@article{HartmanGentzSchilleretal.2018,
  author    = {Hartman, Jan F. and Gentz, Torben and Schiller, Amanda and Greule, Markus and Grossart, Hans-Peter and Ionescu, Danny and Keppler, Frank and Martinez-Cruz, Karla and Sepulveda-Jauregui, Armando and Isenbeck-Schroeter, Margot},
  title     = {A f ast and sensitive method for the continuous in situ determination of dissolved methane and its delta C-13-isotope ratio in surface waters},
  series = {Limnology and Oceanography-methods},
  volume    = {16},
  journal   = {Limnology and Oceanography-methods},
  number    = {5},
  publisher = {Wiley},
  address   = {Hoboken},
  issn      = {1541-5856},
  doi       = {10.1002/lom3.10244},
  pages     = {273 -- 285},
  year      = {2018},
  abstract  = {A fast and sensitive method for the continuous determination of methane (CH4) and its stable carbon isotopic values (delta C-13-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 delta C-13-CH4 with synthetic water standards. The detection limit of the M-CRDS for the simultaneous determination of CH4 and delta C-13-CH4 is 3.6 nmol L-1 CH4. A measurement precision of CH4 concentrations and delta C-13-CH4 in the range of 1.1\%, respectively, 1.7 parts per thousand (1 sigma) and accuracy (1.3\%, respectively, 0.8 parts per thousand [1 sigma]) was achieved for single measurements and averaging times of 10 min. The response time tau of 57 +/- 5 s allow determination of delta C-13-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 L-1) and lightest delta C-13-CH4 (-35.2 parts per thousand) 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 delta C-13-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.},
  language  = {en}
}