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| Titel |
Unique Laser Spectroscopic Approach for High-Precision Compound-Specific Isotope Analysis of 13C/12C and D/H of Combustion Products |
| VerfasserIn |
Nabil Saad, Doug Kuramoto, Herb Tobias, Bruce Richman, Tom Brenna, Richard Zare |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250056980
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| Zusammenfassung |
| The conventional method to perform high-precision stable carbon isotope measurements with
continuous flow-IRMS requires the conversion of the organic sample to carbon dioxide via
combustion which is then analyzed to determine the ratio of 13C to 12C. This process also
results in the formation of water which can be problematic for measurements using IRMS
since the presence of H2O and CO2 simultaneously in the gas stream can lead to the
protonation product HCO2+ in the mass spectrometer’s ion source which interferes with the
accurate determination of carbon isotope ratios. To address such a detrimental interference,
water is invariably eliminated through the use of permeable membranes or cold traps at the
expense of the loss of hydrogen isotopes information embedded in the trapped
water molecules. This wasted D/H information is rather valuable due to its large
variation in nature and to the additional isotopic dimension it provides for various
applications.
Optical spectroscopy-based instruments measure light stable isotope ratios using the shift in
the rovibrational absorption peaks generated from the different masses of the isotopomers.
The isotopes most commonly measured spectroscopically are 12C, 13C in CO2 and 1H, 2H
and 16O, 18O in H2O molecules. The ability to spectroscopically measure carbon isotopes in
CO2 and hydrogen isotopes in H2O leads to the possibility of measuring carbon and
hydrogen isotopes from the combustion products, which is the focus of this work. We present
here a unique laser spectroscopic approach for making high-precision compound-specific
isotope analysis (CSIA) measurements of both the 13C/12C and the 2H/1H isotope ratios of
the CO2 and H2O combustion products. The instrument consists of a single front-end
comprised of a gas chromatograph (GC) for the separation of the organic mixture coupled to
a novel micro-fabricated micro-reactor (MFMR) for combustion of each organic
compound into carbon dioxide, water, and other oxidation products, and the precise
measurement of the 13C/12C in the carbon dioxide gas and 1H/2H in the water vapor
from the well established infrared spectrum of both gases, using an isotopic CO2
Cavity Ring-Down Spectroscopy (CRDS) analyzer and an isotopic water vapor
CRDS analyzer, respectively. Light hydrocarbons are used as our test compounds in
this study, owing to their mud-logging diagnostic significance in exploratory and
routine oil drilling and their suitability for our present work on a pilot instrumental
setup. The analyses of methane, ethane and propane for 13C/12C and D/H values
achieved a precision level better than 1 permil and 2 permil, respectively. These
CRDS results were further compared to the incumbent GC-C-IRMS (13C/12C)
and GC-Py-IRMS (D/H) techniques and showed excellent agreements in isotopic
measurements. |
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