Isotope Ratio Monitoring Research Articles

Stable Carbon Isotopic Analysis of Low-Level Methane in Water and Gas

Sample introduction systems have been developed to significantly extend existing isotope ratio monitoring gas chromatography/mass spectrometry techniques to allow the simultaneous determination of the concentration and stable carbon isotopic ratio of methane from low-concentration aqueous and gaseous samples. These systems, along with improvements to the basic analytical system, are used with 1−50 nM water samples and 45 ppb−250 ppm (v/v) gas samples. The detection limit of the technique is 200 pmol of CH4 in either aqueous or gaseous samples. The precision of isotopic measurements of replicate samples is ≤0.8‰ for water and 0.9‰ when 200-pmol gas samples are analyzed, the latter improving to less than 0.1‰ with 4-nmol or larger gas samples.

Maleimides (1 H-pyrrole-2,5-diones) as molecular indicators of anoxygenic photosynthesis in ancient water columns

Maleimides (1 H-pyrrole-2,5-diones), degradation products of photosynthetic tetrapyrrole pigments, have been found for the first time in the polar fraction of the solvent extracts of two marine sediments deposited in restricted basins: Kupferschiefer (Permian) and Serpiano shale (Mid-Triassic). GC and GC—MS analyses of the TBDMS ( tertiary-butyldimethylsilyl) derivatives show a simple component distribution, dominated by Me Et maleimide, mainly of planktonic origin; Me n-Pr and Me i-Bu maleimides, present in low abundance, are thought on structural grounds to be derived from the bacteriochlorophylls c, d, or e of Chlorobiaceae (anoxygenic green sulfur bacteria). This is confirmed for Kupferschiefer by isotope ratio monitoring (irm) GCMS which shows them to be enriched in 13C as a result of their photosynthetic carbon assimilation, which takes place by the reversed tricarboxylic acid (TCA) cycle. The structurally more specific Me i-Bu maleimide is, however, slightly more enriched in 13C than Me n-Pr maleimide, suggesting that the latter is derived in part from reduction of the C 3-acid substituent at C-17 of phytoplanktonic chlorophyll. These results provide evidence for the existence in both depositional settings of microbial communities containing Chlorobiaceae. In turn, this indicates that there must have been periods when the water column was highly stratified and anoxia extended into the zone of light penetration.

New Methods for Fully Automated Isotope Ratio Determination from Hydrogen at the Natural Abundance Level

Abstract A variety of methods for measurement of (2)H/(1)H from H(2) are evaluated for their ability to be fully automated and for applicability to automated isotopic analysis of water and organic compounds. Equilibration of water with H(2) gas 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 H(2) to the mass spectrometer. Any such method has to address the problem of He ions corrupting the measurement of the H(2) ions. One such approach uses a heated palladium membrane for selective introduction of H(2) into 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 before irm techniques can be used in routine applications, in particular for measuring hydrogen isotopes from GC effluents (irm-GCMS).

Microcombustion of ng Amounts of Carbon in Non-Volatile Materials for isotope Ratio Evaluation

Abstract 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. CO(2) removed from the carrier gas stream by cryogenic trapping is transferred onto a GC column. Following GC separation, the CO(2) is 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 δ(13)C = -28.8‰.

High precision isotope ratio monitoring techniques in mass spectrometry.

Isotope ratio monitoring following on-line combustion is a new method in gas chromatography/mass spectrometry (GC/MS) that allows the ratios of abundances of stable isotopes of elements such as carbon and nitrogen to be determined for individual compounds introduced via a gas chromatograph. It is the first combustion method that allows direct measurement of isotope ratios of individual molecular components of mixtures. Unlike traditional gas isotope ratio measurements made on pure samples, the method requires only very small samples, viz. several picolitres of the vapour. This paper reviews the principles and history of gas isotope ratio monitoring, focusing on the instrumentation used in isotope ratio monitoring GC/MS, including the combustion procedures used for atomization. Standardization methods and data manipulation techniques are described, as are applications to geochemistry, biology, medicine and other areas of science.

Performance and optimization of a combustion interface for isotope ratio monitoring gas chromatography/mass spectrometry.

Conditions and systems for on-line combustion of effluents from capillary gas chromatographic columns and for removal of water vapor from product streams were tested. Organic carbon in gas chromatographic peaks 15 s wide and containing up to 30 nanomoles of carbon was quantitatively converted to CO2 by tubular combustion reactors, 200 x 0.5 mm, packed with CuO or NiO. No auxiliary source of O2 was required because oxygen was supplied by metal oxides. Spontaneous degradation of CuO limited the life of CuO reactors at T > 850 degrees C. Since NiO does not spontaneously degrade, its use might be favored, but Ni-bound carbon phases form and lead to inaccurate isotopic results at T < 1050 degrees C if gas-phase O2 is not added. For all compounds tested except CH4, equivalent isotopic results are provided by CuO at 850 degrees C, NiO + O2 (gas-phase mole fraction, 10(-3)) at 1050 degrees C and NiO at 1150 degrees C. The combustion interface did not contribute additional analytical uncertainty, thus observed standard deviations of 13C/12C ratios were within a factor of 2 of shot-noise limits. For combustion and isotopic analyses of CH4, in which quantitative combustion required T approximately 950 degrees C, NiO-based systems are preferred, and precision is approximately 2 times lower than that observed for other analytes. Water must be removed from the gas stream transmitted to the mass spectrometer or else protonation of CO2 will lead to inaccuracy in isotopic analyses. Although thresholds for this effect vary between mass spectrometers, differential permeation of H2O through Nafion tubing was effective in both cases tested, but the required length of the Nafion membrane was 4 times greater for the more sensitive mass spectrometer.

Analysis of kerogens and kerogen precursors by flash pyrolysis in combination with isotope‐ratio‐monitoring gas chromatography‐mass spectrometry (irm‐GC‐MS)

AbstractThis study describes the application of isotope ratio monitoring gas chromatography‐ mass spectrometry (irm‐GC‐MS) for compound‐specific stable carbon isotopic analysis of aliphatic hydrocarbon and phenolic products from flash pyrolysis (800 °C, 20s) of natural biopolymers and sedimentary kerogens. As part of this work, we provide a detailed description of the analysis of complex samples, including approaches for peak integration, data handling and correction for derivative carbons. Several aliphatic and aromatic biopolymers are analyzed by irm‐GC‐MS in order to establish relationships between the isotopic signatures of pyrolysis products and those of their parent macromolecules. We also analyze a select group of kerogens and kerogen precursors of different ages and biopolymer compositions to evaluate the applicability of combined pyrolysis/irm‐GC‐MS to complex geochemical mixtures. Our findings suggest that, in spite of the wide degree of heterogeneity, the isotopic values of individual aliphatic and phenolic pyrolysis products determined by irm‐GC‐MS can be related to the isotopic composition of the total organic carbon in kerogens and used to trace its biological sources. This study also highlights the need for optimum chromatographic separation in order to fully realize the potential of compound specific isotope analyses.

Stable carbon isotopic correlation of individual biolipids in aquatic organisms and a lake bottom sediment

Isotope ratio monitoring GC-MS (irm-GCMS) analyses of individual hydrocarbons isolated from the surface sediment of a small eutrophic lake (Priest Pot, U.K.) and a phytoplankton tow sample from the upper water column have been undertaken in order to assign sources of sedimentary lipids. The δ 13C values for the sedimentary n-alkanes showed no clear distinction between lower molecular weight components (C 17–C 19) of presumed algal origin and higher members (C 23–C 33) of presumed higher plant origin, thus indicating a limitation to the use of compound specific isotope analysis in the freshwater system studied. In contrast, the δ 13C values of sedimentary hopenes were markedly depleted (by over 20%) compared with those in the phytoplankton sample from the upper water column. Hop-22(29)-ene in the surficial sediment had a δ 13C value of −55.2%, which contrasts witht he value of the phytoplankton component (−32.7%), attributed to cyanobacteria. These values indicate that a highly 13C-depleted source provides a major contribution to the sedimentary component. It is proposed that the sedimentary hopenes originate in part from methanotrophic bacteria, demonstrating the presence of a bacterial methane cycle in this environment.

Application of isotope ratio monitoring gas chromatography–mass spectrometry to the analysis of organic residues of archaeological origin

Isotope ratio monitoring gas chromatography–mass spectrometry (irm GC–MS) was used to determine the δ13C values of individual compounds present in organic residues preserved in archaeological potsherds. The lipid fractions of the preserved organic residues were extracted from potsherds recovered from excavations performed as part of the Raunds Area Project, Northamptonshire, UK. The lipid extracts were analysed by high temperature GC and GC–MS. The leaf wax of a contemporary wild-type Brassica was also extracted and analysed for comparative purposes. Prior to irm GC–MS analysis, lipid extracts were fractionated; alkane and ketone fractions were obtained from the wild-type Brassica leaf wax, the total lipid extracts of two of the potsherds, and an archaeological soil sample using small-scale flash chromatography. The δ13C values obtained for the samples were consistent with the individual lipids investigated being of C3 plant origin. These data support the hypothesis that the lipids preserved in the potsherds were derived from a Brassica species such as cabbage. These results confirm the potential value of irm GC–MS to derive stable isotope ratios for individual lipid species of archaeological interest, and, more specifically, the potential for application of the technique to palaeodietary investigations.