title: No influence of CO2on stable isotope analyses of soil waters with off-axis integrated cavity output spectroscopy (OA-ICOS).
abstract: BACKGROUND
  It was recently shown that the presence of CO2affects the stable isotope (δ2H and δ18O values) analysis of water vapor via Wavelength-Scanned Cavity Ring-Down Spectroscopy. Here, we test how much CO2is emitted from soil samples and if the CO2in the headspace influences the isotope analysis with the direct equilibration method by Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS).
  
  
  METHODS
  The headspace above different amounts of sparkling water was sampled, and its stable isotopic composition (δ2H and δ18O values) and CO2concentration were measured by direct equilibration and by gas chromatography, respectively. In addition, the headspace above soil samples was analyzed in the same way. Furthermore, the gravimetric water content and the loss on ignition were measured for the soil samples.
  
  
  RESULTS
  The experiment with the sparkling water showed that CO2does not influence the stable isotope analysis by OA-ICOS. CO2was emitted from the soil samples and correlated with the isotopic fractionation signal, but no causal relationship between the two was determined. Instead, the fractionation signal in pore water isotopes can be explained by soil evaporation and the CO2can be related to soil moisture and organic matter which both enhance microbial activity.
  
  
  CONCLUSIONS
  We found, despite the high CO2emissions from soil samples, no need for a post-correction of the pore water stable isotope analysis results, since there is no relation between CO2concentrations and the stable isotope results of vapor samples obtained with OA-ICOS. © 2016 The Authors. Rapid Communications in Mass Spectrometry Published by John Wiley & Sons Ltd.
authors.affiliation: Northern Rivers Institute, School of Geosciences, University of Aberdeen, Elphinstone Road, Aberdeen, AB24 3UF, UK.
authors.forename: Matthias
authors.identifier: http://orcid.org/0000-0003-1221-2767
authors.initials: M
authors.lastname: Sprenger
authors.affiliation: Northern Rivers Institute, School of Geosciences, University of Aberdeen, Elphinstone Road, Aberdeen, AB24 3UF, UK.
authors.forename: Doerthe
authors.identifier: http://orcid.org/0000-0002-7183-8674
authors.initials: D
authors.lastname: Tetzlaff
authors.affiliation: Northern Rivers Institute, School of Geosciences, University of Aberdeen, Elphinstone Road, Aberdeen, AB24 3UF, UK.
authors.forename: Chris
authors.initials: C
authors.lastname: Soulsby