Measurements of water vapor isotope ratios with wavelength-scanned cavity ring-down spectroscopy technology: new insights and important caveats for deuterium excess measurements in tropical areas in comparison with isotope-ratio mass spectrometry.

The new infrared laser spectroscopic techniques enable us to measure the isotopic composition (δ(18)O and δ(2)H) of atmospheric water vapor. With the objective of monitoring the isotopic composition of tropical water vapor (West Africa, South America), and to discuss deuterium excess variability (d=δ(2)H - 8δ(18)O) with an accuracy similar to measurements arising from isotope-ratio mass spectrometry (IRMS), we have conducted a number of tests and calibrations using a wavelength-scanned cavity ring-down spectroscopy (WS-CRDS) technique. We focus in this paper on four main aspects regarding (1) the tubing material, (2) the humidity calibration of the instrument, (3) the water vapor concentration effects on δ, and (4) the isotopic calibration of the instrument.