Keita Yamada

Tokyo Institute of Technology, Edo, Tokyo, Japan

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Publications (47)225.99 Total impact

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    ABSTRACT: We have developed a novel method for analyzing intramolecular carbon isotopic distribution of propane as a potential new tracer of its origin. The method is based on on-line pyrolysis of propane followed by analysis of carbon isotope ratios of the pyrolytic products methane, ethylene and ethane. Using propane samples spiked with 13C at the terminal methyl carbon, we characterize the origin of the pyrolytic fragments. We show that the exchange between C-atoms during the pyrolytic process is negligible, and thus that relative intramolecular isotope composition can be calculated. Preliminary data from 3 samples show that site-preference (SP) values, defined as the difference of δ13C values between terminal and sub-terminal C-atom positions of propane, range from -1.8‰ to -12.9‰. In addition, SP value obtained using our method for a thermogenic natural gas sample is consistent with that expected from theoretical models of thermal cracking, suggesting that the isotope fractionation associated with propane pyrolysis is negligible. The method will provide novel insights into the characterization of the origin of propane and will help better understand the biogeochemistry of natural gas deposits.
    No preview · Article · Feb 2016 · Geochimica et Cosmochimica Acta
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    ABSTRACT: The natural carbon isotopic composition of acetone in urine was measured in healthy subjects using gas chromatography–combustion–isotope ratio mass spectrometry combined with headspace solid-phase microextraction (HS-SPME–GC–C–IRMS). Before applying the technique to a urine sample, we optimized the measurement conditions of HS-SPME–GC–C–IRMS using aqueous solutions of commercial acetone reagents. The optimization enabled us to determine the carbon isotopic compositions within ±0.2 ‰ of precision and ±0.3‰ of error using 0.05 or 0.2 mL of aqueous solutions with acetone concentrations of 0.3–121 mg/L. For several days, we monitored the carbon isotopic compositions and concentrations of acetone in urine from three subjects who lived a daily life with no restrictions. We also monitored one subject for 3 days including a fasting period of 24 h. These results suggest that changes in the availability of glucose in the liver are reflected in changes in the carbon isotopic compositions of urine acetone. Results demonstrate that carbon isotopic measurement of metabolites in human biological samples at natural abundance levels has great potential as a tool for detecting metabolic changes caused by changes in physiological states and disease. Graphical abstract The natural carbon isotopic composition of acetone in urine can be determined using HS-SPME-GCC-IRMS and can provide information on changes in the availability of glucose in the liver.
    No preview · Article · Dec 2015 · Analytical and Bioanalytical Chemistry
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    ABSTRACT: RationaleHeadspace solid-phase microextraction (HS-SPME) combined with gas chromatography/pyrolysis-gas chromatography/combustion-isotope ratio mass spectrometry (GC/Py-GC/C-IRMS) was developed for the simultaneous determination of the intramolecular and molecular carbon-isotopic composition (C-13 value) of acetic acid. Methods The C-13 values of carboxyl and methyl carbon were standardized using calibration curves constructed from the regression between the measured C-13 values and the C-13 values of working standards determined in a previous study. We applied this developed HS-SPME-GC/Py-GC/C-IRMS technique to commercial vinegars. ResultsIn one injection analysis, the bulk and intramolecular C-13 values of pure acetic acid standards can be obtained. The repeatability (1 sigma) of the bulk C-13 values is within 0.4 parts per thousand, and that of the C-13(carboxyl) and C-13(methyl) values is within +/- 0.6 parts per thousand. The intramolecular C-13 values of acetic acid in vinegars exhibit a similar pattern. The average value (C-13(COOH) - C-13(CH3)) is 4.3 +/- 2.0 parts per thousand. Conclusions The approach presented herein for the molecular and intramolecular C-13 determination of acetic acid avoids switching between configuration systems and thereby reduces systematic errors. It is expected to be useful for examining isotope fractionation associated with processes related to organic acid (bio)transformations. Copyright (c) 2015 John Wiley & Sons, Ltd.
    No preview · Article · Dec 2015 · Rapid Communications in Mass Spectrometry
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    ABSTRACT: The natural xanthines caffeine, theobromine and theophylline are of major commercial importance as flavor constituents in coffee, cocoa, tea and a number of other beverages. However, their exploitation for authenticity-a requirement in these commodities that have a large origin-based price-range-by the standard method of isotope ratio monitoring by mass spectrometry (irm-MS) is limited. We have now developed a methodology that overcomes this deficit that exploits the power of isotopic quantitative 13C nuclear magnetic resonance (NMR) spectrometry combined with chemical modification of the xanthines to enable the determination of positional intramolecular 13C/12C ratios (δ13Ci) with high precision. However, only caffeine is amenable to analysis: theobromine and theophylline present substantial difficulties due to their poor solubility. However, their N-methylation to caffeine makes spectral acquisition feasible. The method is confirmed as robust, with good repeatability of the δ13Ci values in caffeine appropriate for isotope fractionation measurements at natural abundance. It is shown that there is negligible isotope fractionation during the chemical N-methylation procedure. Thus, the method preserves the original positional δ13Ci values. The method has been applied to measure the position-specific variation of the 13C/12C distribution in caffeine. Not only is a clear difference between caffeine isolated from different sources observed, but theobromine from cocoa is found to show a 13C pattern distinct from that of caffeine.
    No preview · Article · Jun 2015 · Analytical Chemistry
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    ABSTRACT: The ceramide (Cer) content of skin and glucosylceramide (GlcCer) intake affect skin moisture conditions, but their mutual relation in skin remains unclear. For clarification of that mutual relation, carbon stable isotopes ((12)C and (13)C) are useful as a tracer. However, carbon isotopic measurement has not been applied to the study of clarifying their skin moisturizing effects. Therefore, we used gas chromatography / combustion / isotope ratio mass spectrometry (GC-C-IRMS) to ascertain the appropriate conditions for carbon isotopic measurements using synthesized Cer (SCer) in substitution for very low concentrations of Cer in skin. SCer was derivatized to trimethylsilylated SCer (TMS-SCer) quantitatively using N-trimethylsilylimidazole (TMSI) depending on the amount of SCer. The derivatization rates were 75-85%. Excess TMSI was removed using three cycles of hexane-water distribution. Under these conditions, carbon isotopic measurements of TMS-SCer conducted using GC-C-IRMS showed high repeatability and good inter-day variation (S.D. < 0.3‰). The carbon stable isotope ratio value (δ(13)C) of SCer calculated using a mass balance equation was compared with δ(13)C of underivatized SCer, which was regarded as the actual δ(13)C of SCer obtained using sealed tube combustion method. The difference between the calculated δ(13)C of SCer and δ(13)C of the underivatized SCer depended on the TMSI reagent supplier and on the number of hydroxyl groups to be derivatized in SCer. For accurate δ(13)C of Cer in skin using GC-C IRMS, the measured δ(13)C of a target TMS-Cer must be calculated using a correction factor representing the difference in δ(13)C of underivatized standard SCer from that of TMS-standard SCer having a structure resembling that of the target Cer in skin. In addition, we show that the same lot of TMSI reagent from a specific supplier must be used throughout the experiments.
    No preview · Article · Nov 2014 · Journal of oleo science
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    ABSTRACT: Stable hydrogen isotope systematics among H2, H2O, and CH4 during hydrogenotrophic methanogenesis were investigated by growing a thermophilic methanogen, Methanothermobacter thermautotrophicus strain ΔH, in batch cultures spiked with deuterium-labeled H2 and/or H2O. The hydrogen isotope ratio of the product, CH4, reflected not only the isotope ratio of the H2O in the medium but also that of the substrate, H2. The D/H ratios of the CH4 were highest during the early phase of growth, and the growth-phase-dependent changes were greatest in the deuterium-enriched H2 cultures. The hydrogen isotope systematics among H2, H2O, and CH4 during growth of the methanogen could be described with the following equations: δDCH4=a×δDH2O+b×δDH2-cδDCH4=a×δDH2O+b×δDH2-c a=0.71-0.55×ba=0.71-0.55×b 0.17⩽b⩽0.380.17⩽b⩽0.38 c=1000×(a+b-1)c=1000×(a+b-1) δDH2δDH2 δDCH4δDCH4 b b Methanothermobacter marburgensis M. thermautotrophicus M. marburgensis b 2 2 4 4 2 2 2 2 δDCH4δDCH4 4
    No preview · Article · Oct 2014 · Geochimica et Cosmochimica Acta
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    ABSTRACT: Intramolecular (13)C composition gives access to new information on the (bio) synthetic history of a given molecule. Isotopic (13)C NMR spectrometry provides a general tool for measuring the position-specific (13)C content. As an emerging technique, some aspects of its performance are not yet fully delineated. This paper reports on (i) the conditions required to obtain satisfactory trueness and precision for the determination of the internal (13)C distribution, and (ii) an approach to determining the "absolute" position-specific (13)C content. In relation to (i), a precision of <1% can be obtained whatever the molecule on any spectrometer, once quantitative conditions are met, in particular appropriate proton decoupling efficiency. This performance is a prerequisite to the measurement of isotope fractionation either on the transformed or residual compound when a chemical reaction or process is being studied. The study of the trueness has revealed that the response of the spectrometer depends on the (13)C frequency range of the studied molecule, i.e. the chemical shift range. The "absolute value" and, therefore, the trueness of the (13)C NMR measurements has been assessed on acetic acid and by comparison to the results obtained on the fragments from COOH and CH3 by isotopic mass spectrometry coupled to a pyrolysis device (GC-Py-irm-MS), this technique being the reference method for acetic acid. Of the two NMR spectrometers used in this work, one gave values that corresponded to those obtained by GC-Py-irm-MS (thus, the "true" value) while the other showed a bias, which was dependent to the range covered by the resonance frequencies of the molecule. Therefore, the former can be used directly for studying isotope affiliations, while the latter can only be used directly for comparative data, for example in authenticity studies, but can also be used to obtain the true values by applying appropriate correction factors. The present study assesses several key protocol steps required to enable the determination of position-specific (13)C content by isotopic (13)C NMR, irrespective of the NMR spectrometer: parameters to be adjusted, performance test using [1,2-(13)C2]acetic acid, generation of correction factors.
    Full-text · Article · Oct 2014 · Analytica Chimica Acta

  • No preview · Article · Sep 2014 · Earth and Planetary Science Letters
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    ABSTRACT: RATIONALERecent advances in analytical techniques for the intramolecular carbon isotopic ratio measurement of some organic compounds have provided important information on carbon cycles in biochemistry, organic geochemistry and food chemistry. These advances have made it necessary to prepare intramolecular isotopic reference materials (RMs) to use for inter-laboratory calibration and/or inter-calibration among different analytical methods.METHODS We evaluated the feasibility of preparing RMs using commercially available reagents for intramolecular carbon isotopic ratio measurement of acetic acid. The intramolecular carbon isotopic distribution of nine acetic acid and four sodium acetate reagents was determined with high precision using off-line pyrolysis combined with gas chromatography–combustion–isotope ratio mass spectrometry (GC-C-IRMS). We also evaluated the potential alteration in the isotopic signature of acetic acid reagents by evaporation.RESULTSThe intramolecular carbon isotopic distributions for the acetic acid and sodium acetate reagents were determined with a precision of better than 0.45‰. We found that the isotopic values of these reagents spanned the carbon isotopic range of acetic acid in biological and environmental samples. We also found that the isotope fractionation associated with the evaporation of acetic acid occurs solely on the methyl position, the carboxyl position being unaffected.CONCLUSIONS These commercially available reagents will be used as RMs in the future for inter-laboratory calibration and/or inter-calibration with another intramolecular isotopic measurement technique, namely quantitative 13C NMR. In cases where acetic acid is being used as a RM, its storage must be carefully controlled to prevent evaporation. Copyright © 2014 John Wiley & Sons, Ltd.
    Full-text · Article · Aug 2014 · Rapid Communications in Mass Spectrometry
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    ABSTRACT: We analyzed the concentration and stable carbon isotopic ratio (δ13C-CH4) of methane in the atmosphere and in dissolved methane in the water column along 17°S in the subtropical South Pacific. Additionally, the hydrogen isotopic ratios (δD-CH4) of some water samples were analyzed. The sea-air CH4 flux is high in the eastern region and off the west coast of Australia, which is related to the high concentrations of dissolved CH4 and high wind speeds. Moreover, there is a positive correlation between the CH4 and chlorophyll a concentrations at the surface. This consistency suggested that active CH4 productions related to the primary production cause surface CH4 accumulation. CH4 shows a decrease in concentration and an increase in δ13C-CH4 and δD-CH4 values from the surface to the depth of about 1000 m. The relationship between δ13C-CH4 values and CH4 concentration indicates that the isotopic enrichment of CH4 reflects microbial oxidation of CH4 with isotopic fractionation during vertical transport via vertical sinking and/or zooplankton migration. East of 120°W, δ13C-CH4 values at around 1000 m exceed -30.0‰. The relationships among the δ13C-CH4 values, CH4 concentrations, and oxygen concentrations indicate that the 13C-enriched CH4 originates not only from in situ CH4 production and oxidation but also from CH4 transported from the eastern margin off Peru. Furthermore, at a site near the Central Lau Spreading Centers in the Lau Basin, high δ13C-CH4 values (up to -21.4‰) are observed in the benthic water, suggesting a hydrothermal field source.
    Full-text · Article · Aug 2014 · Chemical Geology
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    ABSTRACT: Environmental investigation of the Early Cambrian is assessed through molecular fossils based from three core samples drilled in the Three Gorges area, South China. The core samples record environmental information dating from the earliest part of the Early Cambrian to nearly the end of the Early Cambrian, making this investigation unique from previous investigations because a more far-reaching temporal record is assessed. This record includes unusually high abundance of longer chain n-alkanes (LCA) around C-n(27), having been recognized in the earliest Cambrian strata. Based on no odd-even preference of the LCA and comparisons to hydrocarbons in cultured cells of microorganisms, the LCA may be derived from hydrocarbons of sulfate reducing bacteria. On the other hand, a phototrophic origin of the SCA is supported by a clear positive correlation with pristane. The relative abundance of LCA to SCA increased in the earliest Cambrian, suggesting predominance of sulfate reducers against phototrophs. This LCA abundance with simultaneous decrease of pristane/phytane may indicate a euxinic condition, where sulfate reducers remineralize organic carbon in the water column. This is consistent with the concomitant decrease of the delta C-13(carb) value probably due to a massive reoxidation of organic carbon. The inferred reducing condition is further supported by the enhanced appearance of squalane that could be derived from an Archeal lipid. Our results suggest the euxinia of the Yangtze platform in the earliest Cambrian that may relate to the emergence of the large nektonic animals which require sufficient level of free oxygen.
    Full-text · Article · Apr 2014 · Gondwana Research
  • Keita YAMADA · Alexis GILBERT · Naohiro YOSHIDA
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    ABSTRACT: A stable isotopic composition in materials has provided useful information concerning their origins and histories. For a carbon isotopic measurement of a single organic molecule, the total organic carbon contained in the molecule should be converted quantitatively to carbon dioxide. Therefore, the carbon isotopic composition represents an average value for the molecules. In principle, information concerning the origins and histories of organic molecules exists at the specific position of the molecule. To retrieve diverse, complex, and specific information of their origins and histories from organic molecules, a measurement of the position-specific carbon isotopic composition is required. So far, a difficult measurement have prevented any development for the position-specific isotopic measurement. Here, we describe recent advances in analytical instrumentation concerning a position-specific carbon isotopic measurement for organic molecules using gas chromatography-combustion-isotope ratio mass spectrometry combined with an on-line pyrolysis technique.
    No preview · Article · Mar 2014 · Bunseki kagaku
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    ABSTRACT: Serpentinite-hosted hydrothermal systems have attracted considerable attention as sites of abiotic organic synthesis and as habitats for the earliest microbial communities. Here, we report a systematic isotopic study of a new serpentinite-hosted system: the Hakuba Happo hot spring in the Shiroumadake area, Japan ( , ). We collected water directly from the hot spring from two drilling wells more than 500 m deep; all water samples were strongly alkaline ( ) and rich in H2 (201-664 μmol/L) and CH4 (124-201 μmol/L). Despite the relatively low temperatures (50-60 °C), thermodynamic calculations suggest that the H2 was likely derived from serpentinization reactions. Hydrogen isotope compositions for Happo #1 (Happo #3) were found to be as follows: ( ), ( ), and ( ). The carbon isotope compositions of methane from Happo #1 and #3 were found to be and , respectively. The CH4-H2-H2O hydrogen isotope systematics indicate that at least two different mechanisms were responsible for methane formation. Happo #1 has a similar hydrogen isotope compositions to other serpentinite-hosted systems reported previously. The elevated (with respect to the equilibrium relationship) suggests that the hydrogen of the Happo #1 methane was not sourced from molecular hydrogen but was derived directly from water. This implies that the methane may not have been produced via the Fischer-Tropsch-type (FTT) synthesis but possibly by the hydration of olivine. Conversely, the depleted (with respect to the equilibrium relationship) in Happo #3 suggests the incorporation of biological methane. Based on a comparison of the hydrogen isotope systematics of our results with those of other serpentinite-hosted hydrothermal systems, we suggest that abiotic CH4 production directly from H2O (without mediation by H2) may be more common in serpentinite-hosted systems. Hydration of olivine may play a more significant role in abiotic methane production than previously thought.
    No preview · Article · Jan 2014 · Earth and Planetary Science Letters
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    ABSTRACT: A method for carbon isotope ratio (δ(13)C) analysis was developed for compound-specific isotope analysis of volatiles to compare with the δ(13)C from bulk isotope analyses. The cabon isotope ratio of 2-phenylethanol liberated via enzymatic hydrolysis of 2-phenylethyl β-primeveroside standard was examined first. Isotope fractionations for 2-phenylethyl β-primeveroside from preparative HPLC were also analyzed. Based on the GC-C-IRMS data from 2-phenylethanol, it was possible to derived the conditions for enzyme treatment and preparative HPLC of the glycoconjugates of 2-phenylethanol, (Z)-3-hexenol, and benzyl alcohol isolated from green tea leaves. The volatile compounds liberated by enzymatic hydrolysis were then subjected to GC-C-IRMS. Large variation in the δ(13)C were found for individual volatile compounds compared with bulk analytical data from the bulk leaves. The order of the isotope ratio value was the same for the three compounds isolated from tea leaves produced in seven different geographic areas.
    No preview · Article · Nov 2013 · Journal of Agricultural and Food Chemistry
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    Alexis Gilbert · Keita Yamada · Naohiro Yoshida
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    ABSTRACT: abstract: n-Alkanes are ubiquitous and useful biomarkers in the biogeochemistry field. Their carbon isotope com-position in sedimentary organic matter is therefore of particular importance for inferring their origin. The commonly used technique for d 13 C determination, isotope ratio mass spectrometry (IRMS), gives access to the isotope composition of n-alkanes at the molecular level, but does not provide information on their intramolecular isotope distribution. Here, we evaluate the potential of isotopic 13 C nuclear magnetic resonance (NMR) spectrometry for the determination of the intramolecular isotope composition of long chain n-alkanes (C 11 –C 31). The relative isotope composition of the three terminal carbon positions can be determined with a precision of 1.2‰ or better. The results from commercially available samples show that (i) the intramolecular 13 C isotope distribution is opposite between odd and even numbered n-alkanes in the C 16 –C 31 range and (ii) those in the C 11 –C 15 range show a 13 C depletion of ca. 12‰ in the methyl position and no difference between odd and even numbered compounds. The results are consis-tent with a biological origin of heavy n-alkanes whereas lighter ones are proposed to originate from abiogenic degradation such as thermal cracking. Overall, although only partial intramolecular 13 C patterns are obtained, the approach appears as promising tool in petroleum exploration and in the bio-geochemistry field.
    Full-text · Article · Sep 2013 · Organic Geochemistry
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    ABSTRACT: Isotopic (13)C NMR spectrometry, which is able to measure intra-molecular (13)C composition, is of emerging demand because of the new information provided by the (13)C site-specific content of a given molecule. A systematic evaluation of instrumental behaviour is of importance to envisage isotopic (13)C NMR as a routine tool. This paper describes the first collaborative study of intra-molecular (13)C composition by NMR. The main goals of the ring test were to establish intra- and inter-variability of the spectrometer response. Eight instruments with different configuration were retained for the exercise on the basis of a qualification test. Reproducibility at the natural abundance of isotopic (13)C NMR was then assessed on vanillin from three different origins associated with specific δ(13)Ci profiles. The standard deviation was, on average, between 0.9 and 1.2‰ for intra-variability. The highest standard deviation for inter-variability was 2.1‰. This is significantly higher than the internal precision but could be considered good in respect of a first ring test on a new analytical method. The standard deviation of δ(13)Ci in vanillin was not homogeneous over the eight carbons, with no trend either for the carbon position or for the configuration of the spectrometer. However, since the repeatability for each instrument was satisfactory, correction factors for each carbon in vanillin could be calculated to harmonize the results.
    Full-text · Article · Jul 2013 · Analytica chimica acta
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    ABSTRACT: A new method, combining head-space solid phase micro-extraction (HS-SPME) with an on-line pyrolysis system coupled with Isotope Ratio Mass Spectrometry (IRMS) is developed for the determination of the intramolecular 13C isotope composition of ethanol in aqueous solutions. The δ13C values of the pyrolytic fragments (CO, CH4, C2H4) are shown to be highly reproducible (sd<0.4‰). Furthermore, using 14 ethanol samples of known intramolecular isotope distribution, the CO and CH4 fragments are shown to arise solely from the methylene (CH2OH) and methyl (CH3) carbon atom positions of the starting ethanol, respectively. Although the different steps (extraction and pyrolysis) fractionate between 12C and 13C, the isotopic fractionation is reproducible (sd<0.4‰), allowing correcting factors to be applied in order to back-calculate the original δ13CCH2OH and δ13CCH3 values of ethanol. The method thus allows the determination of the isotope composition of ethanol at the intramolecular and molecular levels, within a single run and a short experimental time (30min), and with a very easy sample preparation. The method is then applied to alcoholic beverages to show its potential for authentication purposes.
    No preview · Article · Jun 2013 · Analytical Chemistry
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    ABSTRACT: The Fukushima nuclear power plant was severely damaged by an earthquake and concomitant tsunami during March 2011. An effect of this disaster was secondary formation of radioactive 35S via the 35Cl(n,p)35S reaction, when neutrons from the partially melted reactor cores activated the coolant sea water. Here we report the first measurements of 35S in sulfate aerosols and rain water collected at six Japanese sampling sites, Hokkaido, Tsukuba, Kashiwa, Fuchu, Yokohama, and Fukushima, during March-September 2011. The measured 35SO42- concentrations in aerosols vary significantly. The Kashiwa (AORI) site shows the highest 35SO42- concentration (6.1 × 104 ± 200 atoms/m3) on 1 April 2011, which is nearly 100 times higher than the natural background activity. Considering the percentage loss of 35SO42- resulting from dry and wet deposition and dilution of the radiation plume in the boundary layer during transport, it was determined that the surface air concentration of 35SO42- at the Fukushima would have been 2.8 × 105 atoms/m3 during the week after the earthquake, which is in agreement with the model prediction [Priyadarshi et al., ]. 35SO42- activity in rain water collected during March-May 2011 at Tokyo Tech Yokohama varies from 1.1 × 105 to 9.8 × 105 atoms/liter, whereas stream water collected near Fukushima was found to have 1.2 × 105 atoms/liter during April. Even after 6 months, 35SO42- activity remains very high (9.9 × 104 ± 770 atoms/m3) in the marine boundary layer in the Fukushima region, which implies that the reactor core was producing radioactive sulfur.
    No preview · Article · Jan 2013 · Journal of Geophysical Research Atmospheres
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    ABSTRACT: The ratio of the measured abundance of (13) C-(18) O bonding CO(2) to its stochastic abundance, prescribed by the δ(13) C and δ(18) O values from a carbonate mineral, is sensitive to its growth temperature. Recently, clumped-isotope thermometry, which uses this ratio, has been adopted as a new tool to elucidate paleotemperatures quantitatively. Clumped isotopes in CO(2) were measured with a small-sector isotope ratio mass spectrometer. CO(2) samples digested from several kinds of calcium carbonates by phosphoric acid at 25 °C were purified using both cryogenic and gas-chromatographic separations, and their isotopic composition (δ(13) C, δ(18) O, Δ(47) , Δ(48) and Δ(49) values) were then determined using a dual-inlet Delta XP mass spectrometer. The internal precisions of the single gas Δ(47) measurements were 0.005 and 0.02 ‰ (1 SE) for the optimum and the routine analytical conditions, respectively, which are comparable with those obtained using a MAT 253 mass spectrometer. The long-term variations in the Δ(47) values for the in-house working standard and the heated CO(2) gases since 2007 were close to the routine, single gas uncertainty while showing seasonal-like periodicities with a decreasing trend. Unlike the MAT 253, the Delta XP did not show any significant relationship between the Δ(47) and δ(47) values. The Delta XP gave results that were approximately as precise as those of the MAT 253 for clumped-isotope analysis. The temporal stability of the Delta XP seemed to be lower, although an advantage of the Delta XP was that no dependency of δ(47) on Δ(47) was found. Copyright © 2012 John Wiley & Sons, Ltd.
    No preview · Article · Jan 2013 · Rapid Communications in Mass Spectrometry
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    ABSTRACT: Highlight: Stable carbon isotope ratio in soil CH4 was examined in alpine ecosystems in China. Alpine meadow and shrub showed different fractionation factors in methane oxidation. In alpine wetland, acetate fermentation was major contributor on methane production. Hummocks showed plant-mediated transport of CH 4 from deep soil to the atmosphere. In hollows, fractional oxidation rate on methane production was estimated at 7-36%. Abstract: To understand the mechanisms of soil CH 4 consumption and production in alpine ecosystems, we for the first time examined the stable carbon isotope ratio of CH 4 (d 13 C-CH 4) at three major grassland vegetation types, alpine meadow, alpine shrub, and alpine wetland, on the Qinghai-Tibetan Plateau. The alpine meadow and shrub showed net CH 4 absorption in their vertical profiles of CH 4 concentration in summer, but a difference in their processes. Whereas the alpine shrub was dominated by CH 4 con-sumption in its soil profile, CH 4 production in the alpine meadow could slightly cancel consumed CH 4 in shallow soil from À0.3 to À0.1 m. This potential CH 4 production can be attributed to the relatively wet soil type of that ecosystem, which might allow methanogenesis to act in moist soil lumps in the shallow layer. The alpine wetland differed in methane production, consumption, and transport pathways between hummock and hollow plots. In summer, both plots were enriched in 13 CeCH 4 in dissolved CH 4 in soil pore water, suggesting that CH 4 production was conducted mainly by acetate fermentation. In autumn, CH 4 production was shifted toward CO 2 /H 2 reduction. Furthermore, in hummocks, plant-mediated transport of CH4 by vascular plants appeared to perform passive CH 4 flow from deep soil to atmosphere, which allowed the produced CH4 to bypass the oxidation zone in shallow soil. In hollows, however, CH 4 produced in shallow soil was subject to simultaneous oxidation. The fractional oxidation rate on gross CH4 production in hollows was estimated by simple mass balance model at 7-17% in summer and 35-36% in autumn.
    Full-text · Dataset · Jan 2013