Carbon isotope analysis of dissolved organic carbon in fresh and saline (NaCl) water via continuous flow cavity ring-down spectroscopy following wet chemical oxidation

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.

Keywords:

• cavity ring-down spectroscopy
• carbon-13
• isotope hydrology
• isotope measurements
• methods and equipment
• wet chemical oxidation

Acknowledgements

The authors thank Jeff Lane, David Hart, Noel Bauman, and John Welsh for software support for the TOC analyser; Guillermo Jiménez, Steve Silva, and Gilles St-Jean for analytical support and advice; and George Aiken for a review of the manuscript. The authors graciously acknowledge the support of the USGS National Research Program. Any use of trade names is for descriptive purposes only and does not imply endorsement by the USA Government.

Disclosure statement

Dr N Saad, who contributed novel software and hardware design for this study, is employed by Picarro, Inc., the manufacturer of the USGS-owned CRDS instrument used in this study. All experimental work, data analysis, and interpretation were principally performed by CH Conaway and the other USGS coauthors, who declare no conflict of interest.