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.     

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.     

Automated system for simultaneous analysis of delta(13)C, delta(18)O and CO(2) concentrations in small air samples

In this paper we present an automated system for simultaneous measurement of CO(2) concentration, delta(13)C and delta(18)O from small (<1 mL) air samples in a short period of time (approximately 1 hour). This system combines continuous-flow isotope ratio mass spectrometry (CF-IRMS) and gas chromatography (GC) with an inlet system similar to conventional dual-inlet methods permitting several measurement cycles of standard and sample air. Analogous to the dual-inlet method, the precision of this system increases with the number of replicate cycles measured. The standard error of the mean for a measurement with this system is 0.7 ppm for the CO(2) concentration and 0.05 per thousand for the delta(13)C and delta(18)O with four replicate cycles and 0.4 ppm and 0.03 per thousand respectively with nine replicate cycles. The mean offset of our measurements from NOAA/CMDL analyzed air samples was 0.08 ppm for the CO(2) concentration, 0.01 per thousand for delta(13)C and 0.00 per thousand for delta(18)O. A specific list of the parts and operation of the system is detailed as well as some of the applications for micrometeorological and ecophysiological applications.     

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.     

A gas chromatography/combustion/isotope ratio mass spectrometry system for high-precision delta13C measurements of atmospheric methane extracted from ice core samples

Past atmospheric composition can be reconstructed by the analysis of air enclosures in polar ice cores which archive ancient air in decadal to centennial resolution. Due to the different carbon isotopic signatures of different methane sources high-precision measurements of delta13CH4 in ice cores provide clues about the global methane cycle in the past. We developed a highly automated (continuous-flow) gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) technique for ice core samples of approximately 200 g. The methane is melt-extracted using a purge-and-trap method, then separated from the main air constituents, combusted and measured as CO2 by a conventional isotope ratio mass spectrometer. One CO2 working standard, one CH4 and two air reference gases are used to identify potential sources of isotope fractionation within the entire sample preparation process and to enhance the stability, reproducibility and accuracy of the measurement. After correction for gravitational fractionation, pre-industrial air samples from Greenland ice (1831 +/- 40 years) show a delta13C(VPDB) of -49.54 +/- 0.13 per thousand and Antarctic samples (1530 +/- 25 years) show a delta13C(VPDB) of -48.00 +/- 0.12 per thousand in good agreement with published data.     

Mobile, outdoor continuous-flow isotope-ratio mass spectrometer system for automated high-frequency 13C- and 18O-CO2 analysis for Keeling plot applications

A continuous-flow isotope-ratio mass spectrometer (CF-IRMS, custom-made GasBenchII and Delta(plus)Advantage, ThermoFinnigan) was installed on a grassland site and interfaced with a closed-path infrared gas analyser (IRGA). The CF-IRMS and IRGA were housed in an air-conditioned travel van. Air was sampled at 1.5 m above the 0.07-m tall grassland canopy, drawn through a 17-m long PTFE tube at a rate of 0.25 L s(-1), and fed to the IRGA and CF-IRMS in series. The IRMS was interfaced with the IRGA via a stainless steel capillary inserted 0.5 m into the sample air outlet tube of the IRGA (forming an open split), a gas-tight pump, and a sample loop attached to the eight-port Valco valve of the continuous-flow interface. Air was pumped through the 0.25-mL sample loop at 10 mL s(-1) (a flushing frequency of 40 Hz). Air samples were analysed at intervals of approx. 2.8 min. Whole system precision was tested in the field using air mixed from pure CO2 and CO2-free air by means of mass flow controllers. The standard deviation of repeated single measurements was 0.21-0.07 per thousand for delta13C and 0.34-0.14 per thousand for delta18O of CO2 in air with mixing ratios ranging between 200-800 micromol mol(-1). The CO2 peak area measured by the IRMS was proportional to the CO2 mixing ratio (r2 = 1.00), allowing estimation of sample air CO2 mixing ratio from IRMS data. A 1-day long measurement cycle of CO2, delta13C and delta18O of air sampled above the grassland canopy was used to test the system for Keeling plot applications. Delta18O exhibited a clear diurnal cycle (4 per thousand range), but short-term (1-h interval) variability was small (average SD 0.38 per thousand). Yet, the correlation between delta18O and CO2 mixing ratio was relatively weak, and this was true for both the whole data set and 1-h subsets. Conversely, the delta13C of all 541 samples measured during the 25.2-h interval fitted well the Keeling regression (r2 = 0.99), yielding an intercept of -27.40 per thousand (+/-0.07 per thousand SE). Useful Keeling regressions (r2 > 0.9, average r2 = 0.96) also resulted from data collected over 1-h intervals of the 12-h long twilight and dark period. These indicated that 13C content of ecosystem respiration was approx. constant near -27.6 per thousand. The precision of the present system is similar to that of current techniques used in ecosystem studies which employ flask sampling and a laboratory-based CF-IRMS. Sampling (and measurement) frequency is greatly increased relative to systems based on flask sampling, and sampling time (0.025 s per sample) is decreased. These features increase the probability for sampling the entire CO2 range which occurs in a given time window. The system obviates sample storage problems, greatly minimises handling needs, and allows extended campaigns of high frequency sampling and analysis with minimal attendance.     

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.     

High-precision, automated stable isotope analysis of atmospheric methane and carbon dioxide using continuous-flow isotope-ratio mass spectrometry

Small-scale developments have been made to an off-the-shelf continuous-flow gas chromatography/isotope-ratio mass spectrometry (CF-GC/IRMS) system to allow high-precision isotopic analysis of methane (CH(4)) and carbon dioxide (CO(2)) at ambient concentrations. The repeatability (1sigma) obtainable with this system is 0.05 per thousand for delta(13)C of CH(4), 0.03 per thousand for delta(13)C of CO(2), and 0.05 per thousand for delta(18)O of CO(2) for ten consecutive analyses of a standard tank. An automated inlet system, which allows diurnal studies of CO(2) and CH(4) isotopes, is also described. The improved precision for CH(4) analysis was achieved with the use of a palladium powder on quartz wool catalyst in the combustion furnace, which increased the efficiency of oxidation of CH(4) to CO(2). The automated inlet further improved the precision for both CH(4) and CO(2) analysis by keeping the routine constant. The method described provides a fast turn-around in samples, with accurate, reproducible results, and would allow a long-term continuous record of CH(4) or CO(2) isotopes at a site to be made, providing information about changing sources of the gases both seasonally and interannually.     

Optimization of automated gas sample collection and isotope ratio mass spectrometric analysis of delta(13)C of CO(2) in air

The application of (13)C/(12)C in ecosystem-scale tracer models for CO(2) in air requires accurate measurements of the mixing ratios and stable isotope ratios of CO(2). To increase measurement reliability and data intercomparability, as well as to shorten analysis times, we have improved an existing field sampling setup with portable air sampling units and developed a laboratory setup for the analysis of the delta(13)C of CO(2) in air by isotope ratio mass spectrometry (IRMS). The changes consist of (a) optimization of sample and standard gas flow paths, (b) additional software configuration, and (c) automation of liquid nitrogen refilling for the cryogenic trap. We achieved a precision better than 0.1 per thousand and an accuracy of 0.11 +/- 0.04 per thousand for the measurement of delta(13)C of CO(2) in air and unattended operation of measurement sequences up to 12 h.     

Extraction of CO2 from air samples for isotopic analysis and limits to ultra high precision delta18O determination in CO2 gas.

The determination of delta18O values in CO2 at a precision level of +/-0.02 per thousand (delta-notation) has always been a challenging, if not impossible, analytical task. Here, we demonstrate that beyond the usually assumed major cause of uncertainty - water contamination - there are other, hitherto underestimated sources of contamination and processes which can alter the oxygen isotope composition of CO2. Active surfaces in the preparation line with which CO2 comes into contact, as well as traces of air in the sample, can alter the apparent delta18O value both temporarily and permanently. We investigated the effects of different surface materials including electropolished stainless steel, Duran glass, gold and quartz, the latter both untreated and silanized. CO2 frozen with liquid nitrogen showed a transient alteration of the 18O/16O ratio on all surfaces tested. The time to recover from the alteration as well as the size of the alteration varied with surface type. Quartz that had been ultrasonically cleaned for several hours with high purity water (0.05 microS) exhibited the smallest effect on the measured oxygen isotopic composition of CO2 before and after freezing. However, quartz proved to be mechanically unstable with time when subjected to repeated large temperature changes during operation. After several days of operation the gas released from the freezing step contained progressively increasing trace amounts of O2 probably originating from inclusions within the quartz, which precludes the use of quartz for cryogenically trapping CO2. Stainless steel or gold proved to be suitable materials after proper pre-treatment. To ensure a high trapping efficiency of CO2 from a flow of gas, a cold trap design was chosen comprising a thin wall 1/4" outer tube and a 1/8" inner tube, made respectively from electropolished stainless steel and gold. Due to a considerable 18O specific isotope effect during the release of CO2 from the cold surface, the thawing time had to be as long as 20 min for high precision delta18O measurements. The presence of traces of air in almost all CO2 gases that we analyzed was another major source of error. Nitrogen and oxygen in the ion source of our mass spectrometer (MAT 252, Finnigan MAT, Bremen, Germany) give rise to the production of NO2 at the hot tungsten filament. NO2+ is isobaric with C16O18O+ (m/z 46) and interferes with the delta18O measurement. Trace amounts of air are present in CO2 extracted cryogenically from air at -196 degrees C. This air, trapped at the cold surface, cannot be pumped away quantitatively. The amount of air present depends on the surface structure and, hence, the alteration of the measured delta18O value varies with the surface conditions. For automated high precision measurement of the isotopic composition of CO2 of air samples stored in glass flasks an extraction interface ('BGC-AirTrap') was developed which allows 18 analyses (including standards) per day to be made. For our reference CO2-in-air, stored in high pressure cylinders, the long term (>9 months) single sample precision was 0.012 per thousand for delta13C and 0.019 per thousand for delta18O.     

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.     

Stable carbon and oxygen isotopic determination of sub-microgram quantities of CaCO3 to analyze individual foraminiferal shells

We have developed an analytical system to determine stable isotopic compositions (delta13C and delta18O) of sub-microgram quantities of CaCO3 for the purpose of analyzing individual foraminiferal shells, using continuous-flow isotope ratio mass spectrometry (CF-IRMS). The system consists of a micro-volume CaCO3 decomposition tube, stainless steel CO2 purification vacuum line with a quantity-regulating unit, helium-purged CO2 purification line, gas chromatograph, and a CF-IRMS system. By using this system, we can determine stable carbon and oxygen isotopic compositions as low as 0.2 microg of CaCO3, with standard deviations of +/-0.10 per thousand for delta13C and +/-0.18 per thousand for delta18O within a 4-h reaction time and 30-min analysis period.     

A rapid and precise technique for measuring delta(13)C-CO(2) and delta(18)O-CO(2) ratios at ambient CO(2) concentrations for biological applications and the influence of container type and storage time on the sample isotope ratios

A simple modification to a commercially available gas chromatograph isotope ratio mass spectrometer (GC/IRMS) allows rapid and precise determination of the stable isotopes ((13)C and (18)O) of CO(2) at ambient CO(2) concentrations. A sample loop was inserted downstream of the GC injection port and used to introduce small volumes of air samples into the GC/IRMS. This procedure does not require a cryofocusing step and significantly reduces the analysis time. The precisions for delta(13)C and delta(18)O of CO(2) at ambient concentration were +/-0.164 and +/-0.247 per thousand, respectively. This modified GC/IRMS was used to test the effects of storage on the (18)O and (13)C isotopic ratios of CO(2) at ambient concentrations in four container types. On average, the change in the (13)C-CO(2) and (18)O-CO(2) ratios of samples after one week of storage in glass vials equipped with butyl rubber stoppers (Bellco Glass Inc.) were depleted by 0.12 and by 0.20 per thousand, respectively. The (13)C ratios in aluminum canisters (Scotty II and IV, Scott Specialty Gasses) after one month of storage were depleted, on average, by 0.73 and 2.04 per thousand, respectively, while the (18)O ratios were depleted by 0.38 and 1.20 per thousand for the Scotty II and IV, respectively. After a month of storage in electropolished containers (Summa canisters, Biospheric Research Corporation), the (13)C-CO(2) and (18)O-CO(2) ratios were depleted, on average, by 0.26 and enriched by 0.30 per thousand, respectively, close to the precision of measurements. Samples were collected at a mature hardwood forest for CO(2) concentration determination and isotopic analysis. A comparison of CO(2) concentrations determined with an infrared gas analyzer and from sample voltages, determined on the GC/IRMS concurrent with the isotopic analysis, indicated that CO(2) concentrations can be determined reliably with the GC/IRMS technique. The (13)C and (18)O ratios of nighttime ecosystem-respired CO(2), determined from the intercept of Keeling plots, were -26.11 per thousand (V-PDB) and -8.81 per thousand (V-PDB-CO(2)), respectively.     

Optimized sample preparation for isotopic analyses of CO2 in air: systematic study of precision and accuracy dependence on driving variables during CO2 purification process

A systematic analysis of efficiency, reproducibility and accuracy of cryogenic purification of CO(2) from air samples for isotopic analyses is presented. The technical characteristics of the cryogenic line are given in detail. To study the cryogenic process, three different operating parameters are considered: flow rate of the gas entering the line, pressure of the gas in the line, and CO(2)-trap shape. Experimental results demonstrate that efficiency, reproducibility and accuracy strongly depend on the CO(2)trap shape. Moreover, a dependence of reproducibility and accuracy on the flow rate of the gas is found, but not on its pressure. High precision (< or =0.02 per thousand for delta(13)C and < or =0.05 per thousand for delta(18)O) and good accuracy (<0.09 per thousand for delta(13)C and <0.14 per thousand for delta(18)O) is achieved after applying the N(2)O correction.     

Evaluating high time-resolved changes in carbon isotope ratio of respired CO2 by a rapid in-tube incubation technique

Recent insights into fractionation during dark respiration and rapid dynamics in isotope signatures of leaf- and ecosystem-respired CO(2) indicate the need for new methods for high time-resolved measurements of the isotopic signature of respired CO(2) (delta(13)C(res)). We present a rapid and simple method to analyse delta(13)C(res) using an in-tube incubation technique and an autosampler for small septum-capped vials. The effect of storage on the delta(18)O and delta(13)C ratios of ambient CO(2) concentrations was tested with different humidity and temperatures. delta(13)C ratios remained stable over 72 h, whereas delta(18)O ratios decreased after 24 h. Storage at 4 degrees C improved the storage time for delta(18)O. Leaves or leaf discs were incubated in the vials, flushed with CO(2)-free air and respired CO(2) was automatically sampled within 5 min on a microGas autosampler interfaced to a GV-Isoprime isotope ratio mass spectrometer. Results were validated by simultaneous on-line gas-exchange measurements of delta(13)C(res) of attached leaves. This method was used to evaluate the short-term (5-60 min) and diurnal dynamics of delta(13)C(res) in an evergreen oak (Quercus ilex) and a herb (Tolpis barbata). An immediate depletion of 2-4 per thousand from the initial delta(13)C(res) value occurred during the first 30 min of darkening. Q. ilex exhibited further a substantial diurnal enrichment in delta(13)C(res) of 8 per thousand, followed by a progressive depletion during the night. In contrast, T. barbata did not exhibit a distinct diurnal pattern. This is in accordance with recent theory on fractionation in metabolic pathways and may be related to the different utilisation of the respiratory substrate in the fast-growing herb and the evergreen oak. These data indicate substantial and rapid dynamics (within minutes to hours) in delta(13)C(res), which differed between species and probably the growth status of the plant. The in-tube incubation method enables both high time-resolved analysis and extensive sampling across different organs, species and functional types.     

Stable carbon and oxygen isotopic analysis of carbon monoxide in natural waters

Techniques have been developed to allow on-line simultaneous analysis of concentration and stable isotopic compositions ((13)C and (18)O) of dissolved carbon monoxide (CO) in natural water, using continuous-flow isotope ratio mass spectrometry (CF-IRMS). The analytical system consisted sequentially of a He-sparging bottle of water, a gas dryer, CO(2)-trapping stage using both Ascarite trap and silica-gel packed gas chromatography (GC), on-line oxidation to CO(2) using the Schütze reagent, cryofocusing, GC purification using a capillary column and measurement by CF-IRMS. Each sample analysis takes about 40 minutes. The detection limit with delta(13)C standard deviation of 0.5 per thousand is 300 pmol and that with delta(18)O deviation of 1.0 per thousand is 750 pmol. Analytical blanks associated with these methods are 21+/-9 pmol. The procedures are evaluated through analyses of temporally varying concentration and isotopic compositions of CO in an artificial lake on the university campus. The delta(13)C and delta(18)O values of CO showed wide variation in accordance with diurnal variation of CO concentration, probably due to significant isotopic effects during photochemical production and microbial oxidation of CO in the aquatic environment. The delta(13)C and delta(18)O values of CO should be a useful tool in studies of the mechanism and pathways of CO production and consumption in natural waters.     

A new method to determine the 17O isotopic abundance in CO2 using oxygen isotope exchange with a solid oxide.

This paper discusses a simple method to determine 17O isotope excess or deficiency ('mass-independent isotopic composition') in CO2 gas. When applying conventional mass spectrometry of CO2 (m/z 44, 45 and 46) to determine the 17O/16O ratio, the 13C/12C ratio has to be established separately. This can be achieved by analysing an aliquot of sample CO2 before and after subjecting it to oxygen isotope exchange with a pool of oxygen with 'normal' 17O/16O ratio, i.e. with Delta17O approximately equal to delta17O-0.516 x delta18O = 0. Cerium oxide has been shown to be practically well suited for the exchange of CO2 oxygen; the reagent is safe and does not produce any contamination. The CO2-CeO2 exchange reaction has 99.8 +/- 0.7% recovery yield. At 650 degrees C this reaction reaches equilibrium in 30 min and, as tested, results in complete oxygen replacement. Delta17O determinations depend on accuracy of CO2 delta measurements: the repeatability of +/-0.015 per thousand (1sigma) in delta(45)R and delta(46)R determination relative to the working reference results in an error of Delta17O as small as +/-0.33 per thousand. Such a precision is sufficient for Delta17O determination in stratospheric CO2. The calculated Delta17O value systematically depends on absolute 17R and 13R ratios in isotopic reference materials, which are presently not yet known with certainty (the 17R value is most important), and may be inadequate for 17O-correction with a = 0.516. Within the present uncertainty, Delta17O determined in 17O-enriched CO2 agrees with the value directly measured in the enriched O2 from which this CO2 was produced. Besides Delta17O determination, investigated CO2-CeO2 equilibration may have several other implications. Fast, complete isotopic exchange of CO2 by reaction with CeO2 may also be employed to get reproducible 17O-correction and, hence, to better monitor small delta13C shifts and to isotopically equilibrate mixtures of CO2 gases.     

Continuous flow stable isotope methods for study of delta(13)C fractionation during halomethane production and degradation

Gas chromatography/mass spectrometry/isotope ratio mass spectrometry (GC/MS/IRMS) methods for delta(13)C measurement of the halomethanes CH(3)Cl, CH(3)Br, CH(3)I and methanethiol (CH(3)SH) during studies of their biological production, biological degradation, and abiotic reactions are presented. Optimisation of gas chromatographic parameters allowed the identification and quantification of CO(2), O(2), CH(3)Cl, CH(3)Br, CH(3)I and CH(3)SH from a single sample, and also the concurrent measurement of delta(13)C for each of the halomethanes and methanethiol. Precision of delta(13)C measurements for halomethane standards decreased (+/-0.3, +/-0.5 and +/-1.3 per thousand) with increasing mass (CH(3)Cl, CH(3)Br, CH(3)I, respectively). Given that carbon isotope effects during biological production, biological degradation and some chemical (abiotic) reactions can be as much as 100 per thousand, stable isotope analysis offers a precise method to study the global sources and sinks of these halogenated compounds that are of considerable importance to our understanding of stratospheric ozone destruction.     

Stable isotope natural abundance of nitrous oxide emitted from Antarctic tundra soils: effects of sea animal excrement depositions

Nitrous oxide (N2O), a greenhouse gas, is mainly emitted from soils during the nitrification and denitrification processes. N2O stable isotope investigations can help to characterize the N2O sources and N2O production mechanisms. N2O isotope measurements have been conducted for different types of global terrestrial ecosystems. However, no isotopic data of N2O emitted from Antarctic tundra ecosystems have been reported although the coastal ice-free tundra around Antarctic continent is the largest sea animal colony on the global scale. Here, we report for the first time stable isotope composition of N2O emitted from Antarctic sea animal colonies (including penguin, seal and skua colonies) and normal tundra soils using in situ field observations and laboratory incubations, and we have analyzed the effects of sea animal excrement depositions on stable isotope natural abundance of N2O. For all the field sites, the soil-emitted N2O was 15N- and 18O-depleted compared with N2O in local ambient air. The mean delta values of the soil-emitted N2O were delta15N = -13.5 +/- 3.2 per thousand and delta18O = 26.2 +/- 1.4 per thousand for the penguin colony, delta15N = -11.5 +/- 5.1 per thousand and delta18O = 26.4 +/- 3.5 per thousand for the skua colony and delta15N = -18.9 +/- 0.7 per thousand and delta18O = 28.8 +/- 1.3 per thousand for the seal colony. In the soil incubations, the isotopic composition of N2O was measured under N2 and under ambient air conditions. The soils incubated under the ambient air emitted very little N2O (2.93 microg N2O--N kg(-1)). Under N2 conditions, much more N2O was formed (9.74 microg N2O--N kg(-1)), and the mean delta15N and delta18O values of N2O were -19.1 +/- 8.0 per thousand and 21.3 +/- 4.3 per thousand, respectively, from penguin colony soils, and -17.0 +/- 4.2 per thousand and 20.6 +/- 3.5 per thousand, respectively, from seal colony soils. The data from in situ field observations and laboratory experiments point to denitrification as the predominant N2O source from Antarctic sea animal colonies.     

Multiple stable isotope (18O, 13C, 15N and 34S) analysis of human hair to identify the recent migrants in a rural community in SW England

Relationships between recent migration and hair delta(18)O values were examined for 40 people living in a rural community in SW England. The isotopic contents of 35 'local' hair samples were compared with those of 5 recently arrived individuals (from Australia, Canada, Chile, Germany and the USA). The hair delta(18)O values of these 'visitors' were +7.9 (Omaha, USA), +11.2 (Jena, Germany), +12.1 (Osorno, Chile), +12.6 (Montreal, Canada) and +14.3 per thousand (Adelaide, Australia). The hair value for the USA visitor (+7.9 per thousand) fell outside the range for the 33 local adult residents, +10.5 to +14.3 per thousand (+12.7 +/- 0.8 per thousand). Hair delta(18)O values did not identify the individuals from Adelaide, Montreal and Osorno as 'visitors', but hair delta(13)C or delta(34)S data did. Combining the hair delta(18)O, delta(13)C and delta(34)S values using principal components analysis (two components explained 89% of the overall variation among the 40 subjects) helped to more clearly distinguish European from non-European individuals, indicating the existence of global overall isotope (geo-origin) relationships.