Water isotope spectroscopy near 2.6 μm for D/H, 18O/16O, and 17O/16O analysis in planetary bodies

A laser absorption spectroscopy method was developed for in situ extraterrestrial sensing of four distinct water isotopologues near 2.64 µm. Infrared rovibrational transitions for H216O, H218O, H217O, and HDO were selected within the overlapping ν1 and ν3 fundamental bands to maximize relative absorption intensity and reduce spectral interference with neighboring water lines and other species, such as CO2, while being positioned within the injection current tuning range of a single distributed-feedback diode laser held at one temperature. Temperature-dependent self-broadening parameters were measured from 296-375 K for the target lines via Voigt lineshape fitting of the transitions in the range of 3788.2-3789.9 cm-1 using vaporized water samples in a temperature- and pressure-controlled gas cell. Based on the novel wavelength selection and spectroscopic measurements, quantitative, calibration-free sensing of isotope abundance ratios of D/H, 18O/16O, and 17O/16O in water vapor was demonstrated in a portable form-factor device using a diode laser, compact Herriott cell, and a liquid water injection system. Accuracy and detection limits using scanned-wavelength direct absorption were estimated by comparison to water samples of known isotopic abundance.