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.     

Grain-scale heterogeneities in the stable carbon and oxygen isotopic compositions of the international standard calcite materials (NBS 19, NBS 18, IAEA-CO-1, and IAEA-CO-8)

We determined grain-scale heterogeneities (from 6 to 88 microg) in the stable carbon and oxygen isotopic compositions (delta(13)C and delta(18)O) of the international standard calcite materials (NBS 19, NBS 18, IAEA-CO-1, and IAEA-CO-8) using a continuous-flow isotope ratio mass spectrometry (CF-IRMS) system that realizes a simultaneous determination of the delta(13)C and the delta(18)O values with standard deviations (S.D.) of less than 0.05 per thousand for CO(2) gas. Based on the S.D. of the delta(13)C and delta(18)O values determined for CO(2) gases evolved from the different grains of the same calcite material, we found that NBS 19, IAEA-CO-1, and IEAE-CO-8 were homogeneous for delta(13)C (less than 0.10 per thousand S.D.), and that only NBS 19 was homogeneous for delta(18)O (less than 0.14 per thousand S.D.). On the level of single grains, we found that both IAEA-CO-1 and IAEA-CO-8 were heterogeneous for delta(18)O (1.46 per thousand and 0.76 per thousand S.D., respectively), and that NBS 18 was heterogeneous for both delta(13)C and delta(18)O (0.34 per thousand and 0.54 per thousand S.D., respectively). Closer inspection of NBS 18 grains revealed that the highly deviated isotopic compositions were limited to the colored grains. By excluding such colored grains, we could also obtain the homogeneous delta(13)C and delta(18)O values (less than 0.18 per thousand and less than 0.16 per thousand S.D., respectively) for NBS 18. We conclude that NBS 19, IAEA-CO-1, or pure grains in NBS 18 are suitable to be used as the standard reference material for delta(13)C, and that either NBS 19 or pure grains in NBS 18 are suitable to be used as the reference material for delta(18)O during the grain-scale isotopic analyses of calcite.     

Assessing the use of delta(13)C natural abundance in separation of root and microbial respiration in a Danish beech (Fagus sylvatica L.) forest

Our understanding of forest biosphere-atmosphere interactions is fundamental for predicting forest ecosystem responses to climatic changes. Currently, however, our knowledge is incomplete partly due to inability to separate the major components of soil CO(2) effluxes, viz. root respiration, microbial decomposition of soil organic matter and microbial decomposition of litter material. In this study we examined whether the delta(13)C characteristics of solid organic matter and respired CO(2) from different soil-C components and root respiration in a Danish beech forest were useful to provide information on the root respiration contribution to total CO(2) effluxes. The delta(13)C isotopic analyses of CO(2) were performed using a FinniganMAT Delta(PLUS) isotope-ratio mass spectrometer coupled in continuous flow mode to a trace gas preparation-concentration unit (PreCon). Gas samples in 2-mL crimp seal vials were analysed in a fully automatic mode with an experimental standard error +/-0.11 per thousand. We observed that the CO(2) derived from root-free mineral soil horizons (A, B(W)) was more enriched in (13)C (delta(13)C range -21.6 to -21.2 per thousand ) compared with CO(2) derived from root-free humus layers (delta(13)C range -23.6 to -23.4 per thousand ). The CO(2) evolved from root respiration in isolated young beech plants revealed a value intermediate between those for the soil humus and mineral horizons, delta(13)C(root) = -22.2 per thousand, but was associated with great variability (SE +/- 1.0 per thousand ) due to plant-specific differences. delta(13)C of CO(2) from in situ below-ground respiration averaged -22.8 per thousand, intermediate between the values for the humus layer and root respiration, but variability was great (SE +/- 0.4 per thousand ) due to pronounced spatial patterns. Overall, we were unable to statistically separate the CO(2) of root respiration vs. soil organic matter decomposition based solely on delta(13)C signatures, yet the trend in the data suggests that root respiration contributed approximately 43% to total respiration. The vertical gradient in delta(13)C, however, might be a useful tool in partitioning respiration in different soil layers. The experiment also showed an unexpected (13)C-enrichment of CO(2) (>3.5 per thousand ) compared with the total-C signatures in the individual soil-C components. This may suggest that analyses of bulk samples are not representative for the C-pools actively undergoing decomposition.     

Minimization of carbon addition during derivatization of monosaccharides for compound-specific delta13C analysis in environmental research

Little is known about the delta13C composition of monosaccharides representing the largest carbon reservoir in the biosphere. The main reason for this might be that monosaccharides have to be derivatized prior to gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analyses and that a large isotopic correction is necessary for the carbon that has to be added to the original molecule during derivatization, resulting in large uncertainty of the calculated delta13C values of individual monosaccharides. The amount of added derivatization carbon is twice (alditol acetates) or even three times (trimethylsilyl (TMS) derivatives) as high as the amount of the original monosaccharide carbon. In addition, isotope fractionation occurs during acetylation. Therefore, the objectives of this study were (i) to minimize carbon addition during derivatization for GC/C/IRMS measurements of monosaccharides in soil and sediment samples and (ii) to quantify improvements in accuracy and precision of the final results. Minimization of carbon addition was accomplished by derivatization with methylboronic acid (MBA) and TMS thereafter (MBA method). Monosaccharides derivatized with the MBA method instead of TMS reduced the number of added carbon atoms from 2.2-2.7 to 0.3-0.8 per sugar carbon atom. Although the precision of GC/C/IRMS measurements with both methods is comparable (about 0.3 per thousand), delta13C values of an internal standard indicated that the newly developed MBA method is about 2 per thousand more accurate than the TMS method. delta13C comparison between soil samples that differed only slightly in their bulk carbon isotope signature showed that the MBA method is better in proving these small differences on a significant level. Total precision of the whole MBA method including all analytical and calculation steps is better by a factor of almost three than the TMS method.     

Measuring the 13C content of soil-respired CO2 using a novel open chamber system

Carbon dioxide respired by soils comes from both autotrophic and heterotrophic respiration. 13C has proved useful in differentiating between these two sources, but requires the collection and analysis of CO2 efflux from the soil. We have developed a novel, open chamber system which allows for the accurate and precise quantification of the delta13C of soil-respired CO2. The chamber was tested using online analyses, by configuring a GasBench II and continuous flow isotope ratio mass spectrometer, to measure the delta13C of the chamber air every 120 s. CO2 of known delta13C value was passed through a column of sand and, using the chamber, the CO2 concentration stabilized rapidly, but 60 min was required before the delta13C value was stable and identical to the cylinder gas (-33.3 per thousand). Changing the chamber CO2 concentration between 200 and 900 micromol.mol(-1) did not affect the measured delta13C of the efflux. Measuring the delta13C of the CO2 efflux from soil cores in the laboratory gave a spread of +/-2 per thousand, attributed to heterogeneity in the soil organic matter and roots. Lateral air movement through dry sand led to a change in the delta13C of the surface efflux of up to 8 per thousand. The chamber was used to measure small transient changes (+/-2 per thousand) in the delta13C of soil-respired CO2 from a peaty podzol after gradual heating from 12 to 35 degrees C over 12 h. Finally, soil-respired CO2 was partitioned in a labelling study and the contribution of autotrophic and heterotrophic respiration to the total efflux determined. Potential applications for the chamber in the study of soil respiration are discussed.     

delta(13)C- and delta(18)O-values of glycerol of food fats

The average values of carbon and oxygen isotopic contents (delta(13)C and delta(18)O) of 36 glycerol samples from fats have been determined. The examined samples arise from many fats of animal and plant origin, as well as from the three Italian hard cheeses Parmigiano-Reggiano, Grana Padano and Trentingrana. The total (13)C content allows one to distinguish between glycerol from plants with the C-4 carbon fixation pathway (maize, mean delta(13)C = -14.4 per thousand) and that from plants with the C-3 pathway (mean delta(13)C = -30.7 per thousand). The delta(13)C-values of glycerols of animal origin seem to depend on the diet of the animal, as suggested by the mean values -29.6, -29.0 and -25.1 per thousand, respectively, observed for Parmigiano-Reggiano, Trentingrana and Grana Padano. Additionally, the mean total (18)O content of glycerol samples of vegetable origin is approximately 23.8 per thousand, while that from animal fat is 15.1 per thousand. However, the delta(18)O mean values relative to Parmigiano-Reggiano, Grana Padano and Trentingrana are 11.8, 16.0 and 13.8 per thousand, respectively. The combination of the (13)C and (18)O measurements relative to the fat glycerol of the three cheeses might be considered a potential criterion of authentication.     

Off-line pyrolysis and compound-specific stable carbon isotope analysis of lignin moieties: a new method for determining the fate of lignin residues in soil

Off-line pyrolysis was used to liberate lignin moieties from dung and soil and, after trimethylsilylation, the delta(13)C values of these derivatives were determined by gas chromatography-combustion-isotope ratio mass spectrometry. Initial delta(13)C values determined for 4-vinylphenol, syringol, 4-vinylguaiacol, 4-acetylsyringol, 4-vinylsyringol, 4-(2-Z-propenyl)syringol, 4-(2-E-propenyl)syringol and 4-(2-propenone)syringol pyrolysis products of the lignin polyphenol structure from C(4) (delta(13)C(bulk) = -12.6%) and C(3) (delta(13)C(bulk) = -30.1 per thousand) dung confirmed the robust and reproducible nature of the off-line preparation technique. C(4) dung was used as a treatment in a randomised field experiment to assess the short-term sequestration of dung carbon in managed grasslands. Since lignin was on average 3.5 per thousand depleted in (13)C compared with bulk dung delta(13)C values, this may have resulted in an under-estimation of dung C incorporation based on bulk delta(13)C values. Therefore, an investigation of the compound-specific delta(13)C values of dung-derived lignin moieties extracted from soils sampled up to 372 days was undertaken. Delta(13)C values between lignin moieties extracted from treated and untreated soils showed that dung-derived lignin was not especially resistant to degradation and suggested that individual moieties of the lignin macromolecule must: (i) move into soil, (ii) be degraded, or (iii) be transformed diagenetically at different rates. This adds to a gathering body of evidence that lignin is not particularly stable in soils, which has considerable significance for the perceived role of different biochemical components in the cycling of C in soils.     

Relationship between soil organic C degradability and the evolution of the delta13C signature in profiles under permanent grassland

The main objective of this research was to investigate to what extent the potential C dynamics of soil organic matter (SOM) are related to the degree of 13C enrichment with increasing depth in soil profiles under permanent grassland. The evolution of the C content and the 13C natural abundance (delta13C value) of SOM were investigated in three soil profiles (0-40 cm depth) under permanent grassland of varying texture (a loamy sand, a loam and a clay loam soil). The delta13C value of the SOM showed a gradual increase with increasing depth and decreasing C content in the profiles, ranging from 1.9 per thousand (loamy sand soil), 2.9 per thousand (clay loam soil) and 4 per thousand (loam soil) in relation to the delta13C value of SOM at the surface. The relationship between the 13C enrichment and total organic C content at different depths in the profiles (down to 40 cm depth in the loam and clay loam soil, down to 25 cm depth in the loamy sand soil) could be well described by the Rayleigh equation. The enrichment factors epsilon, associated with the Rayleigh approximation of the data, ranged from -1.57 per thousand (clay loam soil) to -1.64 per thousand (loamy sand soil) and -1.91 per thousand (loam soil). The potential C dynamics in four depth intervals from the profiles (0-10, 10-20, 20-30 and 30-40 cm depth) were determined by means of an incubation experiment. The C decomposition rate constants from the four sampling depths in the profiles showed a significant, positive correlation (y = 0.21x + 0.018, R(2) = 0.66, p < 0.005) with the corresponding Deltadelta13C values (change of the delta13C value per depth increment). A better correlation was obtained when only the data from the upper 20 cm in the profiles (y = 0.21x + 0.019, R(2) = 0.78, p < 0.05) were considered. These results suggest that the Deltadelta13C values in the surface layers of profiles under permanent grassland may serve as an indicator of the potential degradability or the stability of the SOM (in terms of C decomposition rate constants).     

Alteration of the carbon and nitrogen stable isotope composition of beef by substitution of grass silage with maize silage

This study investigated the effect of substituting grass silage (C3 photosynthetic plant product) with maize silage (C4 photosynthetic plant product) on the natural abundance carbon (delta13C) and nitrogen (delta15N) stable isotope composition of bovine muscle tissue. Forty-five continental crossbred heifers were assigned to one of three diets consisting of 3 kg of a barley-based concentrate plus grass silage, maize silage or an equal mixture (dry matter basis) of grass silage and maize silage, fed ad libitum, for 167 days. Substitution resulted in less negative delta13C values (P<0.001) in lipid-free muscle and in lipid, and also a lower delta15N (P<0.001) in lipid-free muscle. Feeding of maize silage was clearly reflected in the delta13C of muscle, with each 10% difference in the dietary C4 carbon intake resulting in a 0.9 to 1.0 per thousand shift of delta13C in lipid-free muscle and a 1.0 to 1.2 per thousand in lipid. Minimum detectable mean differences (95% confidence, power 0.80, n=15) in this experiment were about 0.5 per thousand and 1.0 per thousand for delta13C of lipid-free muscle and lipid, respectively, and about 0.5 per thousand for delta15N of lipid-free muscle. The power analysis presented here is useful for estimating minimum isotopic differences that can be detected between any two groups of beef samples with a given number of replicates. It is concluded that carbon stable isotope ratio analysis of meat can be used to quantify C3/C4 dietary constituents in beef production.     

Evolution of the delta13C signature related to total carbon contents and carbon decomposition rate constants in a soil profile under grassland

Evolution of the total carbon (C) content and the (13)C enrichment (delta(13)C signature) of soil organic matter (SOM) with increasing depth in a soil profile under permanent grassland (C(3) vegetation) were investigated. The relationship between the total C content and the delta(13)C signature at different depths in the upper 30 cm of the soil profile could be well fitted by the Rayleigh equation (y = -29.8 - 2.3x, R(2) = 0.95, p < 0.001), describing the enrichment in (13)C as resulting from isotopic fractionation associated with C mineralization (isotope enrichment factor epsilon = -2.3 per thousand). Potential C dynamics of SOM in four depth intervals of the profile (0-10, 10-20, 20-30 and 30-40 cm depth) were investigated through an incubation study. The C decomposition rate constants decreased with increasing sampling depth from 0.0479 yr(-1) (0-10 cm sampling depth) to 0.0256 yr(-1) (30-40 cm sampling depth) and were highly correlated (y = 0.02 + 0.13x, R(2) = 0.93, p < 0.05) with the corresponding deltadelta(13)C values (average change of the delta(13)C signature per depth increment). These results suggest that changes of the delta(13)C signature of SOM in undisturbed soil profiles under continuous C(3) vegetation may serve as an indicator of the variation of SOM quality with increasing depth.     

Methods to adjust for the interference of N2O on delta13C and delta18O measurements of CO2 from soil mineralization

In this paper we present an overview of the present knowledge relating to methods that avoid interference of N2O on delta13C and delta18O measurements of CO2. The main focus of research to date has been on atmospheric samples. However, N2O is predominantly generated by soil processes. Isotope analyses related to soil trace gas emissions are often performed with continuous flow isotope ratio mass spectrometers, which do not necessarily have the high precision needed for atmospheric research. However, it was shown by using laboratory and field samples that a correction to obtain reliable delta13C and delta18O values is also required for a commercial continuous flow isotope ratio mass spectrometer. The capillary gas chromatography column of the original equipment was changed to a packed Porapak Q column. This adaptation resulted in an improved accuracy and precision of delta13C (standard deviation(Ghent): from 0.2 to 0.08 per thousand; standard deviation(Lincoln): from 0.2 to 0.13 per thousand) of CO2 for N2O/CO2 ratios up to 0.1. For delta18O there was an improvement for the standard deviation measured at Ghent University (0.13 to 0.08 per thousand) but not for the measurements at Lincoln University (0.08 to 0.23 per thousand). The benefits of using the packed Porapak Q column compared with the theoretical correction method meant that samples were not limited to small N(2)O concentrations, they did not require an extra N2O concentration measurement, and measurements were independent of the variable isotopic composition of N2O from soil.     

水体中痕量挥发性有机物单体碳同位素组成分析

将固相微萃取(SPME)技术与冷阱富集系统相结合,对水体中痕量挥发性有机物进行了单体碳同位素分析,方法检测限较常规SPME提高了一个数量级。在优化的条件下,对20 μg/L的三氯乙烯/四氯乙烯和10 μg/L的苯/甲苯水溶液进行了单体碳同位素分析,相比于纯溶剂(液相)碳同位素值,顶空(气相)同位素分析误差不超过0.5‰,而样本标准偏差为0.3‰。对某受四氯乙烯污染的北京地下水进行了同位素测定,近污染源点(B408)与远污染源点(B230)四氯乙烯的碳同位素值(δ13C)分别为 -37.8‰和-34.45‰     

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.     

Enhancing the understanding of earthworm feeding behaviour via the use of fatty acid delta13C values determined by gas chromatography-combustion-isotope ratio mass spectrometry

Litter-dwelling (epigeic) Lumbricus rubellus and soil-dwelling (endogeic) Allolobophora chlorotica earthworms were observed aggregating under C(3) (delta(13)C = -31.3 per thousand; delta(15)N = 10.7 per thousand) and C(4) (delta(13)C = -12.6 per thousand; delta(15)N = 7.5 per thousand) synthetic dung pats applied to a temperate grassland (delta(13)C = -30.3 per thousand; delta(15)N = 5.7 per thousand) in an experiment carried out for 372 days. Bulk delta(13)C values of earthworms collected from beneath either C(3) or C(4) dung after 28, 56, 112 and 372 days demonstrated that (i) L. rubellus beneath C(4) dung were significantly (13)C-enriched after 56 days (delta(13)C = -23.8 per thousand) and 112 days (delta(13)C = -22.4 per thousand) compared with those from C(3) dung treatments (56 days, delta(13)C = -26.5 per thousand; 112 days, delta(13)C = -27.0 per thousand), and (ii) A. chlorotica were 2.1 per thousand (13)C-enriched (delta(13)C = -24.2 per thousand) relative to those from C(3) dung (delta(13)C = -26.3 per thousand) treatments after 372 days. Bulk delta(15)N values did not suggest significant uptake of dung N by either species beneath C(3) or C(4) dung, but showed that the endogeic species (total mean delta(15)N = 3.3 per thousand) had higher delta(15)N values than the epigeic species (total mean delta(15)N = 5.4 per thousand). Although the two species exhibited similar fatty acid profiles, individual fatty acid delta(13)C values revealed extensive routing of dietary C into body tissue of L. rubellus, but minor incorporation into A. chlorotica. In particular, the direct incorporation of microbial biomarker fatty acids (iC(17:0), aC(17:0)) from (13)C-labelled dung in situ, the routing of dung C into de novo synthesised compounds (iC(20:4)(omega)(6),C(20:5)(omega)(3), and the assimilation of essential fatty acids ((C(18:1)(omega)(9), C(18:1)(omega(7), C(18:2)(omega(6), C(18:3)(omega)(3)) derived from dung, were determined.     

A selective unsaturated hydrocarbon subtraction technique for stable carbon isotopic analysis of atmospheric methyl chloride, methyl bromide, and C2-C5 saturated hydrocarbons using continuous-flow isotope ratio mass spectrometry

Using continuous-flow isotope ratio mass spectrometry, we have developed a new analytical system which enables us to determine the stable carbon isotopic composition of CH3Cl, CH3Br, and C2-C5 saturated hydrocarbons in gas samples even if they contain substantial amounts of unsaturated hydrocarbons, using an I2O5 reagent for their selective subtraction. The analytical precision of the delta13C determinations is better than 0.5 per thousand for >300 pmolC injections and better than 5 per thousand for 20 pmolC injections. Using the system, delta13C values for CH3Cl and CH3Br were found in burning exhaust that contain a substantial quantity of unsaturated hydrocarbons. CH3Cl and CH3Br measured in exhaust from burning rice plants exhibit highly 13C-depleted values of -56.6 +/- 1.3 per thousand and -48.6 +/- 3.9 per thousand, respectively, while saturated hydrocarbons exhibit delta13C values (-26.4 to -28.9 per thousand) that are comparable with the total delta13C value of the parent material (rice plant; -28.0 per thousand). Using the system, we can determine the delta13C values of methyl halides and hydrocarbons in many kinds of gas samples.     

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.     

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.     

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.     

Two new organic reference materials for delta13C and delta15N measurements and a new value for the delta13C of NBS 22 oil

Analytical grade L-glutamic acid is chemically stable and has a C/N mole ratio of 5, which is close to that of many of natural biological materials, such as blood and animal tissue. Two L-glutamic acid reference materials with substantially different 13C and 15N abundances have been prepared for use as organic reference materials for C and N isotopic measurements. USGS40 is analytical grade L-glutamic acid and has a delta13C value of -26.24 per thousand relative to VPDB and a delta15N value of -4.52 per thousand relative to N2 in air. USGS41 was prepared by dissolving analytical grade L-glutamic acid with L-glutamic acid enriched in 13C and 15N. USGS41 has a delta13C value of +37.76 per thousand and a delta15N value of +47.57 per thousand. The delta13C and delta15N values of both materials were measured against the international reference materials NBS 19 calcium carbonate (delta13C=+1.95 per thousand ), L-SVEC lithium carbonate (delta13C=-46.48 per thousand ), IAEA-N-1 ammonium sulfate (delta15N=0.43 per thousand ), and USGS32 potassium nitrate (delta15N=180 per thousand ) by on-line combustion continuous-flow and off-line dual-inlet isotope-ratio mass spectrometry. Both USGS40 and USGS41 are isotopically homogeneous; reproducibility of delta13C is better than 0.13 per thousand, and that of delta15N is better than 0.13 per thousand in 100-microg amounts. These two isotopic reference materials can be used for (i) calibrating local laboratory reference materials, and (ii) quantifying drift with time, mass-dependent fractionations, and isotope-ratio-scale contraction in the isotopic analysis of various biological materials. Isotopic results presented in this paper yield a delta13C value for NBS 22 oil of -29.91 per thousand, in contrast to the commonly accepted value of -29.78 per thousand for which off-line blank corrections probably have not been quantified satisfactorily.     

Delta13C analyses of calcium carbonate: Comparison between the GasBench and elemental analyzer techniques

Measurements of stable carbon isotopic composition (delta13C) of carbonates or carbonate-rich soils are seldom performed in a continuous-flow isotope ratio mass spectrometer (IRMS) using an elemental analyzer (EA) as an online sample preparation device. Such analyses are routinely carried out with an external precision better than 0.1 per thousand using a GasBench II (GB) sample preparation device coupled online with a continuous-flow IRMS. In this paper, we report and compare delta13C analyses (86 total analyses) of calcium carbonates obtained by using both the GB and the EA. Using both techniques, the delta13C compositions of two in-house carbonate standards (MERCK carbonate and NR calcite) and ten selected carbonate-rich paleosol samples (of variable CaCO3 content) were analyzed, and data are reported in the VPDB scale calibrated against international standards, NBS 18 and 19. For the in-house standards analyzed by both techniques, a precision better than 0.08 per thousand is achieved. The analytical errors (1sigma) computed from multiple analyses of the delta13C of both the MERCK and NR obtained by the above two techniques are nearly identical. In general, the 1sigma (internal error) of paleosol analyses obtained in the GB is better than 0.06 per thousand, whereas that for the analyses in the EA (three repetitive analyses of the same sample) varies in the range 0.05-0.21 per thousand. However, for paleosols having more than 85% CaCO3, 1sigma is better than 0.15 per thousand (similar to the instrument precision), and in this case the delta13C(VPDB) of samples obtained by the GB is similar to that obtained by the EA. Our results suggest that the delta13C of pure calcium carbonate samples can also be analyzed using the EA technique.