The potential use of high sensitivity laser absorption spectroscopy for measuring the 13C/12C isotope ratio in atmospheric CO2 has been demonstrated, using a GaSb-based diode laser at 2.05 microm. In this spectral region, the overlapping between relatively strong 12CO2 and 13CO2 absorption features gives rise to several line pairs which are well suitable for a spectroscopic determination of the isotope ratio. Preliminary results have demonstrated that a short-term precision better than 1 per thousand can be easily obtained, for a CO2 concentration of 1000 ppm. We extensively discuss the influence of a possible non-linearity in the detectors' response on the delta-value and suggest an instrumental development that would allow to eliminate this effect.
A prototype off-axis integrated cavity output spectrometer (OA-ICOS) utilizing two identical cavities together with a near-infrared (1.63 microm) external cavity tunable diode laser is described. The two-cavity design-one for a reference gas and one for a sample gas-takes advantage of classical double-beam infrared spectrometer characteristics in reducing uncertainties due to laser scan or power instabilities and major temperature variations by a factor of three or better compared with a single-cavity scheme. This is the first OA-ICOS instrument designed to determine 13C/12C and (18)O/(16)O ratios from CO2 rotation/vibration fine structure in three different combination bands. Preliminary results indicate that at 0.8 Hz a precision of 3.3 and 2.8 per thousand is obtained for delta13C and delta(18)O, respectively, over a period of 10 h and a pure CO2 gas sample at 26 hPa. By averaging 100 spectra over a subset of the data, we achieved a precision of 1.6 and 0.8 \permil\ for delta13C and delta(18)O, respectively.
A recently developed laser spectroscopic instrument allows real-time continuous measurements of the stable isotopologues of carbon dioxide at ambient concentrations. This compact instrument offers sufficient precision (0.2 per thousand in 1 s, 0.02 per thousand in 60 s) and stability (drift in 1 h of<0.2 per thousand), to allow isotopic studies on a variety of timescales and to study a variety of processes. During the development of the instrument, a prototype was set up to sample ambient air nearly continuously for more than 10 months, in a heterogeneous urban area northwest of Boston, MA. During this long sampling experiment, we continued to improve and modify the instrument and sampling system. In this paper, we present data collected during that long sampling experiment in order to demonstrate some of the possibilities provided by such real-time continuous monitoring. We have observed distinct isotopic signatures in CO(2) variations from timescales of seconds to seasons. We also present a method of performing continuous Keeling regressions on a cascade of timescales and show some results in application of that method to the continuous sampling data set.
Breath tests using 13C-labelled substrates require the measurement of 13CO2/12CO2 ratio in breath gas samples. Next to isotope ratio mass spectrometry (IRMS), which is very sensitive but also complex and expensive, alternatively isotope selective nondispersive infrared spectrometry (NDIRS) can be used to determine the 13CO2/12CO2 ratio in expired breath. In this study we compared NDIRS- with IRMS-results to investigate whether the less expensive and very simply to operate NDIRS works as reliable as IRMS. For this purpose we applicated 1- 13C-Phenylalanine to patients with advanced liver cirrhosis and healthy volunteers and took duplicated breath samples for IRMS and NDIRS at selected time points. Our data show a good correlation between these two methods for a small number of samples as required for simple breath tests. Longer series, where repeated measurements are required on the NDIRS instrument lead to a decreasing correlation. This indicates the superiority of IRMS concerning 13CO2-kinetics over longer time periods.
The study of natural isotopic abundance signatures is useful to gain further insights in the processes resulting in depthwise changes in the composition of soil organic matter (SOM). Objectives were to describe the delta 13C and delta 15N abundances of SOM with depth in soils from a 153-year old beech (B1), a 119-year old spruce (F1) and a 61-year old spruce (F2) stand at Solling, north-west Germany, and to study, how podzolisation affects the isotopic abundances of 13C and 15N in the SOM. The degree of podzolisation decreased in the order F1 > B1 > F2. At the surface of the humus layer of all three sites, delta 13C values are approximately 1 to 4/1000 higher than in the leaves and needles, probably mainly due to the discrimination of 13C by microbial decomposition. 13C abundances in the organic layers of B1 and F2 increased only slightly from -27.6/1000 PDB (B1, L) to -27.2/1000 PDB (B1, Oh) and from -26.3/1000 PDB (F2, L) to -25.9/1000 PDB (F2, Oh), suggesting that biotic activity resulted in mixing of organic matter. At F1, however, 13C abundance increased from -27.5/1000 PDB (L) to -26.0/1000 PDB (Oh) which reflects the lack of mixing by animals. In the upper 2-4 cm of the mineral soil, i.e., in the eluvial horizons Aeh, 13C values showed a minimum at the spruce sites which was presumably related to a translocation of 13C enriched fulvic acids. Depthwise changes in delta 15N values were not related to podzolisation processes. At all three sites, a 13N enrichment with depth occurred in the mineral soil which is the result of the discrimination of 15N by microbial decomposition.
The back-calculation of the diet is a common application of stable isotopes in animal ecology. The method is based on a predictable relation between the isotopic signature of the diet and the animal's tissues. Frequently, the assumption of a constant difference in isotopic signatures (trophic shift) is made. Carbon isotopic ratios of C(3) and C(4) plants differ by approximately 10 per thousand, making wheat (C(3)-plant) and corn (C(4)-plant) ideal materials for isotopic studies in nutritional ecology and especially for testing the back-calculation method. In this experiment, red flour beetles, Tribolium castaneum, were reared on wheat flour, corn flour and three different mixtures thereof, either in pure flour or with the addition of yeast inoculum or yeast grains. Development of T. castaneum on these experimental diets was monitored, and isotopic signatures of carbon and nitrogen in emerging adults were analysed. The values of trophic shift of C and N isotopes for wheat and corn flour were different, and the values for the mixtures did not correspond to those expected from a linear mixing model. The latter can be taken as an indication that the tiny larvae of T. castaneum may be capable of differentiating between particles of wheat and corn flour, making this animal model unsuitable for testing the back-calculation method.
The increasing application of 13C-labelled urea in medicine requires simple and reasonable methods for measuring highly enriched C in urea. The combination: ultimate organic analysis--mass spectrometry so far prescribed is complicated and expensive. For medical diagnosis, however, isotope selective nondispersive infrared spectrometers (NDIRS) have been available for many years. One of these tools is FANci2 which is very reasonable and easily to be operated. By means of such devices also urea highly enriched in 13C can be analysed, provided that the samples are first diluted with a defined amount of urea of natural isotopic composition and then transformed into carbon dioxide by means of urease. The relative abundance of 13C in this carbon dioxide, measured by nondispersive infrared spectrometry, is then a measure of the 13C abundance in the initial urea sample. Comparison of results of such measurements with those attained by mass spectrometry proves that this procedure is feasible and yields precis results.
We analysed 13carbon and deuterium discrimination in Opuntia atacamensis PHIL. at three different sites in the Atacama desert in Northern Chile: At the western Andean slopes, influenced by summer rainfall, in the very arid Chilean central valley, and at coastal fog mountains. At the most arid site, the central valley, discrimination of 13C and D was less (delta-values less negative) and also discrimination more against deuterium. This is an aridity, not an altitude effect. The differences in 13C content may be due to some carbon fixation via the C3 photosynthetic pathway at the more humid sites. Deuterium enrichment at the arid sites might be due to greater discrimination of HDO against H2O during transpirational water loss.
Abstract For verifying catabolic states in insulin-dependent patients and dogs the method estimating urea production rates with (13)C and with doubly (15)N labeled urea, respectively, has been established. For a fast steady state of urea tracer dilution, a prime of 600 times the continuous infusion rate had to be injected. Urea was isolated from plasma samples by protein precipitation and cation exchange chromatography with a consecutive derivatization of the dried urea fraction (trimethylsilyl derivatives). The masses of the fragment ions m/z 189 ((14)N(14)N), 190 ((14)N(15)N) and 191 ((15)N(15)N) urea are monitored to estimate the [(15)N(2)]urea frequency in the overall body urea pool in mol percent excess (MPE). 1 to 15 ng of derivatized urea were measured efficiently. An excellent correlation between expected standard and measured MPE (r = 0.9977) was achieved from solutions containing 1 to 7% [(15)N(2)]urea. The interassay coefficient of variation amounted to < 10% for a [(15)N(2)]urea portion of ≥ 3%. Normoglycemic diabetic patients who were treated with insulin overnight showed significantly higher urea production compared to healthy controls (9.22 ± 2.07 vs. 5.4 ± 0.32 μmol·kg(-1) · min(-1); p < 0.05). Measurements in chronic diabetic dogs proved an increased rate of amino acid catabolism (+ 20% urea production) in systemic versus portal application of insulin in paired studies. This increased nitrogen load in diabetics may accelerate progression of diabetic nephropathy. - Thus, the established stable isotope technique may serve as a sensitive and useful indicator of amino acid catabolism in clinical and experimental research.
In estuarine ecosystems, large spatial and seasonal variations in delta13C values of primary producers can occur, and knowledge of these variations may be crucial when interpreting stable isotope data of higher trophic levels. Obtaining clean phytoplankton samples for isotope analysis is usually impossible in such systems, and analysis of total suspended matter is not a simple proxy for phytoplankton delta13C variations. Based on a few simple assumptions regarding the C and N content of the two end-members (terrestrial detritus and phytoplankton) and the delta13C of the terrestrial component, we here present a simple model to estimate the phytoplankton delta13C variations using an existing dataset on the delta13C and elemental (C:N) composition of suspended organic matter from an estuarine mangrove ecosystem in southeast India. These variations are related to the monthly rainfall pattern during the sampling period. It is stressed that this method estimates approximate phytoplankton delta13C values, which should not be used in e.g., mixing models. However, we propose that in cases where sufficiently large datasets are available, the described procedure could provide a valuable method to semi-quantitatively estimate the seasonal or spatial variations of the phytoplankton delta13C signal.
It is essential to establish whether and how environmental factors affect the reliability of [(13)C]methacetin breath test ((13)C-MBT). In 12 healthy volunteers (smokers), a standard (13)C-MBT with 75 mg [(13)C]methacetin was performed twice in random order: on a control day without smoking and on another day with smoking two cigarettes antecedently. A considerable flattening of the curve of the momentary (13)C recovery within the expiratory air was observed when the (13)C-MBT was performed after smoking. The maximum of the momentary (13)C recovery, D(max), decreased from 37.20±2.58 to 25.39±2.29% dose/h (p=0.00052). Moreover, the time to reach D(max) was prolonged after cigarette smoking (26.5±3.1 vs. 16.5±1.9 min, p=0.0199). The curve of the cumulative (13)C recovery on the cigarette smoking day appeared to be shifted downwards, and statistically significant differences relative to the control situation were found between the 24th and 75th minute following [(13)C]methacetin administration. Smoking cigarettes immediately prior to the (13)C-MBT diminishes the ability of the liver to handle methacetin, and hence a possibility of such an interaction should be excluded in order to interpret the results of the test correctly.
Stable Isotopes (strontium-87, deuterium and oxygen-18, carbon-13) have been used to reveal different sources of groundwater and mixing processes in the aquifer of the Silao-Romita Valley in the state of Guanajuato, Mexico. Calcite dissolution appeared to be the main process of strontium release leading to relatively equal (87)Sr/(86)Sr ratios of 0.7042-0.7062 throughout the study area which could be confirmed by samples of carbonate rocks having similar Sr ratios (0.7041-0.7073). delta(13)C values (-11.91- -6.87 per thousand VPDB) of groundwaters confirmed the solution of carbonates but indicated furthermore influences of soil-CO(2). Deuterium and (18)O contents showed a relatively narrow range of-80.1- -70.0 per thousand VSMOW and -10.2- -8.8 per thousand, VSMOW, respectively but are affected by evaporation and mixing processes. The use of delta(13)C together with (87)Sr/(86)Sr revealed three possible sources: (i) carbonate-controlled waters showing generally higher Sr-concentrations, (ii) fissure waters with low-strontium contents and (iii) infiltrating water which is characterized by low delta(13)C and (87)Sr/(86)Sr ratios. The third component is affected by evaporation processes taking place before and during infiltration which might be increased by extraction and reinfiltration (irrigation return flow).
C4 plant species were proposed to generally represent inferior food sources compared to C3 plants thus are avoided by herbivores, particularly insects. This was tested in semi-aquatic and terrestrial arthropods from Amazonian river-floodplains by carbon isotope discrimination (delta13C). Two semi-aquatic grasshopper species (Stenacris f. fissicauda, Tucavaca gracilis-Acrididae) obtain their carbon during development from specific C4 macrophytes and two semi-aquatic species (Cornops aquaticum-Acrididae, Paulinia acuminata-Pauliniidae) from specific C3 macrophytes. The terrestrial millipede Mestosoma hylaeicum (Paradoxosomatidae) obtains about 45% of its carbon from roots of one C4 macrophyte during the development of immatures whereas adults use other food sources, including C3 trees. Results suggest, that (1) both C4 and C3 plants represent distinct hosts for terrestrial arthropods in Amazonia; (2) immatures may use plant species with a different photosynthetic pathway than adults.
Stable isotope breath tests offer a new approach to the study of digestion and fermentation of carbohydrates in man. In this study, 13C labelled peas were grown by pulsing 250 ml 13CO2 into a sealed growth chamber. A second pulse was added to a portion of the peas to increase the 13C enrichment. This generated pea flour with an enrichment of 2.36 at.% excess (range 2.09-2.71 n = 3) and 8.64 atom % excess (range 7.37-9.78 n = 3) respectively. This represented incorporation of an absolute yield of 3.8% of the 13CO2 into peas in the 'once-labelled' treatment and 7.5% in the 'twice-labelled' treatment. Ingestion of a mixture of the labelled pea flour (300 mg) by two volunteers generated measurable 13CO2 excretion for breath test analysis. The profile of breath 13CO2 enrichment increased to a maximum within three hours after consuming the pea flour followed by a decrease almost back to baseline by 13 hours. Breath 13CO2 appeared to rise again after this apparent nadir at 13 hours until the end of the sampling period. Mathematical analysis of the data suggested that two peaks best described the profile of breath 13CO2 up to 13 hours. A third peak was necessary to describe the late rise in breath 13CO2 enrichment. This use of 13C enriched pea flour may provide a useful non invasive method for measurement of digestion and fermentation in vivo.
This study determined the within-subject and between-subject variability of different ways of expressing the results of the (13)C-aminopyrine breath test ((13)C-ABT) and the effect of shortening the test duration. The (13)C-ABT was conducted on three separate occasions in 10 healthy volunteers and on a single occasion in 22 patients with established liver cirrhosis. The within-subject variability of cumulative percentage dose recovered (cPDR), using measured CO(2) production rate (VCO(2)), in the reference group over three trials was 15% over 120 min. Higher within-subject variability in cPDR would have been evident if the test was terminated at either 30 or 60 min. Substitution of predicted VCO(2) to calculate cPDR yielded comparable values at all time points. Significant differences between cirrhotics and reference group were evident after just 10 min using PDR/h, cPDR or enrichment (all P<0.05). The ABT demonstrates clinically acceptable reproducibility. Shortening of the duration may make the test more acceptable clinically, but it is associated with increasing imprecision.
Naturally produced methane shows different delta 13C-values with respect to its origin, e.g., geological or biological. Methane-production of ruminants is considered to be the dominant source from the animal kingdom. Isotopic values of rumen methane--given in literature--range between -80/1000 and -50/1000 and are related to feed composition and also sampling techniques. Keeping cows, camels and sheep under identical feed conditions and sampling rumen gases via implanted fistuale we compared delta PDB 13C-values of methane and CO2 between the species. Referring to mean values obtained from 4 or 5 samples at different times of 11 animals (n = 47) we calculated delta PDB 13C-medians resulting in small but not significant differences within and significant differences between the species for CO2 and methane. The delta PDB 13C-differences between methane and CO2 were statistically equal within and also between the species. Therefore a linear regression of methane values on CO2 is appropriate and leads to: delta PDB 13C(methane)/1000 = 1.57 * delta PDB 13C(CO2)/1000 - 47/1000 with a correlation coefficient of r = 0.87.
To simplify the L-[1-13C]phenylalanine breath test which is used to assess liver function the tracer is usually given orally, and CO2 production rate is estimated. In 12 healthy volunteers and 10 liver cirrhotics we compared the oral approach with i.v. tracer administration combined with measurement of individual CO2 production rate. The 13CO2/12CO2 enrichment was assessed by isotope-ratio mass spectrometry. After i.v. [1-13C]phenylalanine application exhaled 13C recovery per minute peaked within 10 minutes (controls: 0.17 +/- 0.06%; cirrhotics: 0.05 +/- 0.02%, p < 0.01). The oral approach yielded comparable separation between 30-60 minutes, with average peak values being 0.18 +/- 0.03% and 0.06 +/- 0.03% (p < 0.01), respectively. Variable gastrointestinal resorption kinetics after oral application probably causes this difference.
Dual stable isotope analysis in the regulated Colorado River through Grand Canyon National Park, USA, revealed a food web that varied spatially through this arid biome. Down-river enrichment of delta13C data was detected across three trophic levels resulting in shifted food webs. Humpack chub delta13C and delta15N values from muscle plugs and fin clips did not differ significantly. Humpback chub and rainbow trout trophic position is positively correlated with standard length indicating an increase in piscivory by larger fishes. Recovery of the aquatic community from impoundment by Glen Canyon Dam and collecting refinements for stable isotope analysis within large rivers are discussed.
Waters were sampled monthly from a profile at the wastewater outlet and a reference point in the Bay of Vidy (Lake Geneva) for a year. The samples were analyzed for (18)O/(16)O of water, (13)C/(12)C of dissolved inorganic carbon (DIC), major ions, and selected micropollutant concentrations. δ(18)O values, combined with the major ion concentrations, allowed discharged waste and storm-drainage water to be traced within the water column. On the basis of δ(18)O values, mole fractions of wastewater (up to 45 %), storm-drainage (up to 16 %), and interflowing Rhône River water (up to 34 %) could be determined. The results suggest that the stormwater fractions do not influence micropollutant concentrations in a measurable way. In contrast, the Rhône River interflow coincides with elevated concentrations of Rhône River-derived micropollutants in some profiles. δ(13)C values of DIC suggest that an increase in micropollutant concentrations at the sediment-water interface could be related to remineralization processes or resuspension.
Measurement of soil-respired CO(2) at high temporal resolution and sample density is necessary to accurately identify sources and quantify effluxes of soil-respired CO(2). A portable sampling device for the analysis of δ(13)C values in the field is described herein. CO(2) accumulated in a soil chamber was batch sampled sequentially in four gas bags and analysed by Wavelength-Scanned Cavity Ring-down Spectrometry (WS-CRDS). A Keeling plot (1/[CO(2)] versus δ(13)C) was used to derive δ(13)C values of soil-respired CO(2). Calibration to the δ(13)C Vienna Peedee Belemnite scale was by analysis of cylinder CO(2) and CO(2) derived from dissolved carbonate standards. The performance of gas-bag analysis was compared to continuous analysis where the WS-CRDS analyser was connected directly to the soil chamber. Although there are inherent difficulties in obtaining absolute accuracy data for δ(13)C values in soil-respired CO(2), the similarity of δ(13)C values obtained for the same test soil with different analytical configurations indicated that an acceptable accuracy of the δ(13)C data were obtained by the WS-CRDS techniques presented here. Field testing of a variety of tropical soil/vegetation types, using the batch sampling technique yielded δ(13)C values for soil-respired CO(2) related to the dominance of either C(3) (tree, δ(13)C=-27.8 to-31.9 ‰) or C(4) (tropical grass, δ(13)C=-9.8 to-13.6 ‰) photosynthetic pathways in vegetation at the sampling sites. Standard errors of the Keeling plot intercept δ(13)C values of soil-respired CO(2) were typically<0.4 ‰ for analysis of soils with high CO(2) efflux (>7-9 μmol m(-2) s(-1)).
Hyperoxaluria is the most important risk factor for a formation of calcium oxalate-urinary stones. Usually, the bulk of oxalate will be formed in the human body, but in many patients the oxalate from food plays the decisive role. Conventionally, in urine the endogenous oxalate can not be distinguished from food derived oxalate. We have developed a standardized oxalate-absorption test, applying a physiological dose (50 mg disodium salt of [13C2]oxalic acid) of labelled oxalate. The assay has been published. Now we report on the first extensive applications of this test in 86 volunteers and 135 patients from different groups with calcium oxalate stones or an increased risk of the formation of such stones. In one-third of the patients with calcium oxalate-urinary stones an oxalate hyperabsorption was diagnosed. For these patients, a dietetic stone prophylaxis and/or therapy is indicated.
Bile acid kinetics involve the measurement of pool sizes and turnover rates of individual bile acids. The technique is based on isotope dilution and was first described in the 1950s using radioactive (14)C-labelled cholic acid (CA). It took until the 1970s before stable isotopes were introduced for this purpose ((13)C, (2)H) and isotope analysis methods were developed for CA and chenodeoxycholic acid (CDCA) applying gas chromatography/electron impact mass spectrometry. Until the 1980s, the isotope enrichment measurements were performed in bile samples aspirated from the duodenum. Thereafter, methodology became available allowing measurements to be performed in blood requiring at least 2 ml serum samples. Simultaneous measurement of kinetics of metabolically dependent CA and deoxycholic acid using (13)C and (2)H labels was introduced. Until the 1990s, this technique was only possible in adult humans due to the large sample sizes. Introduction of pentafluorobenzyl bromide derivatisation and electron capture negative ion mass spectrometry (GC/ECN-MS) reduced the sample volume to 50 microl serum. This allowed isotope abundance measurement of CA in rats and in mice. However, repetitive collection of 100 microl blood samples in mice is too invasive (collection via the orbita) and exhaustive. Therefore, the method development is now focussing on enhanced sensitivity and reduction of blank effects originating from the sample preparation. The final goal is to determine CA isotope enrichments in 20 microl mouse blood obtained from the tail vein. This paper shows the feasibility of reaching this goal.
δ(13)C values of gaseous acetaldehyde were measured by gas chromatograph-combustion-isotope ratio mass spectrometer (GC-C-IRMS) via sodium bisulfite (NaHSO(3)) adsorption and cysteamine derivatisation. Gaseous acetaldehyde was collected via NaHSO(3)-coated Sep-Pak(®) silica gel cartridge, then derivatised with cysteamine, and then the δ(13)C value of the acetaldehyde-cysteamine derivative was measured by GC-C-IRMS. Using two acetaldehydes with different δ(13)C values, derivatisation experiments were carried out to cover concentrations between 0.009×10(-3) and 1.96×10(-3) mg·l(-1)) of atmospheric acetaldehyde, and then δ(13)C fractionation was evaluated in the derivatisation of acetaldehyde based on stoichiometric mass balance after measuring the δ(13)C values of acetaldehyde, cysteamine and the acetaldehyde-cysteamine derivative. δ(13)C measurements in the derivertisation process showed good reproducibility (<0.5 ‰) for gaseous acetaldehyde. The differences between predicted and measured δ(13)C values were 0.04-0.31 ‰ for acetaldehyde-cysteamine derivative, indicating that the derivatisation introduces no isotope fractionation for gaseous acetaldehyde, and obtained δ(13)C values of acetaldehyde in ambient air at the two sites were distinct (-34.00 ‰ at an urban site versus-31.00 ‰ at a forest site), implying potential application of the method to study atmospheric acetaldehyde.
Customary 13CO2 breath tests--and also 15N urine tests--always start with an oral administration of a test substrate. The test person swallows a stable isotope labelled diagnostic agent. This technique has been used to study several pathophysiological changes in gastrointestinal organs. However, to study pathophysiological changes of the bronchial and lung epithelium, the inhalative administration of a stable isotope labelled agent appeared more suitable to us. [1-13C]Hexadecanol and [1-13C]glucose were chosen. Inhaled [1-13C]hexadecanol did not yield 13CO2 in the exhaled air, but [1-13C]glucose did. To study the practicability of the [1-13C]glucose method and the reproducibility of the results, 18 inhalation tests were performed with healthy subjects. In 6 self-tests, the optimum inhalative dose of [13C]glucose was determined to be 205 mg. Using the APS aerosol provocation system with the nebulizer 'Medic Aid' (Erich Jaeger Würzburg), a 25% aqueous solution was inhaled. Then, breath samples were collected at 15 min. intervals and analysed for 13CO2. 75-120 min after the end of inhalation a well-reproducible maximum delta13C value of 6%o over baseline (DOB) was detected for 12 healthy probands. Speculating that the pulmonary resorption of the [13C]glucose is the rate-limiting step of elimination, decompensations in the epithelium ought to be reflected in changed [1-13C]glucose resorption rates and changed 13CO2 output. Therefore, we speculate that the inhalation of suitable 13C-labelled substrates will pave the way for a new group of 13CO2 breath tests aiding investigations of specific pathophysiological changes in the pulmonary tract, such as inflammations of certain sections and decompensations of cell functions.
The urea breath test (UBT) is a non-invasive diagnostic test to detect the presence of Helicobacter pylori in the stomach, and is the simplest way to confirm eradication after treatment. The test is based on the capacity of H. pylori to secrete the enzyme urease, which hydrolyses urea to ammonia and carbon dioxide. The aim of this study was to determine whether there is an advantage in expressing the results of UBTs in terms of urea hydrolysis rate (UHR), rather than breath 13C enrichment alone. Retrospective analysis of data collected between 1995 and 2002 from 260 patients undergoing UBTs was performed. The cut-offs for positive tests using breath 30-minute enrichment (E30), UHR calculated using VCO2 estimated from height and weight (H/WT) and VCO2 estimated from weight only were determined using two-graph receiver operator characteristic (TG-ROC) analysis. The cut-off points were 3.5/1000 or 38.7 ppm 13C excess, 7.04 micromol/h and 7.08 micromol/h, respectively. There was no advantage in expressing the results as UHR (theta0, Theta-zero, where sensitivity = specificity = 0.97 (UHR H/WT), 0.98 (UHR WT) and 1.00 (E30)) rather than breath 13CO2 enrichment alone. Differences in the extent of H. pylori colonisation and urease activity are more important than variation in VCO2 in determining breath 13CO2 enrichment in the UBT.
To determine the 13C abundance of combustion and break down products formed in cigarette smoke, especially CO and CO2, a simple and fast analytical method is needed. Taking into account the knowledge about the determination of the natural 13C abundance in air, an online method - based on gas chromatography-reaction-continuous flow mass spectrometry (GC-R-CF-MS) - has been developed, which enables the determination of the 13C abundance of CO and CO2 in the vapour phase of cigarette smoke with a relative standard deviation of< or =0.5% in one analytical run. Additionally, in a second step, the 13C abundance of total volatile carbon can be determined.
The [13C]aminopyrine breath test ([13C]ABT) measures the global activity of cytochrome P450 in vivo and is a sensitive indicator of liver metabolic dysfunction. The present study aims to determine whether gender and cigarette smoking influence the results of [13C]ABT as well as to confirm the effect of oral contraceptive steroids (OCS) intake on this metabolic test. Hundred and ten healthy subjects, including men and women, smoker and non-smoker, women taking OCS or not, were phenotyped for CYP1A2 using the [13C]caffeine breath test and underwent a [13C]ABT. Both tests showed large inter-individual variations in accordance with that of CYP450 liver content. [13C]ABT was sensitive enough to point out a significant induction or inhibition related to cigarette smoking habits or OCS. The combined effect of smoking and OCS resulted in an overall unchanged metabolic activity. Consequently, the impact of the studied conditions on the [13C]ABT parameters must be considered by clinicians or clinical investigators.
Abstract We report the first isotopic study of an animal host-parasite system. Parasitic, intestinal nematodes, Graphidium strigosum and Passalurus ambiguus, were (15)N-enriched relative to their host, the European rabbit Oryctolagus cuniculus, while parasitic cestodes, Cittataenia denticulata and Mosgovoyia pectinata, were (15)N-depleted, suggesting different trophic relationships. Host embryos were more similar in their δ(13)C and δ(15)N values to maternal muscle than were any of the parasites. Coprophagy, the direct recycling of food by the rabbit eating its own faeces, did not lead to isotopic differences between stomach contents and faeces, suggesting that the major point for isotopic discrimination in lagomorph nitrogen metabolism is in the animal rather than in the gut. We conclude that bulk δ(13)C and δ(15)N can reveal valuable new information about host-parasite relationships, and these could be explored further at the biochemical level using compound-specific isotopic analyses.
Two novel characteristic parameters, the latency time (t(lat)) and the ascension time (t(asc)), are proposed for evaluation of non-invasive [13C]octanoic acid breath tests for assessment of the gastric emptying of solids. In breath tests performed in control subjects (n = 30) and diabetic patients (n = 100), the usefulness of these parameters was compared to conventional parameters, i.e., gastric half emptying-time (t1/2,b) and lag phase (t(lag),b). The proposed parameters were only loosely correlated (controls, r = 0.199; diabetics, 0.616). A strong correlation was found between the conventional parameters (controls, r = 0.891; diabetics, r = 0.962). Based on the conventional method, 36 patients were suspicious of delayed gastric emptying including 24 patients which exhibited a simultaneous delay in both parameters. Using the new parameters, a total of 46 patients were suspicious of delayed gastric emptying with 15 and 20 having isolated delay in t(lat) and t(asc), respectively. We conclude that the novel parameters may be more appropriate for examination of the different phases of gastric emptying and for evaluation of gastric emptying disturbances in diabetic patients than the parameters conventionally used for this purpose.
An important prerequisite for the effective use of stable isotopes in animal ecology is the accurate assessment of isotopic discrimination factors linking animals to their diets for a multitude of tissue types. Surprisingly, these values are poorly known in general and especially for mammalian carnivores and omnivores in particular. Also largely unknown are the factors that influence diet-tissue isotopic discrimination such as nutritional quality and age. We raised adult and juvenile striped skunks (Mephitis mephitis) in captivity on a constant omnivore diet (Mazuri Omnivore A 5635). Adults (n=6) and juveniles (n=3) were kept for 7 months and young (n=7) to the age of 50 days. We then examined individuals for stable carbon (δ(13)C) and nitrogen (δ(15)N) isotope values of hair, nails, lipid, liver, muscle, bone collagen and the plasma, and cellular fractions of blood. Discrimination values differed among age groups and were significantly higher for young compared with their mothers, likely due to the effects of weaning. Δ(15)N isotopic discrimination factors ranged from 3.14 (nails) to 5.6‰ (plasma) in adults and 4.3 (nails) to 5.8‰ (liver) for young. For Δ(13)C, values ranged from-3.3 (fat) to 3.0‰ (collagen) in adults and from-3.3 (fat) to 2.0‰ (collagen) in young. Our data provide an important tool for predicting diets and source of feeding for medium-sized mammalian omnivorous adults integrated over short (e.g. liver, plasma) through long (e.g. collagen) periods and underline the potential effects of age on isotopic values in omnivore diets.
The dynamics of C and N in terrestrial ecosystems are not completely understood and the use of stable isotopes may be useful to gain further insight in the pathways of CO2 emissions and leaching of dissolved organic carbon (DOC) and nitrogen (DON) during decomposition of litter. Objectives were (i) to study the decomposition dynamics of Calamagrostis epigeios, a common grass species in forests, using 13C-depleted and 15N-enriched plants and (ii) to quantify the effect wood ash addition on the decomposition and leaching of DOC and DON. Decomposition was studied for 128 days under aerobic conditions at 8 degrees C and moisture close to field capacity in a spodic dystric Cambisol with mor-moder layer. Variants included control plots and additions of (i) Calamagrostis litter and (ii) Calamagrostis litter plus 4 kg ash m-2. (i) Decomposition of Calamagrostis resulted in a CO2 production of 76.2 g CO2-C m-2 (10% of added C) after 128 days and cumulative DOC production was 14.0 g C m-2 out of which 0.9 g C m-2 was Calamagrostis-derived (0.1% of added C). The specific CO2 formation and specific DOC production from Calamagrostis were 6 times higher (CO2) and 4 times smaller (DOC) than those from the organic layer. The amount of Calamagrostis-derived total N (NH4+, NO3-, DON) leached was 0.7 g N m-2 (4.8% of added N). Cumulative DON production was 0.8 g N m-2 which was slightly higher than for the control. During soil passage, much of the DOC and DON was removed due to sorption or decomposition. DOC and DON releases from the mineral soil (17 cm depth) were 6.3 g C m-2 and 0.5 g N m-2. (ii) Addition of ash resulted in a complete fixing of CO2 for 40 days due to carbonatisation. Afterwards, the CO2 production rates were similar to the variant without ash addition. Production of DOC (98.6 g C m-2) and DON (2.5 g N m-2) was marked, mainly owing to humus decay. However, Calamagrostis-derived DOC and Calamagrostis-derived total N were only 3.9 g C m-2 (0.5% of added C) and 0.5 g N m-2 (3.4% of added N). The specific DOC production rate from the organic layer was 6 times higher than that from Calamagrostis. The results suggest that with increasing humification from fresh plant residues to more decomposed material (OF and OH layers) the production ratio of DOC/CO2-C increases. Addition of alkaline substances to the forest floor can lead to a manifold increase in DOC production.
A combined system consisting of a TOC analyser connected to a quadrupole MS was recently described as a way of measuring the N content and the 15N abundance of total dissolved nitrogen in aqueous samples. This work examines whether this combination of instruments can also be used for the 13C determination of the total dissolved carbon in aqueous samples. A level of precision good for 13C-enriched samples was achieved with a relative standard deviation of <3%. By using an isotope ratio MS instead of the quadrupole MS employed here, TOC-MS coupling also ought to be suitable for determining natural 13C abundances.
Abstract A seven compartment model was applied for evaluation of oral L-[1-(13)C]leucine loading tests (38 μmol/kg body wt.) in healthy volunteers. The model comprises transport and absorption in stomach and gut into a central L-leucine-compartment which is connected to a protein compartment and to the compartment of the corresponding 2-oxo acid. CO(2) release from the latter occurs in a fast and a slow compartment into the central CO(2) compartment for exhalation. Using the fmins routine of MATLAB for parameter estimation, a good agreement was obtained between calculated and actually measured kinetics of (13)C-labelled metabolites and a mean in vivo L-leucine oxidation of 0.365 ± 0.071 μmol/kg per min (n = 5) was computed. Plausibility of the model was checked by predicting in vivo leucine oxidation rates from primed continuous infusion tests (priming: L-[1-(13)C]leucine, 5 μmol/kg; NaH(13)CO(2), 1.2 μmol/kg; infusion: L-[1-(13)C]leucine, 5 μmol/kg per h). In 5 tested volunteers, the experimental L-leucine oxidation rate amounted to 0.358 ± 0.105 μmol/kg per min versus predicted 0.324±0.099 μmol/kg per min. Possible causes for some observed intraindividual variations are discussed.
Human milk oligosaccharides seem to play an important role in the infant's defense against bacterial and viral infections of the gastrointestinal and the urogenital tract. In this study, we investigated the influence of dietary carbohydrates on the biosynthesis of lactose and oligosaccharides in the human mammary gland and their renal excretion by the human milk-fed infant. For this purpose, a lactating woman was given 27 g galactose (Gal) containing 2 g [13C] Gal (1-13C/99%) immediately after breakfast. In the following 36 h, milk (5-10 ml) was collected before each nursing. Infant's urine was collected over a period of 24 h. 13C-enrichment was measured in total milk, milk fat and protein, in the carbohydrate fraction as well as in urine by isotope ratio mass spectrometry (IRMS). Milk carbohydrates and deproteinized urine samples were fractionated by Sephadex G25 gel filtration and further analyzed by IRMS, high performance thin layer chromatography and and high pH anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD). IRMS revealed that in milk a maximal delta 13CPDB was reached within 8 h after Gal intake which then rapidly declined in the following 8 h. The cumulative 13C-elimination over this first peak was 6.9% of the oral 13C-dose. The highest 13C-enrichment was detectable in the carbohydrate fraction, mainly in lactose and neutral oligosaccharides. Compared to the enrichment of human milk, the delta 13CPDB of infant's urine was delayed. In urine, the highest amount of 13C was found in the Sephadex G25 fractions which mainly contained lactose, fucosyl-lactose, lacto-N-tetraose (LNT), fucosyl-LNT and difucosyl-LNT. For further characterization, individual components were separated by HPAEC-PAD and subsequently analyzed by fast atom bombardment mass spectrometry and IRMS. The data show, that orally applied Gal is incorporated in milk, especially in lactose and neutral oligosaccharides. Obviously, some of these components were absorbed by the infant and then excreted with urine. There, oligosaccharides may serve as analogous receptors for bacterial or viral adhesion molecules, and, hence, may prevent urogenital infections in breastfed infants.
The potential use of non-dispersive infrared spectroscopy for measuring delta(13)C in air is demonstrated. This technique has already been successfully established for breath test analyses in medical diagnostics, where the CO(2) concentration ranges from 1 to 5 vol.% in the exhaled breath of vertebrates. For breath tests, the sensitivity and accuracy has been improved to reach a standard deviation of 0.2 per thousand (delta-value). Further adjustments were necessary to improve the sensitivity of the instrument at concentration levels typical of atmospheric air. The long-term stability is given by a standard deviation of 0.35 per thousand for CO(2) concentrations of about 400 ppm with signal averaging over 60 s.
This study determined the rates of (13)C-aminopyrine metabolism in patients with varying degrees of liver cirrhosis as defined by clinical scores. Twenty-five cirrhotic patients and 18 healthy subjects underwent a (13)C-aminopyrine breath test. The cumulative per cent dose recovery (cPDR) of (13)C on breath expressed as a percentage of the administered dose at 2 h was significantly lower in cirrhotic patients than in healthy subjects (median: 1.7% versus 9.0%; p<.0001). Significant inverse associations between cPDR at 2 h and the model for end-stage liver disease score, Child-Pugh score, international normalised ratio and bilirubin (all p<.05), but not alanine aminotransferase or alkaline phosphatase were observed in the cirrhotic patients. Taking each biochemical marker independently, cirrhotic patients with normal biochemistry had a significantly lower cPDR at 2 h than healthy subjects (all p<.05). Differences in (13)C-aminopyrine metabolism were evident in cirrhotic patients with less severe disease and may mark hepatic dysfunction when conventional biochemical markers appear unchanged.
The aim of this study was to check on the reproducibility of two breath tests intended to test the pancreatic exocrine function accomplished with 13C-mixed triglyceride (13C-MTG) or cornflakes naturally enriched in 13C (13C-CF). The 13CO2 content within breath samples was determined with isotope-selective non-dispersive infrared spectrometry. A 72-h monitoring performed in healthy subjects revealed that a statistically significant rise in breath 13CO2 occurs between the 1st and the 9th hour and between the 1st and the 24th hour after intake of a test meal containing 300 mg 13C-MTG (n=10) or 100 g 13C-CF (n=12), respectively. In another two groups of 12 healthy volunteers each, short-term reproducibility of the two tests was assessed with paired examinations taken at a median interval of two days, whereas paired examinations separated by a median of 20 days served for the medium-term reproducibility assessment. In the case of either test, the medium-term reproducibility was not any worse than the short-term one. The reproducibility of the 13C-CF breath test tended to be slightly worse than that of the 13C-MTG breath test: a least detectable difference in 6-h cumulative 13C breath excretion (which is expressed as the percentage of the administered dose of the substrate) amounted to 2.7 and 4.4 % (short-term reproducibility) and to 3.5 and 4.4 % (medium-term reproducibility) in the case of the 13C-MTG breath test and the 13C-CF breath test, respectively. It is concluded that both tests offer a satisfactory reproducibility for use within a clinical setting. In case the lipolytic and the amylolytic activity would be required to be examined in the same patient, the 13C-CF breath test can be executed on the next day following the 13C-MTG breath test, whereas reciprocally, a 1-day break is recommended before accomplishment of a 13C-MTG breath test following a 13C-CF breath test.
The protocols for 13C and 15N H. pylori tests stipulate that the diagnostic agent should be taken on an empty stomach. It is presumed that food intake prior to the tests leads to less reliable test results due to a prolongation of the gastric residence time of the diagnostic agent urea. This might allow the bacteria to split a higher proportion of urea, resulting in an increased number of false positives. 12 probands received 150mg [15N]urea and 75 mg sodium [13C]acetate in 75 ml orange juice as a test drink. [15N]Urea served as an agent to diagnose gastric H. pylori colonization. The 15N tests were evaluated using a urine sample of the second hour after test start. [13C]Acetate served as a marker of the gastric emptying of water-soluble food including the urea under the influence of food intake. Breath air samples were taken to calculate the gastric emptying half life (EHL) and the apparent resorption time (RT) of the urea. The double tests were carried out four times within four weeks using identical test protocols but different standardized time periods of pretest fasting: overnight, two hours prior to test, one hour prior to test, and no fasting at all. The food intake amount was standardized. Five probands testing positive in the overnight fasting test were also found to be positive in the other test variants with more or less empty stomachs. Seven other probands testing negative after overnight fasting tested negative in the other test variants as well. It is concluded that food intake prior to the test drink does not have much of an influence on the gastric residence time of urea and so on the qualitative H. pylori test results. Due to identical behaviour of [13C]urea and [15N]urea in the stomach, this influence is believed to be independent on the labelling isotope. For survey purpose, no fasting conditions are required for the H. pylori tests.
A sufficiently stable rate of 13CO2 exhalation is necessary when the diagnostic 13CO2 breath tests are performed in healthy subjects and patients. The aim of the research was to define prerequisite conditions for kinetic breath tests in order to ensure a stable 13CO2 background. A 3-part protocol was developed. Part I: a study of the one-day variation of 13CO2 abundance in expired CO2 confirmed that shifts of the basal 13C abundance in breath are inherent in nature. Part II: a study of the variations of 13C enrichment after the ingestion of different meals and beverages showed that ingestion of food items containing C4 plant sugars, such as maize, induces a significant increase in isotopic abundance. Part III: a new test breakfast containing rice grain cereal, milk and orange juice was tested. This test meal induces no significant change on the basal 13CO2 abundance in healthy subjects. This new finding allows to avoid the fasting period normally required prior to a breath test which is sometimes difficult for children and pregnant women.
The use of isotopic carbon dioxide lasers for determination of carbon (and oxygen) isotope ratios was first demonstrated in 1994. Since then a commercial device called LARA, has been manufactured and used for Helicobacter pylori breath tests using (13)C-labelled urea. The major advantages of the optogalvanic effect compared with other infrared absorption isotope ratio measurement techniques are its lack of optical background and its high sensitivity resulting from a signal gain proportional to laser power. Continuous normalisation using two cells, a standard and sample, lead to high accuracy as well as precision. Recent advances in continuous flow measurement of (13)C/(12)C ratios of CO(2) in air and extensions of the technique to (14)C, which can be analysed as a stable isotope, are described.
A continuous dual 13CO2 and 15NH4(15)NO3 labelling experimental set-up is presented that was used to investigate the C and N uptake and allocation within 3-year old beech (Fagus sylvatica L.) during one growing season. The C and N allocation pattern was determined after six, twelve and eighteen weeks of growth. The carbon uptake was distinctly different in the three phases examined: The first six weeks after budbreak were dedicated to leaf growth with a R/S (root to shoot) ratio of 0.14 for the new carbon. The second growth phase showed a balanced R/S ratio of C allocation and after week 13, the root compartment was the main carbon sink (R/S = 6.97). Nitrogen allocation was more basipetal as compared to carbon. In the second growth phase, R/S of Nnew was 5.57 but fell to 3.54 for the third growth phase probably due to formation of reserves in buds and stem.
Abstract The factors for (18)O/(16)O fractionation between carbonates and CO(2) gas produced by the dissolution of the carbonates in phosphoric acid (sealed vessel method) have been investigated as a function of reaction temperature (20-90°C) and cationic substitution in the solid. Synthetic CaCO(3), Ca(0.75) Mn(0.25) CO(3), MnCO(3), BaCO(3) and SrCO(3) powders, and a natural kutnahorite sample were used as solids. The δ(18)O values of the gaseous CO(2) liberated by the reaction with phosphoric acid decrease with increasing temperature and seem to be a linear function of T(°K)(-2). The slopes are specific for different carbonates. No temperature-depended (13)C/(12)C fractionation seems to exist.
A methylchloroformate derivative was used for the simultaneous determination of plasma enrichments of 1-13C-phenylalanine, 1-13C-tyrosine, 15N-phenylalanine and 15N-tyrosine by gas chromatography/mass spectrometry. All four tracer enrichments could be measured in a single GC run. A specific ion fragment was obtained for each tracer. This approach allowed an easy determination of the "tracer to tracee ratios". Each ion fragment could be measured with an appropriate single-to-noise ratio and precision in samples obtained from 100 microliters plasma. The derivatization consists of a fast one-step reaction. Therefore it is well suited for studies involving a large number of samples, such as non-steady state bolus studies.
Most regions in the tropics undergo high seasonal precipitation that produces cyclic patterns of riverine discharge, resulting in periods characterized by low and high water levels. Many chemical and bio-logical factors are affected by this hydrologic seasonality, and it therefore appeared to be very likely that aquatic food webs would also differ during the low and high water periods. Available carbon sources for fish are thought to be less varied during low water periods, but flooding during high water periods could bring fish into contact with a greater abundance and diversity of food sources such as terrestrial plants or the biofilms that grow on submerged terrestrial plants. At low water levels, higher fish densities may lead to more piscivory and less omnivory when compared with the high water periods. Therefore, trophic links within the fish communities may then be modified by water level changes in tropical reservoirs. To address this prediction, we performed stable isotope analyses of the most common species in Sélingué and Manantali, two large reservoirs in Mali (West Africa). Allochthonous and littoral carbon sources were shown to support fish production to a significant extent, even during low water periods. However, the allochthonous or littoral carbon contributions that sustained the top-predators production were indeed greater during the high water periods as expected. The expected higher omnivory in the high water period might have shortened the food chain when compared with the low water period. Some carnivorous fish species were shown to feed at lower trophic levels during high water periods in both reservoirs, but this was not a general pattern. Flooding did not, therefore, necessarily result in a shorter food chain when water levels were high.
A new, low-temperature sealed tube technique for combustion of organic carbon prior to subsequent off-line isotope analysis is proposed. Complete oxidation is achieved with potassium peroxodisulfate and silver permanganate as oxidants at temperatures not exceeding 500 degrees C. The combustion of gaseous (methane), solid (cane sugar, vanilla, N-thiazolyl-2-sulfamide, ascorbic acid, phenanthrene, thiourea, polyethylenefilm, tetrafluoropolyethylene, polyetheretherketone, graphite, and Suwannee River Fulvic Acid), and liquid (tetrachloroethene, toluene, and oil) model compounds and international standards was tested. A 24 h combustion at 500 degrees C was sufficient for complete oxidation in all cases. The time required for complete oxidation of Suwannee River Fulvic Acid, typical of refractory freshwater dissolved organic carbon, as a function of combustion temperature was 2 h at 500 degrees C, 6 h at 400 degrees C, and 24 h at 300 degrees C. Preparation of saline solution parallels of cane sugar, vanilla, N-thiazolyl-2-sulfanilamide, and ascorbic acid gave consistent results. For reproducible delta13C analyses using a Thermoquest MAT 252 MS, a minimum of 5 microg C had to be combusted. Reliable 14C results, measured at an accelerator mass spectrometer facility, were obtained from coal and from cane sugar combusted for 24 h at 500 degrees C by the proposed method.
Abstract Locust nymphs were raised from hatching to adult locusts on either seedling wheat (C(3)) or maize (C(4)), to determine whether relative enrichments/depletions of (15)N and (13)C within body tissues are influenced by diet. The maize contained less hexose sugars and protein per gram than wheat. The isotopic spacing between the food and the whole insect was found to differ between the two diets. The lower quality maize diet showed an overall +5.1‰ enrichment in δ(15)N compared to + 2.8‰ for wheat, possibly due to increased fractionation due to protein recycling. The maize diet resulted in increased depletion in lipid and trehalose and depletion in chitin relative to diet. The results for both δ(15)N and δ(13)C suggest that substrate recycling was occurring on the low quality maize diet. Therefore diet quality determines the enrichment/depletion in δ(15)N and δ(13)C within organisms.
Between 2008 and 2010 various batch experiments were carried out to study the stable carbon isotopic composition of biogas (CH4 and CO2) produced from (i) pure sludge and (ii) sludge including maize. From the evolution of the natural isotopic signature, a temporal change of methanogenic pathways could be detected for the treatment with maize showing that a dominance in acetotrophic methanogenesis was replaced by a mixture of hydrogenotrophic and acetotrophic methanogenesis. For pure sludge, hydrogenotrophic methanogenesis was the dominant or probably exclusive pathway. Experiments with isotopically labelled acetate (99% ¹³CH3COONa and 99%
COONa) indicated a significant contribution of syntrophic acetate oxidation (SAO) for all the investigated treatments. In the case of pure sludge, experiments from 2008 showed that acetate was almost entirely oxidised to CO2, i.e. acetotrophic methanogenesis was negligible. However, in 2010, the sludge showed a clear dominance in acetotrophic methanogenesis with a minor contribution by SAO indicating a significant change in the metabolic character. Our results indicate that SAO during anaerobic degradation of maize might be a significant process that needs to be considered in biogas research.
We investigated turnover of methane (CH4) in soils from a poorly drained UK forest. In situ, this forest exhibited a negligible soil-atmosphere CH4 flux, whereas adjacent grassland plots were sources of CH4. We hypothesised that the forest plots exhibited reduced anaerobic CH4 production through water-table draw down. Consequently, we exposed soil cores from under oak to high and low water-table conditions in the laboratory. Methane fluxes increased significantly in the high water-table (1925+/-1702 mug CH4 m(-2) h(-1)) compared to the low one (-3.5+/-6.8 microg CH4 m(-2) h(-1)). Natural abundance delta13C values of CH4 showed a strong depletion in high water-table cores (-56.7+/-2.9 per thousand) compared to methane in ambient air (-46.0 per thousand) indicative of methanogenic processes. The delta13C values of CH4 from low water-table cores (delta13C-46.8+/-0.2 per thousand) was similar to ambient air and suggested little alteration of headspace CH4 by the soil microbial community. In order to assess the CH4 oxidizing activity of the two treatments conclusively, a 13CH4 spike was added to the cores and 13CO2 production was measured as the by-product of CH4 oxidation. 13CH4 oxidation rates were 57.5 (+/-12.7) and 0.5 (+/-0.1) microg CH4 m(-2) h(-1) for high and low water-tables, respectively. These data show that the lower water-table hydrology treatment impacted methanogenic processes without stimulating methanotrophy.