A fast and sensitive method for the continuous in situ determination of dissolved methane and its d13C-isotope ratio in surface waters


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Torben.Gentz [ at ] awi.de

Abstract

A fast and sensitive method for the continuous determination of methane (CH4) and its stable carbon isotopic values (d13C-CH4) in surface waters was developed by applying a vacuum to a gas/liquid exchange membrane and measuring the extracted gases by a portable cavity ring-down spectroscopy analyser (M-CRDS). The M-CRDS was calibrated and characterized for CH4 concentration and d13C-CH4 with synthetic water standards. The detection limit of the M-CRDS for the simultaneous determination of CH4 and d13CCH4 is 3.6 nmol L21 CH4. A measurement precision of CH4 concentrations and d13C-CH4 in the range of 1.1%, respectively, 1.7& (1r) and accuracy (1.3%, respectively, 0.8& [1r]) was achieved for single measurements and averaging times of 10 min. The response time s of 5765 s allow determination of d13C-CH4 values more than twice as fast than other methods. The demonstrated M-CRDS method was applied and tested for Lake Stechlin (Germany) and compared with the headspace-gas chromatography and fast membrane CH4 concentration methods. Maximum CH4 concentrations (577 nmol L21) and lightest d13C-CH4 (235.2&) were found around the thermocline in depth profile measurements. The M-CRDS-method was in good agreement with other methods. Temporal variations in CH4 concentration and d13C-CH4 obtained in 24 h measurements indicate either local methane production/oxidation or physical variations in the thermocline. Therefore, these results illustrate the need of fast and sensitive analyses to achieve a better understanding of different mechanisms and pathways of CH4 formation in aquatic environments.



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Published
Eprint ID
47556
DOI 10.1002/lom3.10244

Cite as
Hartmann, J. F. , Gentz, T. , Schiller, A. , Greule, M. , Grossart, H. P. , Ionescu, D. , Keppler, F. , Martinez-Cruz, K. , Sepulveda-Jauregui, A. and Isenbeck-Schröter, M. (2018): A fast and sensitive method for the continuous in situ determination of dissolved methane and its d13C-isotope ratio in surface waters , Limnol. Oceanogr.: Methods . doi: 10.1002/lom3.10244


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