Isotopic composition of inorganic carbon as an indicator of benzoate degradation by pseudomonas putida: temperature, growth rate and pH effects

Degradation experiments of benzoate by Pseudomonas putida resulted in enzymatic carbon isotope fractionations. However, isotopic temperature effects between experiments at 20 and 30 degrees C were minor. Averages of the last three values of the CO(2) isotopic composition (delta(13)C(CO2(g))) were more negative than the initial benzoate delta(13)C value (-26.2 per thousand Vienna Pee Dee Belenite (VPDB)) by 3.8, 3.4 and 3.2 per thousand at 20, 25 and 30 degrees C, respectively. Although the maximum isotopic temperature difference found was only 0.6 per thousand, more extreme temperature variations may cause larger isotope effects. In order to understand the isotope effects on the total inorganic carbon (TIC), a better measure is to calculate the proportions of the inorganic carbon species (CO(2)(g), CO(2)(aq) and HCO(3)(-)) and to determine their cumulative delta(13)C(TIC). In all three experiments delta(13)C(TIC) was more positive than the initial isotopic composition of the benzoate at a pH of 7. This suggests an uptake of (12)C in the biomass in order to match the carbon balance of these closed system experiments.     

Deuterium and oxygen-18 determination of microliter quantities of a water sample using an automated equilibrator

We describe a modified version of the equilibration method and a correction algorithm for isotope ratio measurements of small quantities of water samples. The deltaD and the delta(18)O of the same water sample can both be analyzed using an automated equilibrator with sample sizes as small as 50 microL. Conventional equilibration techniques generally require water samples of several microL. That limitation is attributable mainly to changes in the isotope ratio ((18)O/(16)O or D/H) of water samples during isotopic exchange between the equilibration gas (CO(2) or H(2)) and water, and therefore the technique for microL quantities of water requires mass-balance correction using the water/gas (CO(2) or H(2)) mole ratio to correct this isotopic effect. We quantitatively evaluate factors controlling the variability of the isotopic effect due to sample size. Theoretical consideration shows that a simple linear equation corrects for the effects without determining parameters such as isotope fractionation factors and water/gas mole ratios. Precisions (1-sigma) of 50-microL meteoric water samples whose isotopic compositions of -1.4 to -396.2 per thousand for deltaD are +/-0.5 to +/-0.6 per thousand, and of -0.37 to -51.37 per thousand for delta(18)O are +/-0.01 to +/-0.11 per thousand.     

A simple method for oxygen-18 determination of milligram quantities of water using NaHCO3 reagent

This paper presents a modified H(2)O-CO(2) equilibration method for stable oxygen isotopic composition (delta(18)O) analysis of water. This method enables rapid and simple delta(18)O analysis of milligram quantities of water, by employing solid reagent NaHCO(3) as the CO(2) source, a small (0.6 mL) glass vial for the equilibration chamber, and an isotope-monitoring gas chromatography/mass spectrometry (irm-GC/MS) system for delta 18O(CO2) analysis. This method has several advantages, including simple handling for the H(2)O-CO(2) equilibration (without purging and/or evacuation treatments), rapid and easy delta(18)O analysis of equilibrated CO(2), and highly sensitive and highly precise delta(18)O analysis of H(2)O, using samples as small as 10 mg and with a precision of less than +/-0.12 per thousand. The time needed to attain oxygen isotopic equilibration between CO(2) and water is also comparable (17 h for 10 mg H(2)O and 10 h for 100 mg H(2)O) to other previous methods using CO(2) gas for the CO(2) source. The extent of delta(18)O variation of sample water from its initial delta(18)O value due to isotope exchange with added NaHCO(3) is also discussed. It is concluded that the correction needed is negligible (less than 0.1 per thousand ) as long as the oxygen atom ratio (O(NaHCO3)/O(H2O)) is less than 3.3 +/- 10(-3) and provided the delta18O(H2O) determination is made by comparing delta(18)O of CO(2) equilibrated with sample water and that equilibrated with standard water of a moderately close delta(18)O value, less than 30 per thousand difference.     

GasBench/isotope ratio mass spectrometry: a carbon isotope approach to detect exogenous CO(2) in sparkling drinks

A new procedure for the determination of carbon dioxide (CO(2)) (13)C/(12)C isotope ratios, using direct injection into a GasBench/isotope ratio mass spectrometry (GasBench/IRMS) system, has been developed to improve isotopic methods devoted to the study of the authenticity of sparkling drinks. Thirty-nine commercial sparkling drink samples from various origins were analyzed. Values of delta(13)C(cava) ranged from -20.30 per thousand to -23.63 per thousand, when C3 sugar addition was performed for a second alcoholic fermentation. Values of delta(13)C(water) ranged from -5.59 per thousand to -6.87 per thousand in the case of naturally carbonated water or water fortified with gas from the spring, and delta(13)C(water) ranged from -29.36 per thousand to -42.09 per thousand when industrial CO(2) was added. It has been demonstrated that the addition of C4 sugar to semi-sparkling wine (aguja) and industrial CO(2) addition to sparkling wine (cava) or water can be detected. The new procedure has advantages over existing methods in terms of analysis time and sample treatment. In addition, it is the first isotopic method developed that allows (13)C/(12)C determination directly from a liquid sample without previous CO(2) extraction. No significant isotopic fractionation was observed nor any influence by secondary compounds present in the liquid phase.     

Dynamic calibration and dissolved gas analysis using membrane inlet mass spectrometry for the quantification of cell respiration

A membrane inlet mass spectrometer connected to a miniaturized reactor was applied for dynamic dissolved gas analysis. Cell samples were taken from 7 mL shake flask cultures of Corynebacterium glutamicum ATCC 13032, and transferred to the 12 mL miniaturized reactor. There, oxygen uptake and carbon dioxide and its mass isotopomer production rates were determined using a new experimental procedure and applying nonlinear model equations. A novel dynamic method for the calibration of the membrane inlet mass spectrometer using first-order dynamics was developed. To derive total dissolved concentration of all carbon dioxide species (C(T)) from dissolved carbon dioxide concentration ([CO(2)](aq)), the ratio of C(T) to [CO(2)](aq) was determined by nonlinear parameter estimation, whereas the mass transfer coefficient of CO(2) was determined by the Wilke-Chang correlation. Subsequently, the suitability of the model equations for respiration measurements was examined using residual analysis and the Jarque-Bera hypothesis test. The resulting residuals were found to be random with normal distribution, which proved the adequacy of the application of the model for cell respiration analysis. Hence, dynamic changes in respiration activities could be accurately analyzed using membrane inlet mass spectrometry with the novel calibration method.     

An automated system for stable isotope and concentration analyses of CO2 from small atmospheric samples

We have developed an automated, continuous-flow isotope ratio mass spectrometry (CF-IRMS) system for the analysis of delta(13)C, delta(18)O, and CO(2) concentration (micromol mol(-1)) ([CO(2)]) from 2 mL of atmospheric air. Two replicate 1 mL aliquots of atmospheric air are sequentially sampled from fifteen 100 mL flasks. The atmospheric sample is inserted into a helium stream and sent through a gas chromatograph for separation of the gases and subsequent IRMS analysis. Two delta(13)C and delta(18)O standards and five [CO(2)] standards are run with each set of fifteen samples. We obtained a precision of 0.06 per thousand, 0.11 per thousand, and 0.48 micromol mol(-1) for delta(13)C, delta(18)O, and [CO(2)], respectively, by analyzing fifty 100 mL samples filled from five cylinders with a [CO(2)] range of 275 micromol mol(-1). Accuracy was determined by comparison with established methods (dual-inlet IRMS, and nondispersive infrared gas analysis) and found to have a mean offset of 0.00 per thousand, -0.09 per thousand, and -0.26 micromol mol(-1) for delta(13)C and delta(18)O, and [CO(2)], respectively.     

Synthesis and characterization of reduced heme and heme/copper carbonmonoxy species

Carbon monoxide readily binds to heme and copper proteins, acting as a competitive inhibitor of dioxygen. As such, CO serves as a probe of protein metal active sites. In our ongoing efforts to mimic the active site of cytochrome c oxidase, reactivity toward carbon monoxide offers a unique opportunity to gain insight into the binding and spectroscopic characteristics of synthetic model compounds. In this paper, we report the synthesis and characterization of CO-adducts of ((5/6)L)Fe(II), [((5/6)L)Fe(II)...Cu(I)](B(C(6)F(5))(4)), and [(TMPA)Cu(I)(CH(3)CN)](B(C(6)F(5))(4)), where TMPA = tris(2-pyridylmethyl)amine and (5/6)L = a tetraarylporphyrinate tethered in either the 5-position ((5)L) or 6-position ((6)L) to a TMPA copper binding moiety. Reaction of ((5/6)L)Fe(II) [in THF (293 K): UV-vis 424 (Soret), 543-544 nm; (1)H NMR delta(pyrrole) 52-59 ppm (4 peaks); (2)H NMR (from ((5)L-d(8))Fe(II)) delta(pyrrole) 53.3, 54.5, 55.8, 56.4 ppm] with CO in solution at RT yielded ((5/6)L)Fe(II)-CO [in THF (293 K): UV-vis 413-414 (Soret), 532-533 nm; IR nu(CO)(Fe) 1976-1978 cm(-1); (1)H NMR delta(pyrrole) 8.8 ppm; (2)H NMR (from ((5)L-d(8))Fe(II)-CO) delta(pyrrole) 8.9 ppm; (13)C NMR delta((CO)Fe) 206.8-207.1 ppm (2 peaks)]. Experiments repeated in acetonitrile, acetone, toluene, and dichloromethane showed similar spectroscopic data. Binding of CO resulted in a change from five-coordinate, high-spin Fe(II) to six-coordinate, low-spin Fe(II), as evidenced by the upfield shift of the pyrrole resonances to the diamagnetic region ((1)H and (2)H NMR spectra). Addition of CO to [((5/6)L)Fe(II)...Cu(I)](B(C(6)F(5))(4)) [in THF (293 K): UV-vis ((6)L only) 424 (Soret), 546 nm; (1)H NMR delta(pyrrole) 54-59 ppm (multiple peaks); (2)H NMR (from [((5)L-d(8))Fe(II).Cu(I)](B(C(6)F(5))(4))) delta(pyrrole) 53.4 ppm (br)] gave the bis-carbonyl adduct [((5/6)L)Fe(II)-CO...Cu(I)-CO](B(C(6)F(5))(4)) [in THF (293 K): UV-vis ((6)L only) 413 (Soret), 532 nm; IR nu(CO)(Fe) 1971-1973 cm(-1), nu(CO)(Cu) 2091-2093 cm(-1), approximately 2070(sh) cm(-1); (1)H NMR delta(pyrrole) 8.7-8.9 ppm; (2)H NMR (from [((5)L-d(8))Fe(II)-CO...Cu(I)-CO](B(C(6)F(5))(4))) delta(pyrrole) 8.9 ppm; (13)C NMR delta((CO)Fe) 206.8-208.1 ppm (2 peaks), delta((CO)Cu) 172.4 ((5)L), 178.2 ((6)L) ppm]. Experiments in acetonitrile, acetone, and toluene exhibited spectral features similar to those reported. The [((5/6)L)Fe(II)-CO.Cu(I)-CO](B(C(6)F(5))(4)) compounds yielded (CO)(Fe) spectra analogous to those seen for ((5/6)L)Fe(II)-CO and (CO)(Cu) spectra similar to those seen for [(TMPA)Cu(I)-CO](B(C(6)F(5))(4)) [in THF (293 K): IR nu(CO)(Cu) 2091 cm(-1), approximately 2070(sh) cm(-1); (13)C NMR delta((CO)Cu) 180.3 ppm]. Additional IR studies were performed in which the [((5)L)Fe(II)-CO...Cu(I)-CO](B(C(6)F(5))(4)) in solution was bubbled with argon in an attempt to generate the iron-only mono-carbonyl [((5)L)Fe(II)-CO.Cu(I)](B(C(6)F(5))(4)) species; in coordinating solvent or with axial base present, decreases in characteristic IR-band intensities revealed complete loss of CO from copper and variable loss of CO from the heme.     

Determination of stable carbon isotopes of organic acids and carbonaceous aerosols in the atmosphere

A wet oxidation method for the compound-specific determination of stable carbon isotopes (delta(13)C) of organic acids in the gas and aerosol phase, as well as of water-soluble organic carbon (WSOC), is presented. Sampling of the organic acids was done using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to an ion chromatography (IC) system. The method allows for compound-specific stable carbon isotope analysis by collecting different fractions of organic acids at the end of the IC system using a fraction collector. delta(13)C analyses of organic acids were conducted by oxidizing the organic acids with sodium persulfate at a temperature of 100 degrees C and determining the delta(13)C value of the resulting carbon dioxide (CO(2)) with an isotope ratio mass spectrometer. In addition, analysis of delta(13)C of the WSOC was performed for particulate carbon collected on aerosol filters. The WSOC was extracted from the filters using ultrapure water (MQ water), and the dissolved organic carbon was oxidized to CO(2) using the oxidation method. The wet oxidation method has an accuracy of 0.5 per thousand with a precision of +/-0.4 per thousand and provides a quantitative result for organic carbon with a detection limit of 150 ng of carbon.     

Dynamic headspace: a single-step extraction for isotopic analysis of microg/L concentrations of dissolved chlorinated ethenes

In this study a dynamic headspace method was developed to measure the carbon isotope values of dissolved chlorinated ethenes at microg/L concentrations. A gas chromatograph/combustion/isotope ratio mass spectrometer (GC/C/IRMS) was modified to include a headspace extraction system followed by a cryogenic trap. Extracting headspace from a 160 mL vial with 80 mL of aqueous solution and 40 g of NaCl for 8-12 min resulted in accurate and reproducible delta13C values for trichloroethene (TCE) and cis-1,2-dichloroethene (cDCE) at concentrations of 50-75 microg/L. Based on these results a conservative lower limit of quantitation of 38 microg/L can be calculated for these compounds. For more volatile compounds such as tetrachloroethene (PCE) and vinyl chloride (VC), field data analyzed using this method indicate a lower limit of quantitation in the tens of microg /L range.     

A portable automated system for trace gas sampling in the field and stable isotope analysis in the laboratory

A computer-controllable mobile system is presented which enables the automatic collection of 33 air samples in the field and the subsequent analysis for delta13C and delta18O stable isotope ratios of a carbon-containing trace gas in the laboratory, e.g. CO2, CO or CH4. The system includes a manifold gas source input for profile sampling and an infrared gas analyzer for in situ CO2 concentration measurements. Measurements of delta13C and delta18O of all 33 samples can run unattended and take less than six hours for CO2. Laboratory tests with three gases (compressed air with different pCO2 and stable isotope compositions) showed a measurement precision of 0.03 per thousand for delta13C and 0.02 per thousand for delta18O of CO2 (standard error (SE), n = 11). A field test of our system, in which 66 air samples were collected within a 24-hour period above grassland, showed a correlation of 0.99 (r2) between the inverse of pCO2 and delta13C of CO2. Storage of samples until analysis is possible for about 1 week; this can be an important factor for sampling in remote areas. A wider range of applications in the field is open with our system, since sampling and analysis of CO and CH4 for stable isotope composition is also possible. Samples of compressed air had a measurement precision (SE, n = 33) of 0.03 per thousand for delta13C and of 0.04 per thousand for delta18O on CO and of 0.07 per thousand for delta13C on CH4. Our system should therefore further facilitate research of trace gases in the context of the carbon cycle in the field, and opens many other possible applications with carbon- and possibly non-carbon-containing trace gases.     

A field and laboratory method for monitoring the concentration and isotopic composition of soil CO2

The stable isotope composition of nmol size gas samples can be determined accurately and precisely using continuous flow isotope ratio mass spectrometry (IRMS). We have developed a technique that exploits this capability in order to measure delta13C and delta18O values and, simultaneously, the concentration of CO2 in sub-mL volume soil air samples. A sampling strategy designed for monitoring CO2 profiles at particular locations of interest is also described. This combined field and laboratory technique provides several advantages over those previously reported: (1) the small sample size required allows soil air to be sampled at a high spatial resolution, (2) the field setup minimizes sampling times and does not require powered equipment, (3) the analytical method avoids the introduction of air (including O2) into the mass spectrometer thereby extending filament life, and (4) pCO2, delta13C and delta18O are determined simultaneously. The reproducibility of measurements of CO2 in synthetic tank air using this technique is: +/-0.08 per thousand (delta13C), +/-0.10 per thousand (delta18O), and +/-0.7% (pCO2) at 5550 ppm. The reproducibility for CO2 in soil air is estimated as: +/-0.06 per thousand (delta13C), +/-0.06 per thousand (delta18O), and +/-1.6% (pCO2). Monitoring soil CO2 using this technique is applicable to studies concerning soil respiration and ecosystem gas exchange, the effect of elevated atmospheric CO2 (e.g. free air carbon dioxide enrichment) on soil processes, soil water budgets including partitioning evaporation from transpiration, pedogenesis and weathering, diffuse solid-earth degassing, and the calibration of speleothem and pedogenic carbonate delta13C values as paleoenvironmental proxies.     

On the N2O correction used for mass spectrometric analysis of atmospheric CO2

To obtain accurate values of delta(13)C(CO(2)) and delta(18)O(CO(2)) on environmental CO(2) by mass spectrometry, the raw isotope data must be corrected for the isobaric N(2)O contribution. This is one of the analytical problems limiting inter-laboratory delta(13)C(CO(2)) data consistency. The key parameter, the N(2)O relative ionisation efficiency (E(N2O)), cannot be determined with sufficient accuracy by direct measurements of pure N(2)O. The determination of (E(N2O)) by analyses on N(2)O--CO(2) mixtures of known isotope composition and mixing proportions has been recently suggested. In this work we propose a new method of N(2)O correction which uses the m/z 30 signal as a measure of the N(2)O/CO(2) ratio, so that determinations of (E(N2O)) and N(2)O content are not required. The method uses the fact that fragment-ion spectra of N(2)O and CO(2) are very specific. The formalism of the correction is considered. Various tests demonstrate that the new method is robust, stable and easy to implement in practice. The effective value (E(N2O)) (the key parameter for the new correction) has to be calibrated on known N(2)O--CO(2) mixtures by measuring (30)R signals only. The method accuracy we presently achieved is around 2.5% and any error which appears to come mostly from our N(2)O--CO(2) mixture preparation. Based on our tests and error considerations, the error of the proposed method that may be achieved is as low as +/-1.5% (relative to the correction magnitude). For tropospheric CO(2) this means +/-0.003 per thousand and +/-0.005 per thousand for delta(13)C(CO(2)) and delta(18)O(CO(2)), respectively. The proposed method may be valuable for small samples where no separate N(2)O determinations are available (e.g. ice core samples and CF-IRMS measurements) as well as for determination of (E(N2O)) and testing the 'traditional' N(2)O correction based on mass balance calculations.     

Oxygen isotope analysis of carbonates in the calcite-dolomite-magnesite solid-solution by high-temperature pyrolysis: initial results

Accurate and efficient measurement of the oxygen isotope composition of carbonates (delta(C) (18)O) based on the mass spectrometric analysis of CO(2) produced by reacting carbonate samples with H(3)PO(4) is compromised by: (1) uncertainties associated with fractionation factors (alpha(CO)(2)C) used to correct measured oxygen isotope values of CO(2)(delta(CO(2)(18)O) to delta(C) (18)O; and (2) the slow reaction rates of many carbonates of geological and environmental interest with H(3)PO(4). In contrast, determination of delta(C) (18)O from analysis of CO produced by high-temperature (>1400 degrees C) pyrolytic reduction, using an elemental analyser coupled to continuous-flow isotope-ratio mass spectrometry (TC/EA CF-IRMS), offers a potentially efficient alternative that measures the isotopic composition of total carbonate oxygen and should, therefore, theoretically be free of fractionation effects. The utility of the TC/EA CF-IRMS technique was tested by analysis of carbonates in the calcite-dolomite-magnesite solid-solution and comparing the results with delta(C) (18)O measured by conventional thermal decomposition/fluorination (TDF) on the same materials. Initial results show that CO yields are dependent on both the chemical composition of the carbonate and the specific pyrolysis conditions. Low gas yields (<100% of predicted yield) are associated with positive (>+0.2 per thousand) deviations in delta(C(TC/EA) (18)O compared with delta(C(TDF) (18)O. At a pyrolysis temperature of 1420 degrees C the difference between delta(C) (18)O measured by TC/EA CF-IRMS and TDF (Delta(C(TC/EA,TDF) (18)O) was found to be negatively correlated with gas yield (r = -0.785) and this suggests that delta(C) (18)O values (with an estimated combined standard uncertainty of +/-0.38 per thousand) could be derived by applying a yield-dependent correction. Increasing the pyrolysis temperature to 1500 degrees C also resulted in a statistically significant correlation with gas yield (r = -0.601), indicating that delta(C) (18)O values (with an estimated uncertainty of +/-0.43 per thousand) could again be corrected using a yield-dependent procedure. Despite significant uncertainty associated with TC/EA CF-IRMS analysis, the magnitude of the uncertainty is similar to that associated with the application of poorly defined values of alpha(CO)(2), (C) used to derive delta(C) (18)O from delta(CO(2) (18)O measured by the H(3)PO(4) method for most common carbonate phases. Consequently, TC/EA CF-IRMS could provide a rapid alternative for the analysis of these phases without any effective deterioration in relative accuracy, while analytical precision could be improved by increasing the number of replicate analyses for both calibration standards and samples. Although automated gas preparation techniques based on the H(3)PO(4) method (ISOCARB, Kiel device, Gas-Bench systems) have the potential to measure delta(CO)(2) (18)O efficiently for specific, slowly reacting phases (e.g. dolomite), problems associated with poorly defined alpha(CO)(2), (C) remain. The application of the Principle of Identical Treatment is not a solution to the analysis of these phases because it assumes that a single fractionation factor may be defined for each phase within a solid-solution regardless of its precise chemical composition. This assumption has yet to be tested adequately.     

气相色谱-稳定同位素质谱法测定溶解无机碳碳同位素

选用NaHCO3配制了浓度分别为0.24、1.19、2.38和4.76 mmol/L的溶解无机碳(D IC)溶液,经过1h、4h、8h和24h不同平衡时间,建立了一种分析D IC碳同位素的方法。不同浓度的D IC样品与其母质NaHCO3的1δ3C值之间的差值仅为(0.2~0.5)‰。通过对照组的实验结果和对空气CO2的碳同位素测试与研究,证明此方法可有效避免实验过程中大气等物质对样品的污染,确保实验结果的准确性。     

Flushing time and storage effects on the accuracy and precision of carbon and oxygen isotope ratios of sample using the Gasbench II technique

Continuous-flow isotope ratio mass spectrometry interfaced with a Gasbench II is used for automated and faster analyses of delta(13)C and delta(18)O in water, carbonate, and air samples that are accurate and highly precise. Prior to online chemistry and measurement using the Gasbench technique, rubber septa-capped glass vials are routinely flushed to remove air. Due to the small amounts of sample gas required for isotope analyses using current techniques, care should be taken to properly flush these vials to avoid contamination of sample gas with air. Our results indicate that isotopic composition of sample CO(2) gas remains constant when 10 mL vials are flushed (rate of 100 mL/min) for > or =600 s, whereas for vials flushed <600 s, the isotopic composition becomes substantially lighter with decreasing time of flushing, which affects the accuracy of analyses. This largely depends on the isotopic composition (and volume) of air that still remains after flushing. This effect is more pronounced on delta(18)O than on delta(13)C of sample CO(2) gas because there is very little carbon in the air. After 24 h storage in vials with punctured septa, both delta(13)C and delta(18)O of CO(2) become isotopically heavier compared with first day analyses, suggesting time-dependent changes in isotopic composition. The magnitude of shift depends on the concentration and the isotopic composition of CO(2) in laboratory air as well as on fractionation due to outflow of sample gas or inflow of air via punctured septa. Contamination of sample gas with air can be observed as a secondary peak on chromatograms that precedes sample peaks, and the intensity of these peaks depends on the amount of air. Such peaks are always present with short flushing times. For accuracy and better precision, irrespective of the magnitude of the secondary peaks, the analyses should be discarded if these appear in the chromatograms.     

δ18O anchoring to VPDB: calcite digestion with 18O-adjusted ortho-phosphoric acid

For anchoring CO(2) isotopic measurements on the δ(18)O(VPD-CO2) scale, the primary reference material (NBS 19 calcite) needs to be digested using concentrated ortho-phosphoric acid. During this procedure, great care must be taken to ensure that the isotopic composition of the liberated gas is accurate. Apart from controlling the reaction temperature to ±0.1 °C, the potential for oxygen isotope exchange between the produced CO(2) and water must be kept to a minimum. The water is usually assumed to reside on the walls in the headspace of the reaction vessel. We demonstrate here that a large fraction of the exchange may also occur with water inside the acid. Our results indicate that both exchange reactions have a significant impact on the results and may have largely been responsible for scale inconsistencies between laboratories in the past. The extent of CO(2)/H(2)O oxygen exchange depends on the concentration (amount of free water) in the acid. For acids with a nominal H(3)PO(4) mass fraction of less than 102%, oxygen isotope exchange can create a substantial isotopic bias during high-precision measurements with the degree of the alteration being proportional to the effective isotopic contrast between the acid and the CO(2) released from the calcite. Water evaporating from the acid at 25 °C has a δ(18)O value of -34.5‰ relative to the isotopic composition of the whole acid. This large fractionation is likely to occur in two steps; by exchange with phosphate, water inside the acid is decreased in oxygen-18 relative to the bulk acid by ～ -22‰. This water is then fractionated further during evaporation. Oxygen exchange with both water inside the acid and water condensate in the headspace can contribute to the measured isotopic signature depending on the experimental parameters. The system employed for this study has been specifically designed to minimize oxygen exchange with water. However, the amount of altered CO(2) for a 95% H(3)PO(4) at 25 °C still accounts for about 3% of the total CO(2) produced from a 40 mg calcite sample, resulting in a δ(18) O range of about 0.8‰ when varying the δ(18)O value of the acid by 25‰. Least biased results for NBS19-CO(2) were obtained for an acid with a δ(18)O value close to +23‰ vs. VSMOW. In contrast, commercial acids from several sources had an average δ(18)O value of +13‰, amounting to a 10‰ offset from the optimal value. This observation suggests that the well-known scale incompatibilities between laboratories could arise from this difference with measurements that may have suffered systematically from non-optimal acid-δ(18)O values, thus producing variable offsets, depending on the experimental details. As a remedy, we suggest that the δ(18)O of phosphoric acid reacted with calcites for establishing a δ(18)O scale anchor be adjusted, and this should reduce the variability of the δ(18)O of CO(2) evolved in acid digestion to less than ±0.05‰. The adjustment should be made by taking into account the difference in δ(18)O between the calcite-CO(2) and the acid, with a target difference of 16‰. With this strategy, agreement between δ(18)O scales based on water, atmospheric CO(2) , and carbonates as well as data compatibility between laboratories may be substantially improved.     

Evidence of CO(2) molecule acting as an electron acceptor on a nanoporous metal-organic-framework MIL-53 or Cr(3+)(OH)(O(2)C-C(6)H(4)-CO(2))

The adsorption mode of CO(2) at low coverage in the nanoporous metal benzenedicarboxylate MIL-53(Cr) or Cr(3+)(OH)(O(2)C-C(6)H(4)-CO(2)) has been identified using IR spectroscopy; the red shift of the nu(3) band and the splitting of the nu(2) mode of CO(2) in addition to the shifts of the nu(OH) and delta(OH) bands of the MIL-53(Cr) hydroxyl groups provide evidence that CO(2) interacts with the oxygen atoms of framework OH groups as an electron-acceptor via its carbon atom; this is the first example of such an interaction between CO(2) and bridged OH groups in a solid.     

Automated analysis of 13C/12C ratios in CO2 and dissolved inorganic carbon for ecological and environmental applications

Stable carbon isotope ratios (13C/12C) are a valuable tool for studying a wide range of environmental processes, including carbon cycling and subsurface microbial activity. Recent advances in automated analysis provide the opportunity to increase greatly the ease and consistency of isotopic analysis. This study evaluated an automated headspace sampler linked to a commercially available CO2 preconcentration system and continuous flow isotope ratio mass spectrometer. Field sampling and analysis methods are illustrated for delta13C of soil respired CO2, from both tracer and natural abundance experiments, and dissolved inorganic carbon from contaminated groundwater. The automated system demonstrated accuracy, precision, and linearity, with standard errors below 0.1 per thousand for replicate gas standards run at concentrations varying five-fold. It measured 40 samples per 10-hour run, with concentrations ranging from ppb to percentage levels. In the field, gas samples were injected into nitrogen-filled autosampler vials, thereby allowing use of small sample volumes, control of analyte concentration, and direct analysis by the automated system with no further preparation. A significant linear relationship between standard concentrations and peak area allows for accurate estimates of sample CO2 concentration from the mass spectrometric data. The ability to analyze multiple small-volume samples with minimal off-line preparation should enhance the application of isotopes to well-replicated field experiments for process-level studies and spatial and temporal scaling.     

Isotopic ((13)C) analysis of dissolved organic carbon in stream water using an elemental analyzer coupled to a stable isotope ratio mass spectrometer

A technique for measurement of the stable isotope composition of dissolved organic carbon (DOC) in stream water, using an elemental analyzer (EA) coupled to an isotope ratio mass spectrometer (IRMS), is described. Stream water samples were concentrated by rotary evaporation, acidified to remove dissolved inorganic carbon (DIC), and dried in silver cups prior to analysis. Precision was evaluated with standards (alanine and humic acid), and with stream water samples with varying (13)C enrichment. Standards and samples were also prepared in sealed quartz tubes for high-temperature combustion (HTC) and analyzed by dual inlet for comparison. The delta(13)C values of natural abundance standards and samples measured by the two techniques differed by 相似文献   

The International Measurement Evaluation Programme,IMEP-8: carbon and oxygen isotope ratios in CO<Subscript>2</Subscript>

In IMEP-8, two samples of high purity CO(2)(g), with different carbon and oxygen isotope ratios were distributed to 27 participants, originating from 14 countries and from various isotopic measurement domains (geochemistry, atmospheric and food chemistry), but particularly set up for food laboratories. In total 19 laboratories reported results. The outcome of this comparison exercise shows that the laboratories reported carbon and oxygen isotope results in good agreement with the reference values across the domains. The reported results for delta(13)C(VPDB) (carbon) for both materials are within 1 per thousand. However, for the reported results of delta(18)O(VPDB) (oxygen) for both materials the overall spread of the reported results is about 11 per thousand. Within this spread two distinct groups of participants can be identified, where the results within each group vary about 2 per thousand. The latter seems to be caused by calculation errors by participants of the reporting delta(18)O(VPDB) values. As requested, participants also reported the isotope amount ratio for carbon and oxygen in the CO(2) samples. For carbon, all reported results for both materials agree with the isotope ratio value, which can be traced back to the value reported by Craig. For oxygen, all results are in good agreement and deviate by a maximum of 0.5% from the reference values measured at IRMM. Work carried out indicates the carbon isotope ratio, for both samples IMEP-8A and IMEP-8B, differ from those reported by Craig by as much as 1.2%. In the case of oxygen, this deviation is far smaller. Both data sets, i.e. the one realised by Craig and the one realised at IRMM, demonstrate traceability to SI. It is clear that both values significantly disagree.