Carbon isotope analysis of dissolved organic carbon in fresh and saline (NaCl) water via continuous flow cavity ring-down spectroscopy following wet chemical oxidation |
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Authors: | Christopher H Conaway Burt Thomas Nabil Saad James J Thordsen Yousif K Kharaka |
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Institution: | 1. USGS National Research Program, Menlo Park, CA, USAcconaway@usgs.gov;3. USGS National Research Program, Menlo Park, CA, USA;4. Picarro, Inc, Santa Clara, CA, USA |
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Abstract: | This work examines the performance and limitations of a wet chemical oxidation carbon analyser interfaced with a cavity ring-down spectrometer (WCO-CRDS) in a continuous flow (CF) configuration for measuring δ13C of dissolved organic carbon (δ13C-DOC) in natural water samples. Low-chloride matrix (<5?g Cl/L) DOC solutions were analysed with as little as 2.5?mg C/L in a 9?mL aliquot with a precision of 0.5?‰. In high-chloride matrix (10–100?g Cl/L) DOC solutions, bias towards lighter δ13C-DOC was observed because of incomplete oxidation despite using high-concentration oxidant, extended reaction time, or post-wet chemical oxidation gas-phase combustion. However, through a combination of dilution, chloride removal, and increasing the oxidant:sample ratio, high-salinity samples with sufficient DOC (>22.5?µg C/aliquot) may be analysed. The WCO-CRDS approach requires more total carbon (µg C/aliquot) than conventional CF-isotope ratio mass spectrometer, but is nonetheless applicable to a wide range of DOC concentration and water types, including brackish water, produced water, and basinal brines. |
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Keywords: | cavity ring-down spectroscopy carbon-13 isotope hydrology isotope measurements methods and equipment wet chemical oxidation |
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