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Automated, in situ measurements of dissolved CO 2 , CH 4 , and δ 13 C values using cavity enhanced laser absorption spectrometry: Comparing response times of air‐water equilibrators
Author(s) -
Webb Jackie R.,
Maher Damien T.,
Santos Isaac R.
Publication year - 2016
Publication title -
limnology and oceanography: methods
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.898
H-Index - 72
ISSN - 1541-5856
DOI - 10.1002/lom3.10092
Subject(s) - isotopologue , analytical chemistry (journal) , chemistry , absorption (acoustics) , exponential decay , absorption spectroscopy , mass spectrometry , reaction rate constant , volumetric flow rate , spectrometer , kinetics , materials science , molecule , physics , chromatography , optics , organic chemistry , nuclear physics , quantum mechanics , composite material
Combining air‐water equilibrators with a field deployable cavity enhanced laser absorption spectrometer (CELAS) can generate precise, high resolution, measurements of dissolved CO 2 and CH 4 concentrations and δ 13 C values in aquatic systems. However, equilibration response times for combined measurements of CO 2 and CH 4 isotopologues have not been assessed. We performed laboratory step experiments on six different equilibrators to constrain CO 2 and CH 4 equilibration time constants ( τ ; high‐to‐low exponential decay constant). Three equilibrator types were then used in field‐based step experiments to determine τ for the individual isotopologues 12 CO 2 , 13 CO 2 , 12 CH 4 , and 13 CH 4 . In the laboratory experiments, τ ranged from 34–124 s for CO 2 and 117–2041 s for CH 4 among the six equilibrators. The ratio between response times of CO 2 and CH 4 was substantially lower in the membrane type equilibrators ( τ ‐CH 4 ∼5 times > τ ‐CO 2 ) than in the showerhead and marble types ( τ ‐CH 4 ∼15 times > τ ‐CO 2 ). Individual isotopologue time constants under a water flow rate of ∼5.5 L min −1 ranged from 33.7–43.1 s for 12 CO 2 and 13 CO 2 , and 177–347 s for 12 CH 4 , and 13 CH 4 . The τ of CO 2 isotopologues were within 1 s while τ of CH 4 isotopologues were the same. Further investigations into water flow rate revealed an exponential decrease in equilibration time from 1.5 L min −1 to 9 L min −1 in a marble‐type equilibrator. The response time was always longer from high‐to‐low than low‐to‐high concentrations. By taking into consideration the equilibration response time, measurements of CO 2 , CH 4 , δ 13 C‐CO 2 , and δ 13 C‐CH 4 can be resolved in near real‐time using appropriate water‐air equilibration devices.