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Concentration effects on laser-based δ 18 O and δ 2 H measurements and implications for the calibration of vapour measurements with liquid standards

Abstract : Recently available isotope ratio infrared spectroscopy can directly measure the isotopic composition of atmospheric water vapour (del O-18, del H-2), overcoming one of the main limitations of isotope ratio mass spectrometry (IRMS) methods. Calibrating these gas-phase instruments requires the vapourisation of liquid standards since primary standards in principle are liquids. Here we test the viability of calibrating a wavelength-scanned cavity ring-down spectroscopy (CRDS) instrument with vapourised liquid standards. We also quantify the dependency of the measured isotope values on the water concentration for a range of isotopic compositions. In both liquid and vapour samples, we found an increase in del O-18 and del H-2 with water vapour concentration. For del O-18, the slope of this increase was similar for liquid and vapour, with a slight positive relationship with sample del-value. For del H-2, we found diverging patterns for liquid and vapour samples, with no dependence on del-value for vapour, but a decreasing slope for liquid samples. We also quantified tubing memory effects to step changes in isotopic composition, avoiding concurrent changes in the water vapour concentration. Dekabon tubing exhibited much stronger, concentration-dependent, memory effects for del H-2 than stainless steel or perfluoroalkoxy (PFA) tubing. Direct vapour measurements with CRDS in a controlled experimental chamber agreed well with results obtained from vapour simultaneously collected in cold traps analysed by CRDS and IRMS. We conclude that vapour measurements can be calibrated reliably with liquid standards. We demonstrate how to take the concentration dependencies of the del-values into account. Copyright (C) 2010 John Wiley & Sons, Ltd.
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https://hal-cnrs.archives-ouvertes.fr/hal-02389368
Contributor : Philippe Biron <>
Submitted on : Monday, December 2, 2019 - 2:51:27 PM
Last modification on : Wednesday, October 14, 2020 - 4:12:38 AM

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Markus Schmidt, Kadmiel Maseyk, Céline Lett, Philippe Biron, Patricia Richard, et al.. Concentration effects on laser-based δ 18 O and δ 2 H measurements and implications for the calibration of vapour measurements with liquid standards. Rapid Communications in Mass Spectrometry, Wiley, 2010, 24 (24), pp.3553-3561. ⟨10.1002/rcm.4813⟩. ⟨hal-02389368⟩

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