W). Relative standard errors on the regressed value were calculated to evaluate the precision of the method, which is mostly within 10%, and the worst up to 25%. Therefore, the algorithm described in this contribution provides a solution for precise determination of trace element compositions for micrometer-scale ilmenite lamellae in titanomagnetite using LA-ICP-MS, and is potentially practical for other geological materials.

New identification and significance of Early Cretaceous mafic rocks in the interior South China Block.

Early Cretaceous mafic rocks are first reported in the northern Guangxi region from the western Qin-Hang belt in the interior South China Block. A systematic investigation of zircon U-Pb dating, whole-rock geochemistry, Sm-Nd isotopes and zircon Hf-O isotopes for these mafic rocks reveals their petrogenesis and the mantle composition as well as a new window to reconstruct lithospheric evolution in interior South China Block during Late Mesozoic. Zircon U-Pb dating yielded ages of 131 ± 2 Ma to 136 ± 2 Ma for diabase and gabbro from Baotan area, indicating the first data for Early Cretaceous mafic magmatism in the western Qing-Hang belt. These mafic rocks show calc-alkaline compositions, arc-like trace element distribution patterns, low zircon εHf(t) of - 9.45 to - 6.17 and high δ18O values of + 5.72 to + 8.09‰, as well as low whole-rock εNd(t) values of - 14.27 to - 9.53. These data suggest that the studied mafic rocks are derived from an ancient lithospheric mantle source that was metasomatized during Neoproterozoic subduction. Thus, the occurrence of these mafic rocks indicates a reactivation of Neoproterozoic subducted materials during an extension setting at Late Mesozoic in the western Qin-Hang belt, an old suture zone that amalgamates the Yangtze and Cathaysia blocks.

Accurate Determination of Barium Isotopic Compositions in Sequentially Leached Phases from Carbonates by Double Spike-Thermal Ionization Mass Spectrometry (DS-TIMS).

Recent studies have proposed barium isotopes as a novel proxy for studying primary productivity in paleo-oceangraphical studies and elements cycling through the critical zone. Pristine marine carbonates are generally assumed to preserve Ba isotope compositions of ancient seawater. However, Ba incorporated in or adsorbed on detrital minerals such as clays in impure carbonates may limit the accurate application of the Ba isotope proxy for paleo-ocean environmental reconstruction purposes. We present here a sequential extraction procedure and show that a considerable range of Ba concentrations can be associated with the four operationally defined sequential leaching fractions (water-soluble, exchangeable, carbonate, and oxidizable fractions). Chemical separation of Ba from these leachates is achieved with a recovery of >98.6% by our modified ion exchange procedure. Potential instrumental mass bias effects and barium isotope fractionation during the chemical separation are corrected using a carefully optimized 130Ba-135Ba double-spike method. A long-term reproducibility better than ±0.03‰ (2SD) for δ137/134Ba has been achieved by using the double spike-thermal ionization mass spectrometry (DS-TIMS) in this study. We demonstrate that significant variations of δ137/134Ba in the analyzed leachates suggest a considerable Ba isotope fractionation between carbonate mineral phase and noncarbonate phases of marine carbonate rocks. The barium isotope distribution in a set of standard reference materials and natural geological samples under various geological settings has been presented. When utilizing Ba isotopes as a proxy for primary productivity and the biogeochemical cycling of Ba, our new findings from sequential Ba extraction as well as our modified precise DS-TIMS analytical setup should be taken into account.

A simple two-stage column chromatographic separation scheme for strontium, lead, neodymium and hafnium isotope analyses in geological samples by thermal ionization mass spectrometry or multi-collector inductively coupled plasma mass spectrometry.

Here, a two-stage column separation scheme is developed for the concomitant isolation of Sr, Pb, Nd, and Hf from geological samples. The first column, which consists of three resin layers (AG50W-X8 ion exchange resin + Ln specific resin + Sr specific resin), separates the high field strength element + rare earth element, Sr and Pb from the matrices. Subsequently, Nd and Hf are further separated from the high field strength element + rare earth element fraction on the second column using 1 mL of Ln specific resin. The two-stage column process can be completed within about seven and a half hours for a batch of samples (20-30). The separated Sr fraction was ready for isotope ratio measurements by thermal ionization mass spectrometry. The Pb, Nd, and Hf fractions were converted to nitrate prior to isotopic analyses by multi-collector inductively coupled plasma mass spectrometry. The feasibility of this new procedure is confirmed by the analyses of four international rock standards (BCR-2, AGV-2, BHVO-2, and JB-3), which yielded isotope ratios that were in good agreement with other published data.

The origin of rare alkali metals in geothermal fluids of southern Tibet, China: A silicon isotope perspective.

Geothermal waters from the Semi, Dagejia and Kawu hot springs in the Shiquanhe-Yarlung Zangbo geothermal field of southern Tibet (China) are highly enriched in rare alkali metals (RAM). However, the enrichment mechanism is still hotly debated. Here, we report the first silicon isotope data of these geothermal waters to unravel the origin of the extreme RAM enrichments. Sinter precipitation in the spring vents and water-rock interaction in the deep reservoir controlled both the silicon budget and silicon isotope fractionation. The rates of water-rock interaction and sinter precipitation in three spring sites decrease in the sequences Semi > Kawu > Dagejia, and Dagejia > Kawu > Semi respectively. Silicon isotope fractionation during sinter precipitation (i.e. Δ30Siprecipitate-solution < -0.1‰) is less than that due to water-rock interaction (i.e. Δ30Sisolution-rocks at least as high as -0.47‰), which makes it possible to use the δ30Si signatures of springs to evaluate the intensity of water-rock interaction. Based on the available evidence, a conceptual model of RAM enrichment is proposed: (i) persistent magmatic activity in southern Tibet provided the initial enrichment of the RAM in host rocks and a heat sources for the deep reservoirs of geothermal systems; (ii) the high Cl- content and long residence time (thousands of years) promote the leaching of RAM from the silicate host rocks.

Silicon Isotope Geochemistry: Fractionation Linked to Silicon Complexations and Its Geological Applications.

The fundamental advances in silicon isotope geochemistry have been systematically demonstrated in this work. Firstly, the continuous modifications in analytical approaches and the silicon isotope variations in major reservoirs and geological processes have been briefly introduced. Secondly, the silicon isotope fractionation linked to silicon complexation/coordination and thermodynamic conditions have been extensively stressed, including silicate minerals with variable structures and chemical compositions, silica precipitation and diagenesis, chemical weathering of crustal surface silicate rocks, biological uptake, global oceanic Si cycle, etc. Finally, the relevant geological implications for meteorites and planetary core formation, ore deposits formation, hydrothermal fluids activities, and silicon cycling in hydrosphere have been summarized. Compared to the thermodynamic isotope fractionation of silicon associated with high-temperature processes, that in low-temperature geological processes is much more significant (e.g., chemical weathering, biogenic/non-biogenic precipitation, biological uptake, adsorption, etc.). The equilibrium silicon isotope fractionation during the mantle-core differentiation resulted in the observed heavy isotope composition of the bulk silicate Earth (BSE). The equilibrium fractionation of silicon isotopes among silicate minerals are sensitive to the Si-O bond length, Si coordination numbers (CN), the polymerization degrees of silicate unites, and the electronegativity of cations in minerals. The preferential enrichment of different speciation of dissoluble Si (DSi) (e.g., silicic acid H4SiO4° (H4) and H3SiO4- (H3)) in silica precipitation and diagenesis, and chemical weathering, lead to predominately positive Si isotope signatures in continental surface waters, in which the dynamic fractionation of silicon isotope could be well described by the Rayleigh fractionation model. The role of complexation in biological fractionations of silicon isotopes is more complicated, likely involving several enzymatic processes and active transport proteins. The integrated understanding greatly strengthens the potential of δ30Si proxy for reconstructing the paleo terrestrial and oceanic environments, and exploring the meteorites and planetary core formation, as well as constraining ore deposits and hydrothermal fluid activity.

Improved Accuracy for Trace Element Analysis of Al and Ti in Quartz by Electron Probe Microanalysis.

The trace elements in quartz, Al and Ti, contain considerable information about mineral genesis, and determining their concentrations is of great importance in geology. Electron probe microanalysis has the advantages of non-destructive testing and high spatial resolution; however, it is a challenge to improve the accuracy and precision of trace element detection using this method. The important factors affecting accuracy include the fragility of quartz lattices at high beam currents and the methods used to determine the background. In this paper, the peaks of Al-Kα and Ti-Kα, and their backgrounds, were found to exhibit intensity variations at high beam currents and small beam diameters; therefore, it is necessary to select a large beam diameter (up to 20 µm) to avoid variations in intensity at high currents (500 nA). For background determination of Al, a multipoint background method is proposed to determine the background value, which greatly improves the accuracy of the results. For Ti, the choice of background measurement does not affect the result. In addition, it is verified that the background obtained from other quartz samples can be used as the background of an unknown quartz sample, which reduces the analysis time and minimizes sample damage.

Major, trace and rare earth elements of apatite and zircon U-Pb ages of ore-associated and barren granitoids from the Edong ore district, South China.

The data in this article contains major, trace and rare earth element concentrations of apatite, and statistic analysis of apatite morphology in the granitoid rocks from the Edong ore district, South China, together with zircon U-Pb dating results for these rocks. Both the ore-associated and barren granitoids yield similar ages, including quartz diorite in the Fuzishan Cu-Au skarn deposit (138.6 ± 2.9 Ma), quartz monzodiorite in the Niutoushan Cu skarn deposit (137.8 ± 1.8 Ma), quartz diorite in the Ouyangshan Cu skarn deposit (138.4 ± 1.2 Ma), Liujiawan quartz monzodiorite (135.0 ± 2.4 Ma), and Bengqiaodi quartz monzodiorite porphyry (138.7 ± 1.1 Ma). Apatite occurs in all the minerals for each rock sample, and a detailed petrographic analysis shows the majority of them are early formed mineral phase. The F, Cl, SO3 contents in apatite are different, those in ore-associated rocks show higher values than barren ones. Li, Ni, Co, V contents are also higher in apatites from the ore-associated rocks than in barren rocks. Apatite (La/Sm)N and (Yb/Sm)N ratio show a positive correlation for ore-associated rocks but a negative correlation for barren rocks.

Matrix Effects Originating from Coexisting Minerals and Accurate Determination of Stable Silver Isotopes in Silver Deposits.

Except for extensive studies in core formation and volatile-element depletion processes using radiogenic Ag isotopes (i.e., the Pd-Ag chronometer), recent research has revealed that the mass fractionation of silver isotopes is in principle controlled by physicochemical processes (e.g., evaporation, diffusion, chemical exchange, etc.) during magmatic emplacement and hydrothermal alteration. As these geologic processes only produce very minor variations of δ109Ag from -0.5 to +1.1‰, more accurate and precise measurements are required. In this work, a robust linear relationship between instrumental mass discrimination of Ag and Pd isotopes was obtained at the Ag/Pd molar ratio of 1:20. In Au-Ag ore deposits, silver minerals have complex paragenetic relationships with other minerals (e.g., chalcopyrite, sphalerite, galena, pyrite, etc.). It is difficult to remove such abundant impurities completely because the other metals are tens to thousands of times richer than silver. Both quantitative evaluation of matrix effects and modification of chemical chromatography were carried out to deal with the problems. Isobaric inferences (e.g., 65Cu40Ar+ to 105Pd, 208Pb2+ to 104Pd, and 67Zn40Ar+ to 107Ag+) and space charge effects dramatically shift the measured δ109Ag values. The selection of alternative Pd isotope pairs is effective in eliminating spectral matrix effects so as to ensure accurate analysis under the largest possible ranges for metal impurities, which are Cu/Ag ≤ 50:1, Fe/Ag ≤ 600:1, Pb/Ag ≤ 10:1, and Zn/Ag ≤ 1:1, respectively. With the modified procedure, we reported silver isotope compositions (δ109Ag) in geological standard materials and typical Au-Ag ore deposit samples varying from -0.029 to +0.689 ‰ with external reproducibility of ±0.009-0.084 ‰. A systemic survey of δ109Ag (or ε109Ag) variations in rocks, ore deposits, and environmental materials in nature is discussed.

In Situ Carbon Isotope Analysis by Laser Ablation MC-ICP-MS.

Carbon isotopes have been widely used in tracing a wide variety of geological and environmental processes. The carbon isotope composition of bulk rocks and minerals was conventionally analyzed by isotope ratio mass spectrometry (IRMS), and, more recently, secondary ionization mass spectrometry (SIMS) has been widely used to determine carbon isotope composition of carbon-bearing solid materials with good spatial resolution. Here, we present a new method that couples a RESOlution S155 193 nm laser ablation system with a Nu Plasma II MC-ICP-MS, with the aim of measuring carbon isotopes in situ in carbonate minerals (i.e., calcite and aragonite). Under routine operating conditions for δ13C analysis, instrumental bias generally drifts by 0.8‰-2.0‰ in a typical analytical session of 2-3 h. Using a magmatic calcite as the standard, the carbon isotopic composition was determined for a suite of calcite samples with δ13C values in the range of -6.94‰ to 1.48‰. The obtained δ13C data are comparable to IRMS values. The combined standard uncertainty for magmatic calcite is <0.3‰ (1s). No significant matrix effects have been identified in calcite with the amplitude of chemical composition variation (i.e., MnO, SrO, MgO, or FeO) up to 2.5 wt %. Two modern corals were investigated using magmatic calcite as the calibration standard, and the average δ13C values for both corals are similar to the bulk IRMS values. Moreover, coral exhibits significant heterogeneity in carbon isotope compositions, with differences up to 4.85‰ within an individual coral. This study indicates that LA-MC-ICP-MS can serve as an appropriate method to analyze carbon isotopes of carbonate minerals in situ.

Partial Melting of Subducted Sediments Produced Early Mesozoic Calc-alkaline Lamprophyres from Northern Guangxi Province, South China.

There is growing agreement that subducted sediments recycled into the deep mantle could make a significant contribution to the generation of various mantle-derived rocks. However, solid evidence and examples to support this view are few, and whether or not the subducted sediments can act as the dominating material source for the magma is unclear. Here, we report a comprehensive geochemical study that demonstrates that the newly identified Early Mesozoic calc-alkaline lamprophyres in the northern Guangxi Province, southeastern Yangtze Block in South China were likely derived in large part from the partial melting of the subducted terrigenous sediments in the deep mantle. The investigated lamprophyres are SiO2-rich minettes, characterized by moderate TFeO and MgO and high Mg# (>70). The multi-element pattern shows a typical crustal-like signature, such as enrichments in large-ion lithophile elements (LILE) and light rare earth elements (LREE) with troughs in Nb-Ta, Ti and Eu and peaks in Th-U and Pb. These rocks also show sediment-like ratios of Nb/U, Nb/Th and Ce/Pb, together with extremely radiogenic 87Sr/86Sr (0.71499-0.71919), unradiogenic 143Nd/144Nd (0.51188-0.51195) and radiogenic 207Pb/204Pb (15.701-15.718) isotopic compositions.

Adsorption Behavior of Metasilicate on N-Methyl d-Glucamine Functional Groups and Associated Silicon Isotope Fractionation.

Significant isotope fractionation of silicon provides a powerful geochemical tracer for biological and physicochemical processes in terrestrial and marine environments. The exact mechanism involved in silicon uptake as part of the biological process is not well known. The silicon uptake in biological processes is investigated using silicate adsorption onto the N-methylglucamine functional group (sugarlike structure, abbreviated as L) of Amberlite IRA-743 resin as an analogue of the formation of silicate-sugar complexes in plants. This study provides new evidence that certain sugars can react readily with basic silicic acid to form sugar-silicate chelating complexes, and the equilibrium adsorption behavior of silicate can be well described by the Langmuir isotherm with a Gibbs free energy (ΔG) of -11.94 ± 0.21 kJ·mol(-1) at 293 K. The adsorption kinetics corresponds well to a first-order kinetic model in which the adsorption rate constant ka of 1.25 × 10(-4) s(-1) and the desorption rate constant kd of 4.00 × 10(-6) s(-1) are obtained at 293 K. Both ka and kd increase with increasing temperature. The bonding configurations of silicate-sugar complexes imply the principal coordination complex of hexacoordinated silicon (silicon/L = 1:3) in the liquid phase and the dominant tetracoordinated silicon in the solid phase. Similar to those of many natural processes, the biological uptake via the sugar-silicate chelating complexes favors the preferential enrichment of light Si isotopes into solids, and the Rayleigh model controls the dynamic isotope fractionation with an estimated silicon isotope fractionation factor (i.e., αsolid-solution = [Formula: see text]) of 0.9971. This study advanced the fundamental understanding of the dynamic isotope fractionation of silicon during silicon cycling from the lithosphere to the biosphere and hydrosphere in surficial processes.

Matrix effects of calcium on high-precision sulfur isotope measurement by multiple-collector inductively coupled plasma mass spectrometry.

Multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has been successfully applied in the rapid and high-precision measurement for sulfur isotope ratios in recent years. During the measurement, the presence of matrix elements would affect the instrumental mass bias for sulfur and these matrix-induced effects have aroused a lot of researchers' interest. However, these studies have placed more weight on highlighting the necessity for their proposed correction protocols (e.g., chemical purification and matrix-matching) while less attention on the key property of the matrix element gives rise to the matrix effects. In this study, four groups of sulfate solutions, which have different concentrations of sulfur (0.05-0.60mM) but a constant sequence of atomic calcium/sulfur ratios (0.1-50), are investigated under wet (solution) and dry (desolvation) plasma conditions to make a detailed evaluation on the matrix effects from calcium on sulfur isotope measurement. Based on a series of comparative analyses, we indicated that, the matrix effects of calcium on both measured sulfur isotope ratios and detected (32)S signal intensities are dependent mainly on the absolute calcium concentration rather than its relative concentration ratio to sulfur (i.e., atomic calcium/sulfur ratio). Also, for the same group of samples, the matrix effects of calcium under dry plasma condition are much more significant than that of wet plasma. This research affords the opportunity to realize direct and relatively precise sulfur isotope measurement for evaporite gypsum, and further provides some suggestions with regard to sulfur isotope analytical protocols for sedimentary pore water.

Improvements on high-precision measurement of bromine isotope ratios by multicollector inductively coupled plasma mass spectrometry.

A new, feasible procedure for high-precision bromine isotope analysis using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) is described. With a combination of HR mass resolution mode and accurate optimization of the Zoom Optics parameters (Focus Quad: -1.30; Zoom Quad: 0.00), the challenging problem of the isobaric interferences ((40)Ar(38)ArH(+) and (40)Ar(40)ArH(+)) in the measurement of bromine isotopes ((79)Br(+), (81)Br(+)) has been effectively solved. The external reproducibility of the measured (81)Br/(79)Br ratios in the selected standard reference materials ranged from ±0.03‰ to ±0.14‰, which is superior to or equivalent to the best results from previous contributions. The effect of counter cations on the Br(+) signal intensity and the instrumental-induced mass bias was evaluated as the loss of HBr aerosol in nebulizer and potential diffusive isotope fractionations.

Rise to modern levels of ocean oxygenation coincided with the Cambrian radiation of animals.

The early diversification of animals (∼ 630 Ma), and their development into both motile and macroscopic forms (∼ 575-565 Ma), has been linked to stepwise increases in the oxygenation of Earth's surface environment. However, establishing such a linkage between oxygen and evolution for the later Cambrian 'explosion' (540-520 Ma) of new, energy-sapping body plans and behaviours has proved more elusive. Here we present new molybdenum isotope data, which demonstrate that the areal extent of oxygenated bottom waters increased in step with the early Cambrian bioradiation of animals and eukaryotic phytoplankton. Modern-like oxygen levels characterized the ocean at ∼ 521 Ma for the first time in Earth history. This marks the first establishment of a key environmental factor in modern-like ecosystems, where animals benefit from, and also contribute to, the 'homeostasis' of marine redox conditions.

Rapid and high-precision measurement of sulfur isotope and sulfur concentration in sediment pore water by multi-collector inductively coupled plasma mass spectrometry.

We have developed a technique for the rapid, precise and accurate determination of sulfur isotopes (δ(34)S) by MC-ICP-MS applicable to a range of sulfur-bearing solutions of different sulfur content. The 10 ppm Alfa-S solution (ammonium sulfate solution, working standard of the lab of the authors) was used to bracket other Alfa-S solutions of different concentrations and the measured δ(34)SV-CDT values of Alfa-S solutions deviate from the reference value to varying degrees (concentration effect). The stability of concentration effect has been verified and a correction curve has been constructed based on Alfa-S solutions to correct measured δ(34)SV-CDT values. The curve has been applied to AS solutions (dissolved ammonium sulfate from the lab of the authors) and pore water samples successfully, validating the reliability of our analytical method. This method also enables us to measure the sulfur concentration simultaneously when analyzing the sulfur isotope composition. There is a strong linear correlation (R(2)>0.999) between the sulfur concentrations and the intensity ratios of samples and the standard. We have constructed a regression curve based on Alfa-S solutions and this curve has been successfully used to determine sulfur concentrations of AS solutions and pore water samples. The analytical technique presented here enable rapid, precise and accurate S isotope measurement for a wide range of sulfur-bearing solutions - in particular for pore water samples with complex matrix and varying sulfur concentrations. Also, simultaneous measurement of sulfur concentrations is available.

An improved procedure for separation/purification of boron from complex matrices and high-precision measurement of boron isotopes by positive thermal ionization and multicollector inductively coupled plasma mass spectrometry.

In order to eliminate boron loss and potential isotopic fractionation during chemical pretreatment of natural samples with complex matrices, a three-column ion-exchange separation/purification procedure has been modified, which ensures more than 98% recovery of boron from each step for a wide range of sample matrices, and is applicable for boron isotope analysis by both TIMS and MC-ICP-MS. The PTIMS-Cs2BO2(+)-static double collection method was developed, ensuring simultaneous collection of (133)Cs2(11)B(16)O2(+)(m/z 309) and (133)Cs2(10)B(16)O2(+) (m/z 308) ions in adjacent H3-H4 Faraday cups with typical zoom optics parameters (Focus Quad: 15 V, Dispersion Quad: -85 V). The external reproducibilities of the measured (11)B/(10)B ratios of the NIST 951 boron standard solutions of 1000 ng, 100 ng and 10 ng of boron by PTIMS method are ±0.06‰, ±0.16‰ and ±0.25‰, respectively, which indicates excellent precision can be achieved for boron isotope measurement at nanogram level boron in natural samples. An on-peak zero blank correction procedure was employed to correct the residual boron signals effect in MC-ICP-MS, which gives consistent δ(11)B values with a mean of 39.66±0.35‰ for seawater in the whole range of boron content from 5 ppb to 200 ppb, ensuring accurate boron isotope analysis in few ppb boron. With the improved protocol, consistent results between TIMS and MC-ICP-MS data were obtained in typical geological materials within a wide span of δ(11)B values ranging from -25‰ to +40‰.

A rapid and high-precision method for sulfur isotope δ(34)S determination with a multiple-collector inductively coupled plasma mass spectrometer: matrix effect correction and applications for water samples without chemical purification.

RATIONALE: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Precise determination of the absolute isotopic abundance ratio and the atomic weight of chlorine in three international reference materials by the positive thermal ionization mass spectrometer-Cs2Cl+-graphite method.

Because the variation in chlorine isotopic abundances of naturally occurring chlorine bearing substances is significant, the IUPAC Inorganic Chemistry Division, Commission on Isotopic Abundances and Atomic Weights (CIAAW-IUPAC) decided that the uncertainty of atomic weight of chlorine (A(r)(Cl)) should be increased so that the implied range was related to terrestrial variability in 1999 (Coplen, T. B. Atomic weights of the elements 1999 (IUPAC Technical Report), Pure Appl. Chem.2001, 73(4), 667-683; and then, it emphasized that the standard atomic weights of ten elements including chlorine were not constants of nature but depend upon the physical, chemical, and nuclear history of the materials in 2009 (Wieser, M. E.; Coplen, T. B. Pure Appl. Chem.2011, 83(2), 359-396). According to the agreement by CIAAW that an atomic weight could be defined for one specified sample of terrestrial origin (Wieser, M. E.; Coplen, T. B. Pure Appl. Chem.2011, 83(2), 359-396), the absolute isotope ratios and atomic weight of chlorine in standard reference materials (NIST 975, NIST 975a, ISL 354) were accurately determined using the high-precision positive thermal ionization mass spectrometer (PTIMS)-Cs(2)Cl(+)-graphite method. After eliminating the weighing error caused from evaporation by designing a special weighing container and accurately determining the chlorine contents in two highly enriched Na(37)Cl and Na(35)Cl salts by the current constant coulometric titration, one series of gravimetric synthetic mixtures prepared from two highly enriched Na(37)Cl and Na(35)Cl salts was used to calibrate two thermal ionization mass spectrometers in two individual laboratories. The correction factors (i.e., K(37/35) = R(37/35meas)/R(37/35calc)) were obtained from five cycles of iterative calculations on the basis of calculated and determined R((37)Cl/(35)Cl) values in gravimetric synthetic mixtures. The absolute R((37)Cl/(35)Cl) ratios for NIST SRM 975, NIST 975a, and ISL 354 by the precise calibrated isotopic composition measurements are 0.319876 ± 0.000067, 0.319768 ± 0.000187, and 0.319549 ± 0.000044, respectively. As a result, the atomic weights of chlorine in NIST 975, NIST 975a, and ISL 354 are derived as 35.45284(8), 35.45272(21), and 35.45252(2) individually, which are consistent with the issued values of 35.453(2) by IUPAC in 1999.

A new method to determine carbon isotopic composition of dissolved inorganic carbon in seawater and pore waters by CO(2) -water equilibrium.

RATIONALE: Precise measurement of the carbon isotopic compositions of dissolved inorganic carbon (DIC) in water samples is very important for earth and environmental sciences, and the H(3) PO(4) acidification method has long been applied for this purpose. Due to the large variation in DIC concentration in various types of water samples, however, it is difficult to sample the optimized amount of water for high-precision mass spectrometry analysis and in many cases this has led to poor data quality. In addition, when the amount of water sample is very limited or when the water has a complicated composition such as seawater and saline marine pore waters, it is often difficult to obtain high-quality data using the conventional method. METHODS: Here we report a new method of obtaining CO(2) -water equilibrium for high-precision carbon isotope analysis of DIC in water samples. The instrument used was a Delta Plus XP stable isotope mass spectrometer coupled with an on-line a Gas Bench II sample preparation device. In general, 0.5 mL of water is sampled and equilibrated with CO(2) in headspace. The CO(2) is then transferred into the mass spectrometer and δ(13) C values are obtained. RESULTS: The δ(13) C values for four marine pore water samples determined by this new method are only slightly different (<0.15‰) from those determined using the conventional H(3) PO(4) method. The new method of obtaining CO(2) -water equilibrium also allows a constant amount of CO(2) for every sample to finally enter the mass spectrometer and hence better precision can be achieved. CONCLUSIONS: This new method is suitable for the measurement of both oxygen isotopic composition in water and carbon isotopic composition of DIC for seawater and pore water samples. Another benefit of this new method is that the water sample can be re-used for other further analyses and this is critical when the amount of sample is limited.