Abstract
RationaleIt was recently shown that the presence of CO2 affects the stable isotope (δ2H and δ18O values) analysis of water vapor via Wavelength‐Scanned Cavity Ring‐Down Spectroscopy. Here, we test how much CO2 is emitted from soil samples and if the CO2 in the headspace influences the isotope analysis with the direct equilibration method by Off‐Axis Integrated Cavity Output Spectroscopy (OA‐ICOS).MethodsThe headspace above different amounts of sparkling water was sampled, and its stable isotopic composition (δ2H and δ18O values) and CO2 concentration 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.ResultsThe experiment with the sparkling water showed that CO2 does not influence the stable isotope analysis by OA‐ICOS. CO2 was 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 CO2 can be related to soil moisture and organic matter which both enhance microbial activity.ConclusionsWe found, despite the high CO2 emissions from soil samples, no need for a post‐correction of the pore water stable isotope analysis results, since there is no relation between CO2 concentrations 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.
Highlights
The application of the direct equilibration for stable isotope analysis of pore waters[9] is increasingly used.[2,3,4,10,11,12]. Because this direct equilibration method uses laser spectrometry to determine the isotopic composition of the headspace in equilibrium with the soil pore water, volatile compounds can potentially alter the analysis by spectral interferences, when they absorb the laser in similar wavelengths to the isotopologues of water.[13,14]
We conclude that CO2 in vapor samples does not affect the measurement of the stable isotopic composition of the water vapor by off-axis integrated cavity output spectroscopy (OA-ICOS)
No post-correction is needed when applying the direct equilibration method with Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) to determine the isotopic composition of soil water
Summary
The effect of CO2 on stable isotope analysis has only been tested for WS-CRDS[15] and there is a need to assess if or how CO2 affects the analysis with Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) Both analyzer systems use the absorption of a near-infrared laser beam by molecules (i.e. isotopologues) in a gaseous sample in a high-finesse optical cavity.[20,21] directing the laser beam off-axis allows spatial separation of the multiple reflections within the cavity,[22] which results in fully resolved OA-ICOS absorption spectra.[23] unlike WS-CRDS, OA-ICOS does not derive the isotope ratios in the cavity from discrete wavelength sampling measurements. How this different instrumental set up for OA-ICOS may affect the influence of CO2 on the measurements of stable isotopes of water vapor has not yet been studied. The isotope analyses described below were performed within 1 week after soil sampling
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