A field-deployable mid-infrared quantum cascade laser-based spectrometer was designed and developed for measurements of H216O and H218O at 7.12 μm. H216O and H218O absorption features at 1390.52 cm-1and 1389.91 cm-1, respectively, accessible within current tuning range of the laser, were targeted. The target lines were carefully selected to assure similar absorption levels and similar temperature sensitivities of the line strength due to comparable lower state energies. A real-time spectral fitting algorithm was implemented for isotopic concentration retrieval. Detection limits for H216O and H218O of 2.2 ppm and 7.0 ppb, respectively, were achieved at a dew point of 14 °C (volume mixing ratio of 15 766 ppm) in 1 s integration time, which resulted in a δ18O isotopic ratio measurement precision of 0.25‰. The ultimate minimum detection limits obtained after 160 s integration time for H216O and H218O, and δ18O measurements were 0.6 ppm, 1.7 ppb, and 0.05‰, respectively.
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