Real Time Measurement of Concentration and δ¹³C-CH₄ in Water

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• 作者：Jan F. Hartmann^a ; ^{Jan.Hartmann@geow.uni-heidelberg.de} ; Amanda Schiller^a ; Torben Gentz^b ; Markus Greule^a ; Hans-Peter Grossart^c ; Danny Ionescu^c ; Frank Keppler^a ; Karla Martinez-Cruz^c ; Armando Sepulveda-Jauregui^c ; Margot Isenbeck-Schrö ; ter^a
• 关键词：CH4 in water ; δ13C-CH4 in water ; in situ ; CRDS analyser ; Picarro G-2201-i ; membrane module
• 刊名：Procedia Earth and Planetary Science
• 出版年：2017
• 出版时间：2017
• 年：2017
• 卷：17
• 期：Complete
• 页码：460-463
• 全文大小：243 K
• 卷排序：17

文摘

A new, high temporal and spatial resolution, in-situ method for the fast and precise determination of methane (CH4) concentration and its δ13C isotope value (δ13C-CH4) in water was developed. Methane is extracted from water by the application of a vacuum to a membrane gas/liquid exchange module and analysed by a portable cavity ring-down spectroscopy (CRDS) analyser (Picarro® G2201-i). The system was calibrated for concentration and δ13C of CH4 using synthetic water standards. Reference samples were analysed with gas chromatography (GC) and mass spectroscopy (GC-C-IRMS) via the headspace method. All results show good correlations for CH4 concentration (R2 = 0.994) and a measuring accuracy of 0.002 ppm (CH4 concentration) and 0.6‰ (δ13C-CH4). Calculated response times τ were averaged, resulting in 13.5 s. This allows much faster analyses compared to previously published methods. This method was successfully tested in a field campaign at Lake Stechlin (Germany), which has a pronounced CH4 peak at the thermocline in summer. Hypolimnic CH4 concentrations up to 0.6 μmol/l indicate microbial methanogenesis in the well-oxygenated upper 10 m of the water column. Simultaneous measurements by the CRDS-membrane system (M-CRDS), a membrane combined with off-axis integrated cavity output spectrometer system (M-ICOS) and gas chromatography (GC) show similar results. 24-hour measurements were performed at the depth of maximum CH4 concentration and indicate short-term variations of 0.2 μmol/l (CH4 concentration) and 7‰ (δ13C-CH4) within 24 hours. The M-CRDS provides a high temporal and spatial data resolution for accurate detection, mapping and quantification of CH4 distribution in aquatic systems.