摘要:

The effects of earthworm activity on the concentration and isotopic composition of total sulfur in soils was investigated using batch experiments. Two ecologically different lumbricid species, the anecicLumbricus terrestrisand the endogeicAporrectodea caliginosa, were used. The earthworms were fed birch leaves, beech leaves, cattle manure or mixed plant litter. All food sources differed isotopically (δ34S) from the soil (Parabraunerde). As a reference, one experiment was carried out without additional food.

ISSN:1025-6016    

DOI:10.1080/10256019608036313

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

ISSN:1025-6016    

DOI:10.1080/10256019608036314

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

ISSN:1025-6016    

DOI:10.1080/10256019608036315

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A seven compartment model was applied for evaluation of oral L-[1-13C]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. CO2release from the latter occurs in a fast and a slow compartment into the central CO2compartment for exhalation. Using the fmins routine of MATLAB for parameter estimation, a good agreement was obtained between calculated and actually measured kinetics of13C-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-[1-13C]leucine, 5 μmol/kg; NaH13CO2, 1.2 μmol/kg; infusion: L-[1-13C]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.

ISSN:1025-6016    

DOI:10.1080/10256019608036316

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

ISSN:1025-6016    

DOI:10.1080/10256019608036317

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A novel microcombustion technique for carbon isotopic analysis of nanogram amounts of carbon in non-volatile materials based on isotope ratio monitoring (irm) mass spectrometry is described. Liquid or solid samples placed in a quartz sleeve are combusted at 1000°C in a continuous stream of helium and oxygen. CO2removed from the carrier gas stream by cryogenic trapping is transferred onto a GC column. Following GC separation, the CO2is transferred via an open split to the ion source of a gas isotope ratio mass spectrometer. Reproducibility for samples >25 nmol carbon is <1‰. Problems associated with blanks from various sources and with reproducible deposition of small sample amounts led to variable accuracy, which was dependent on the compound class being analysed. Minimum sample size was in the range from 5 to 10 nmol carbon. Measurements of dissolved organic carbon (DOC) of groundwater from Germany yielded consistent values of δ13C = -28.8‰.

ISSN:1025-6016    

DOI:10.1080/10256019608036318

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A variety of methods for measurement of2H/1H from H2are evaluated for their ability to be fully automated and for applicability to automated isotopic analysis of water and organic compounds. Equilibration of water with H2gas with the aid of a platinum catalyst has been commercialized into a fully automated sample preparation device. A second and newer technique, involving injecting water, methane, or other volatile organic compounds onto hot chromium in a reactor attached to the dual inlet system of a gas isotope ratio mass spectrometer, can be integrated with a conventional GC-autosampler to allow automated analysis of a variety of substrates. Both techniques result in precisions around 1‰ (δ notation) on the VSMOW scale, and are fast and accurate, and with appropriate mass spectrometers require only negligible scaling for the SLAP/VSMOW difference. Several experimental methods which show considerable promise employ “isotope ratio monitoring” (irm) inlet systems, in which a carrier gas is used for transport of H2to the mass spectrometer. Any such method has to address the problem of He ions corrupting the measurement of the H2ions. One such approach uses a heated palladium membrane for selective introduction of H2into the mass spectrometer, and a second involves modifications to the ion optics to control the stray helium ions. Both approaches have significant limitations that must be overcome beforeirmtechniques can be used in routine applications, in particular for measuring hydrogen isotopes from GC effluents (irm-GCMS).

ISSN:1025-6016    

DOI:10.1080/10256019608036319

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A Liquid Chromatography-Combustion (LC-C) Interface, based on a moving wire technique, has been built and tested. The LC effluent is deposited onto a transport wire, which carries the sample through solvent evaporation and combustion ovens. CO2from the combustion step is analysed in an isotope ratio mass spectrometer. Performance of the interface was tested by loop injections of sucrose and glucose into a liquid flow of methanol/water (80/20). Accuracy and precision of δ13CPDB< 1‰ was achieved for sample concentrations > 500 ng/ul (5μl loop), sufficient for studies at natural isotope ratios. In case of13C tracer applications the detection limit was determined to be about 20 pg carbon tracer (on wire).

ISSN:1025-6016    

DOI:10.1080/10256019608036320

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The authenticity of natural and synthetic matter can be checked by measurement of the isotope ratios of C, N and S. Controlled substances like drugs of abuse (cocaine, heroin) and explosives (TNT) or, simply, traces of paint can hold information in their isotope pattern. Total combustion of samples in an elemental analyzer followed by on-line determination of the isotopes of the combustion products (CO2, N2, SO2) in an isotope ratio mass spectrometer (EA-IRMS) provides high sample throughout with a minimum of sample preparation.

ISSN:1025-6016    

DOI:10.1080/10256019608036321

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

In the 1950s Hauck introduced a special version of the15N dilution technique (15N flux method) for the determination of N losses from the soil by denitrification. Although this method is very useful and reliable its application has been rather infrequent up to now. This is mainly due to the need to measure the m/z 30 in addition to the usually measured m/z 28 and 29 for dinitrogen, because the15N in the enriched air sample taken from an enclosure (cover box) at the soil surface is nonrandom. The signal from the m/z 30 is very low and difficult to measure with sufficient precision because other species (e.g. NO) also having the m/z 30 often interfere with its measurement. In this study the accuracy and precision of an easy to use CF-IRMS with sample batch operation to measure the ratio 30/28 was investigated. The relative standard deviation (RSD = precision) from natural abundance up to 2 at.% was always <1%. After correction of the mass ratio 30/28 (R30), by means of a formula obtained by linear regression of theoretical R30 against measured R30, the accuracy of the abundance calculated from this corrected R30 was very high. From the achieved precision and assuming a cover box height of 10 cm (headspace volume of 7 1), and a collection time of 2 h, a limit of detection for N2losses by denitrification equivalent to 16 g N/ha*d or 6 kg N/ha*a can be estimated. The performance of the15N dilution method using the equipment and procedure described is demonstrated by means of results from an incubation experiment with [15N]nitrate-amended soils.

ISSN:1025-6016    

DOI:10.1080/10256019608036322

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor