A laboratory comparison of two methods used to estimate the isotopic composition of soil delta13CO2 efflux at steady state

The stable isotopic composition of soil (13)CO(2) flux is important for monitoring soil biological and physical processes. While several methods exist to measure the isotopic composition of soil flux, we do not know how effective each method is at achieving this goal. To provide clear evidence of the accuracy of current measurement techniques we created a column filled with quartz sand through which a gas of known isotopic composition (-34.2 per thousand) and concentration (3,000 ppm) diffused for 7 h. We used a static chamber at equilibrium and a soil probe technique to test whether they could identify the isotopic signature of the known gas source. The static chamber is designed to identify the source gas isotopic composition when in equilibrium with the soil gas, and the soil probe method relies on a mixing model of samples withdrawn from three gas wells at different depths to identify the gas source. We sampled from ports installed along the side of the sand column to describe the isotopic and concentration gradient as well as to serve as a control for the soil probe. The soil probe produced similar isotopic and concentration values as the control ports, as well as Keeling intercepts. The static chamber at equilibrium did not identify the source gas but, when applied in a two end-member mixing model, did produce a similar Keeling intercept produced from the control ports. Neither of the methods was able to identify the source gas via the Keeling plot method probably because CO(2) profiles did not reach isotopic steady state. Our results showed that the static chamber at equilibrium should be used only with a Keeling plot approach and that the soil probe is able to provide estimates of uncertainty for the isotopic composition of soil gas as well as information pertinent to the soil profile.     

A method for monitoring the composition of suspensions

The concentrations of particulates in dispersion media, as well as of true solutions, were analyzed by ratiometric method. The error in determination of the concentrations was estimated for the ratiometric, as well as for the conventional procedure based on the calibration connection equations.     

On the 17O correction for CO2 mass spectrometric isotopic analysis

To calculate delta(13)C from raw CO(2) isotope data, the ion beam ratio of m/z 45 to 44 is corrected for the contribution arising from the contribution of (17)O-bearing molecules. First, a review on the current state of (17)O-corrections for CO(2) mass spectrometry is presented. The three correction algorithms that are generally in use, however, do produce biased delta(13)C values, and the bias is actually larger than the precision of modern isotope ratio mass spectrometers. The origin of this bias is twofold: different values for (17)R(VPDB-CO2) as well as different values for lambda are used in the correction algorithms. Despite both values being of high importance, large discrepancies between the absolute values published for (17)R(VPDB-CO2) appear to be the main reason for the delta(13)C biases. Next, the question of how to choose the value of lambda to best be used is considered. Natural (e.g. tropospheric) CO(2) as well as primary reference materials (PDB and NBS-19), having been in isotope exchange with water, are assumed to lie on the fractionation line for waters. On this ground, lambda = 0.5281 +/- 0.0015, as determined for waters (Meijer and Li, Isot. Environ. Health Stud., 1998; 34: 349-369), is suggested to be a base for the (17)O-correction algorithm. Finally, an approach to determine the absolute value for (17)R(VPDB-CO2), based on data of relative isotope measurements on two CO(2) gases having a large (17)O difference, is discussed and algebraic formulas are considered. Experimental data and new numerical values determined for (17)R(VPDB-CO2) and (17)R(VSMOW) are given in a companion paper.     

Infrared spectra of isotopic CO2-He complexes

Infrared spectra of three isotopic forms of the weakly bound CO(2)-He van der Waals complex have been studied in the region of the CO(2) nu(3) fundamental band around 2300 cm(-1), using a tunable diode laser to probe a pulsed supersonic expansion. The complex is a T-shaped near-oblate asymmetric rotor, and it is found that (18)O isotopic substitution is sufficient to interchange the a and b inertial axes. For the symmetric isotopes, such as the normal species and the (16)O(13)C(16)O and (18)O(13)C(18)O forms studied here, half of the normal rotational levels of the complex are missing due to the effects of (16)O (or (18)O) interchange symmetry. However, for asymmetrically substituted ones, such as (16)O(13)C(18)O, all rotational levels are present. Moreover, for the asymmetric isotope, both a- and b-type transitions were observed, so that the spectrum was much richer. The CO(2)-He system is of interest both as a benchmark for intermolecular potential energy surface calculations, and because CO(2) is a valuable probe molecule for helium cluster spectroscopy.     

A mobile mass spectrometry laboratory for isotopic ratio measurements of uranium and plutonium

A modified quadrupole mass spectrometer is housed in a van that serves as a mobile laboratory to provide on-site determinations of isotopic ratios and concentrations of uranium and plutonium. Each element can be determined at 1–3 ng levels with precisions of 1–2%.     

Assessing the stable carbon isotopic composition of intercellular CO2 in a CAM plant using gas chromatography-combustion-isotope ratio mass spectrometry

Most of the literature focused on internal CO(2) (Ci) determinations in plants has used indirect methods based on gas-exchange estimations. We have developed a new method based on the capture of internal air gas samples and their analysis by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). This method provided a direct measure of intercellular CO(2) concentrations combined with stable carbon isotopic composition in O. ficus-indica plants. Plants were grown at both ambient and elevated CO(2) concentration. During the day period, when the stomata are closed, the Ci was high and was very (13)C-enriched in both ambient and elevated CO(2)-grown plants, reflecting Rubisco's fractionation (this plant enzyme has been shown to discriminate by 29 per thousand, in vitro, against (13)CO(2)). Other enzyme fractionations involved in C metabolism in plants, such as carbonic anhydrase, could also be playing an important role in the diurnal delta(13)C enrichment of the Ci. During the night, when stomata are open, Ci concentrations were higher in elevated (and the corresponding delta(13)C values were more (13)C-depleted) than in ambient CO(2)-grown plants.     

N2O influence on isotopic measurements of atmospheric CO2

In spite of extensive efforts, even the most experienced laboratories dealing with isotopic measurements of atmospheric CO2 still suffer from poor inter-laboratory consistency. One of the complicating factors of these isotope measurements is the presence of N2O, giving rise to mass overlap in the isotope ratio mass spectrometer (IRMS). The aim of the experiment reported here has been twofold: first, the re-establishment of the correction for 'mechanical' interference of N2O in the IRMS, along with its variability and drift, and the best way to quantitatively determine the correction factors. Second, an investigation into secondary effects, i.e. the influence of N2O admitted with the CO2 sample on the "cross contamination" between sample and (pure CO2) working gas. To make the suspected effects better detectable, isotopically enriched CO2 gas with different concentrations of N2O has been measured for the first time. No evidence of secondary effects was observed, from which we conclude that N2O is not a major player in the inter-laboratory consistency problems. Still, we also found that the determination of the 'mechanical' N2O correction needs to be very carefully determined for each individual IRMS, and should be periodically re-determined. We show that the determination of the correction should be performed using CO2/N2O mixtures with concentration ratios around that of the atmosphere, as the extrapolation from pure gas end member behaviour will give erroneous results due to non-linearities. For our IRMS, a VG SIRA series II, we find a correction of 0.23 per thousand for delta45CO2 and 0.30 per thousand for delta46CO2 of atmospheric samples, (with 0.85 per thousand mixing ratio). This implies that the relative ionisation efficiency (E) value associated with this machine is 0.75.     

An improved method for determination of uranium isotopic composition in urine by alpha spectrometry

A comparative study of source preparation techniques to determine uranium isotopic composition by alpha spectrometry, namely electrodeposition and chemical stripping with polymeric membranae containing trioctylphosphine oxide (TOPO), is presented. The mean yield obtained for electrodeposition and TOPO deposition were 85% and 74%, respectively. The mean activity ratio²³⁵U/²³⁸U were 0.044 and 0.042 and the ratio²³⁴U/²³⁸U were 0.994 and 1.009, using electrodeposition and TOPO deposition techniques, respectively. The method of uranium separation from urine using an ion-exchange resin Dowex 1×8, chloride form and citrate form, was also studied. The obtained global yields of these methods were 50% and 41%, respectively.     

A solid molecular basket sorbent for CO2 capture from gas streams with low CO2 concentration under ambient conditions

In this paper, a solid molecular basket sorbent, 50 wt% PEI/SBA-15, was studied for CO(2) capture from gas streams with low CO(2) concentration under ambient conditions. The sorbent was able to effectively and selectively capture CO(2) from a gas stream containing 1% CO(2) at 75 °C, with a breakthrough and saturation capacity of 63.1 and 66.7 mg g(-1), respectively, and a selectivity of 14 for CO(2)/CO and 185 for CO(2)/Ar. The sorption performance of the sorbent was influenced greatly by the operating temperature. The CO(2)-TPD study showed that the sorbent could be regenerated under mild conditions (50-110 °C) and was stable in the cyclic operations for at least 20 cycles. Furthermore, the possibility for CO(2) capture from air using the PEI/SBA-15 sorbent was studied by FTIR and proved by TPD. A capacity of 22.5 mg g(-1) was attained at 75 °C via a TPD method using a simulated air with 400 ppmv CO(2) in N(2).     

A clean laboratory for ultralow concentration heavy metal analysis

Summary Laboratory facilities and methods are described which have been developed in our laboratory in collaboration with C. Patterson's group at the California Institute of Technology for the reliable measurement of various heavy metals at extremely low concentration levels down to the sub pg/g level in Antarctic and Greenland snow and ice. All analytical work is performed inside a clean laboratory pressurized with air filtered through high efficiency particle air filters and equipped with all-plastic laminar flow clean benches. High-purity water is produced by ion-exchange resins, and high purity acids from the US National Institute of Science and Technology are used. The various containers which are in contact with the samples are made of conventional low density polyethylene and FEP teflon. These containers are cleaned by immersion during several weeks in a succession of heated acid baths of increasing purity. Extremely careful blank determinations are made in order to quantitatively determine how much of the investigated metals is added to the samples from each separate reagent, from the walls of the various containers and from the air of the clean laboratory.     

Methane content and isotopic composition of shallow groundwater: implications for environmental monitoring related to shale gas exploitation

The baseline methane for shallow groundwater can provide an important evidence to interpret possible methane stray associated with shale gas exploration. This study investigated and traced methane content and its origin of shallow groundwater in a karst aquifer in the Fuling shale gas block, SW China. The results show that methane contents of shallow groundwater are all less than 0.01 mg L^?1 and volumetric content in dissolved gas ranges from not detected to 0.0064%. The δ¹³C-CH₄ ranges from ?74.4 to ?49.1‰, suggesting biogenic origin. For the first time, the δ¹³C-CH₄ and ³He/⁴He end-numbers were determined.     

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.     

Short‐term dynamics of isotopic composition of leaf‐respired CO2 upon darkening: measurements and implications

Recent advances in understanding the metabolic origin and the temporal dynamics in δ¹³C of dark‐respired CO₂ (δ¹³C_res) have led to an increasing awareness of the importance of plant isotopic fractionation in respiratory processes. Pronounced dynamics in δ¹³C_res have been observed in a number of species and three main hypotheses have been proposed: first, diurnal changes in δ¹³C of respiratory substrates; second, post‐photosynthetic discrimination in respiratory pathways; and third, dynamic decarboxylation of enriched carbon pools during the post‐illumination respiration period. Since different functional groups exhibit distinct diurnal patterns in δ¹³C_res (ranging from 0 to 10‰ diurnal increase), we explored these hypotheses for different ecotypes and environmental (i.e. growth light) conditions. Mass balance calculations revealed that the effect of respiratory substrates on diurnal changes in δ¹³C_res was negligible in all investigated species. Further, rapid post‐illumination changes in δ¹³C_res (30 min), which increased from 2.6‰ to 5‰ over the course of the day, were examined by positional ¹³C‐labelling to quantify changes in pyruvate dehydrogenase (PDH) and Krebs cycle (KC) activity. We investigated the origin of these dynamics with Rayleigh mass balance calculations based on theoretical assumptions on fractionation processes. Neither the estimated changes of PDH and KC, nor decarboxylation of a malate pool entirely explained the observed pattern in δ¹³C_res. However, a Rayleigh fractionation of ¹²C‐discriminating enzymes and/or a rapid decline in the decarboxylation rate of an enriched substrate pool may explain the post‐illumination peak in δ¹³C_res. These results are highly relevant since δ¹³C_res is used in large‐scale carbon cycle studies. Copyright © 2009 John Wiley & Sons, Ltd.     

A calculation method to estimate the concentration and elemental composition of organic substances in natural Waters

The possibility of calculating the elemental composition and concentrations of organic substances in natural water based on the results of determining organic carbon and nitrogen and chemical oxygen demand is substantiated both theoretically and experimentally. Three types of substances differing in the electrochemical valence of carbon were among organic substances. The elemental composition of a number of water objects was analyzed. It was found that the concentrations of organic carbon, oxygen, hydrogen, and nitrogen are close to 50, 40, 4–5, and 2–5%, respectively.     

The effect of N2O on the isotopic composition of air-CO2 samples

The ionisation efficiencies of N2O vs. CO2 as well as their ratios were measured in detail introducing clean N2O and CO2 into the electron impact ion source of an isotope ratio mass spectrometer. Changes in the ionisation efficiency ratio (IER) were found for different electron energy settings and compared with the ratios of literature ionisation cross-section values for pure N2O and CO2. To establish the influence of mixtures of N2O and CO2 in a mass spectrometer, artificial air mixtures were prepared by mixing different amounts of N2O and CO2 from well-calibrated spike cylinders with CO2-free air. The mixing ratios varied from 8-512 ppb for N2O and from 328-744 ppm for CO2. With these mixtures the effects of varying N2O concentrations on apparent CO2 isotope ratios in air samples were determined. After applying a mass balance correction the delta13C results were consistent within small error margins. The data seemed almost independent from a particular choice for the IER of N2O vs. CO2 in the correction algorithm. For delta18O a small effect of the ionisation efficiency ratio of N2O vs. CO2 was found. Several sets of calculations were made varying the IER between 0.88 and 0.62. The dependence of delta18O was the smallest with an adopted IER of 0.68-0.72 in the mass balance correction equation for isotopic analysis of CO2 in air. For high-precision measurements of the CO2 stable isotope ratios in air samples a careful assessment of the mass spectrometer performance is necessary. Different ion sources, even different ion source settings, alter the IER of N2O vs. CO2 which is used in the N2O correction algorithm. Preferably, the specific mass spectrometric behaviour should be established with clean N2O/CO2 mixtures or with air mixtures covering a larger range of N2O concentrations.     

Repeat‐pulse 13CO2 labeling of canola and field pea: implications for soil organic matter studies

Both the quantity and quality of plant residues can impact soil properties and processes. Isotopic tracers can be used to trace plant residue decomposition if the tracer is homogeneously distributed throughout the plant. Continuous labeling will homogeneously label plants but is not widely accessible because elaborate equipment is needed. In order to determine if the more accessible repeat‐pulse labeling method could be used to trace plant residue decomposition, this labeling procedure was employed using ¹³CO₂ to enrich field pea and canola plants in a controlled environment. Plants were exposed weekly to pulses of 33 atom% ¹³CO₂ and grown to maturity. The distribution of the label throughout the plant parts (roots, stem, leaves, and pod) and biochemical fractions (ADF and ADL) was determined. The label was not homogeneously distributed throughout the plant; in particular, the pod fractions were less enriched than other fractions indicating the importance of continuing labeling well into plant maturity for pod‐producing plants. The ADL fraction was also less enriched than the ADF fraction. Because of the heterogeneity of the label throughout the plant, caution should be applied when using the repeat‐pulse method to trace the fate of ¹³C‐labeled residues in the soil. However, root contributions to below‐ground C were successfully determined from the repeat‐pulse labeled root material, as was ¹³C enrichment of soil within the top 15 cm. Canola contributed more above‐ and below‐ground residue C than field pea; however, canola was also higher in ADF and ADL fractions indicating a more recalcitrant residue. Copyright © 2010 John Wiley & Sons, Ltd.