A quantum cascade laser-based water vapor isotope analyzer for environmental monitoring

W E Wang; A P M Michel; L Wang; T Tsai; M L Baeck; J A Smith; G Wysocki

doi:10.1063/1.4894161

A quantum cascade laser-based water vapor isotope analyzer for environmental monitoring

Rev Sci Instrum. 2014 Sep;85(9):093103. doi: 10.1063/1.4894161.

Authors

W E Wang¹, A P M Michel², L Wang³, T Tsai¹, M L Baeck³, J A Smith³, G Wysocki¹

Affiliations

¹ Department of Electrical Engineering, Princeton University, Princeton, New Jersey 08544, USA.
² Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08540, USA.
³ Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544, USA.

PMID: 25273703
DOI: 10.1063/1.4894161

Abstract

A field-deployable mid-infrared quantum cascade laser-based spectrometer was designed and developed for measurements of H2(16)O and H2(18)O at 7.12 μm. H2(16)O and H2(18)O absorption features at 1390.52 cm(-1) and 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 H2(16)O and H2(18)O 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 δ(18)O isotopic ratio measurement precision of 0.25‰. The ultimate minimum detection limits obtained after 160 s integration time for H2(16)O and H2(18)O, and δ(18)O measurements were 0.6 ppm, 1.7 ppb, and 0.05‰, respectively.