Measurement of CO<sub>2</sub> Isotopologue Ratios Using a Hollow Waveguide-Based Mid-Infrared Dispersion Spectrometer
Haojie Zhang, Tao Wu, Qiang Wu, Weidong Chen, Chenwen Ye, Mengyu Wang, Xingdao He
Abstract
We demonstrate for the first time the measurement of CO 2 isotope ratios ( 13 C/ 12 C and 18 O/ 16 O) in a hollow waveguide (HWG) fiber using a mid-infrared heterodyne phase-sensitive dispersion spectrometer (HPSDS). A 4.329 μm interband cascade laser is used to target the absorption lines of three CO 2 isotopes ( 13 C 16 O 2, 18 O 12 C 16 O, and 12 C 16 O 2 ) in a 1 m long and 1 mm inner diameter HWG fiber. The detection limits are 0.29 ppm, 65.78 ppb, and 14.65 ppm with an integration time of 218 s for 13 C 16 O 2, 18 O 12 C 16 O, and 12 C 16 O 2, respectively, at a modulation frequency of 160 MHz and a pressure of 230 mbar. The measurement precisions of δ 13 C and δ 18 O are 0.89 and 0.88 ‰, respectively, corresponding to an integration time of 167 s. An experimental comparison between a HPSDS and a built wavelength modulation system with second-harmonic detection (WMS-2f) is conducted. The results show that compared to the WMS-2f, the developed HPSDS exhibits a greater linear dynamic range and excellent long-term stability. This work aims to demonstrate a detection technique of CO 2 isotope dispersion spectroscopy with a large dynamic range for relevant applications focusing on samples with high concentrations of CO 2 (% volume fraction), such as respiratory analysis in medical diagnostics.