Measurement of CO₂ Isotopologue Ratios Using a Hollow Waveguide-Based Mid-Infrared Dispersion Spectrometer

We demonstrate for the first time the measurement of CO₂ isotope ratios (¹³C/¹²C and ¹⁸O/¹⁶O) 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 CO₂ isotopes (¹³C¹⁶O₂, ¹⁸O¹²C¹⁶O, and ¹²C¹⁶O₂) 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 ¹³C¹⁶O₂, ¹⁸O¹²C¹⁶O, and ¹²C¹⁶O₂, respectively, at a modulation frequency of 160 MHz and a pressure of 230 mbar. The measurement precisions of δ¹³C and δ¹⁸O 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 CO₂ isotope dispersion spectroscopy with a large dynamic range for relevant applications focusing on samples with high concentrations of CO₂ (% volume fraction), such as respiratory analysis in medical diagnostics.