Simultaneous Sensitive Determination of δ13C, δ18O, and δ17O in Human Breath CO2 Based on ICL Direct Absorption Spectroscopy

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.

Portable TDLAS Sensor for Online Monitoring of CO2 and H2O Using a Miniaturized Multi-Pass Cell

We designed a tunable diode laser absorption spectroscopy (TDLAS) sensor for the online monitoring of CO₂ and H₂O concentrations. It comprised a small self-design multi-pass cell, home-made laser drive circuits, and a data acquisition circuit. The optical and electrical parts and the gas circuit were integrated into a portable carrying case (height = 134 mm, length = 388 mm, and width = 290 mm). A TDLAS drive module (size: 90 mm × 45 mm) was designed to realize the function of laser current and temperature control with a temperature control accuracy of ±1.4 mK and a current control accuracy of ±0.5 μA, and signal acquisition and demodulation. The weight and power consumption of the TDLAS system were only 5 kg and 10 W, respectively. Distributed feedback lasers (2004 nm and 1392 nm) were employed to target CO₂ and H₂O absorption lines, respectively. According to Allan analysis, the detection limits of CO₂ and H₂O were 0.13 ppm and 3.7 ppm at an average time of 18 s and 35 s, respectively. The system response time was approximately 10 s. Sensor performance was verified by measuring atmospheric CO₂ and H₂O concentrations for 240 h. Experimental results were compared with those obtained using a commercial instrument LI-7500, which uses non-dispersive infrared technology. Measurements of the developed gas analyzer were in good agreement with those of the commercial instrument, and its accuracy was comparable. Therefore, the TDLAS sensor has strong application prospects in atmospheric CO₂ and H₂O concentration detection and ecological soil flux monitoring.

Fast Simultaneous CO2 Gas Temperature and Concentration Measurements by Quantum Cascade Laser Absorption Spectroscopy

A quantum cascade laser-based sensing technique is presented which allows for in situ high-precision temperature and/or CO2 concentration measurements of gases in the room temperature regime with sampling rates up to about 40 kHz. The method is based on Boltzmann-like thermally populated fundamental and hot-band rovibrational transitions of CO2 with opposite temperature dependence. Single absorption spectra at about 2350 to 2352 cm−1 are recorded by a nanosecond frequency down chirped IR pulse of a pulsed distributed feedback quantum cascade laser (intrapulse mode). The statistical uncertainty (1σ) in the temperature measurement within one laser pulse is about 1 K and can be further reduced down to about 0.1 K by time averaging over 100 ms. Online temperature and CO2 concentration measurements on a breath simulator controlled gas flow were performed to demonstrate response-time and sensitivity for an application-driven test system.