Real-time measurement of CO2 isotopologue ratios in exhaled breath by a hollow waveguide based mid-infrared gas sensor

Artificial Intelligence-Enhanced Waveguide "Photonic Nose"- Augmented Sensing Platform for VOC Gases in Mid-Infrared

On-chip nanophotonic waveguide sensor is a promising solution for miniaturization and label-free detection of gas mixtures utilizing the absorption fingerprints in the mid-infrared (MIR) region. However, the quantitative detection and analysis of organic gas mixtures is still challenging and less reported due to the overlapping of the absorption spectrum. Here,an Artificial-Intelligence (AI) assisted waveguide "Photonic nose" is presented as an augmented sensing platform for gas mixture analysis in MIR. With the subwavelength grating cladding supported waveguide design and the help of machine learning algorithms, the MIR absorption spectrum of the binary organic gas mixture is distinguished from arbitrary mixing ratio and decomposed to the single-component spectra for concentration prediction. As a result, the classification of 93.57% for 19 mixing ratios is realized. In addition, the gas mixture spectrum decomposition and concentration prediction show an average root-mean-square error of 2.44 vol%. The work proves the potential for broader sensing and analytical capabilities of the MIR waveguide platform for multiple organic gas components toward MIR on-chip spectroscopy.

Real-time breath gas analysis of methane using a multipass cell-based near-infrared gas sensor

We demonstrated a near-infrared exhaled breath sensor for real-time methane measurements by using tunable diode laser absorption spectroscopy (TDLAS), which can enable the noninvasive diagnosis of intestinal tract problems. The core component of the near-infrared TDLAS sensor is a two-mirror-based multipass cell with nine-circle patterns. An optical path length of 23.4 m was achieved in a volume of 233.3 cm3, which effectively improved the detection sensitivity and shortened the gas exchange time. The minimum detection limit was 0.37 ppm by applying wavelength modulation spectroscopy, which was 12.4 times greater than that of direct absorption spectroscopy. In addition, combined with wavelength modulation spectroscopy, the two-mirror-based multipass cell enabled sub-second gas exchange time of 0.6 s. Methane breath experiments were conducted with six volunteers, and the real-time measurement results and concentrations at the end of exhalation were analyzed. This study demonstrates that the developed sensor has high sensitivity, high selectivity, and fast response for breath methane measurements and has promising potential for noninvasive, real-time, and point-of-care disease diagnosis in clinical applications.

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

Previous research revealed that isotopes ¹³C and ¹⁸O of exhaled CO₂ have the potential link with Helicobacter pylori; however, the ¹⁷O isotope has received very little attention. We developed a sensitive spectroscopic sensor for simultaneous δ¹³C, δ¹⁸O, and δ¹⁷O analysis of human breath CO₂ based on mid-infrared laser direct absorption spectroscopy with an interband cascade laser (ICL) at 4.33 μm. There was a gas cell with a small volume of less than 5 mL, and the pressure in the gas cell was precisely controlled with a standard deviation of 0.0035 Torr. Moreover, real-time breath sampling and batch operation were achieved in gas inlets. The theoretical drifts for δ¹³C, δ¹⁸O, and δ¹⁷O measurement caused by temperature were minimized to 0.017‰, 0.024‰, and 0.021‰, respectively, thanks to the precise temperature control with a standard deviation of 0.0013 °C. After absolute temperature correction, the error between the system responded δ-value and the reference is less than 0.3‰. According to Allan variance analysis, the system precisions for δ¹³C, δ¹⁸O, and δ¹⁷O were 0.12‰, 0.18‰, and 0.47‰, respectively, at 1 s integration time, which were close to the real-time measurement errors of six repeated exhalations.

A Laser-Locked Hollow Waveguide Gas Sensor for Simultaneous Measurements of CO2 Isotopologues with High Accuracy, Precision, and Sensitivity

A laser frequency-locked hollow waveguide (HWG) gas sensor is demonstrated for simultaneous measurements of three isotopologues (¹²CO₂, ¹³CO₂, and ¹⁸OC¹⁶O) using wavelength modulation spectroscopy with a 2.73 μm distributed feedback laser. The first harmonic (1f) signal at the sampling point where the peak of the second harmonic (2f) signal was located was employed as the locking point to lock the laser frequency to the transition center of ¹³CO₂, while the absorption lines of ¹²CO₂ and ¹⁸OC¹⁶O were being scanned. Continuous measurements of the three isotopologues of 4.7% CO₂ samples over 103 min under free running and frequency locking conditions were performed. The measurement accuracy and precision of the three isotopologues achieved under the frequency locking condition were at least 3 times and 1.3 times better than those obtained under the free running condition, respectively. The Allan variance plot of the developed laser-locked HWG gas sensor shows a detection limit of 0.72‰ for both δ¹³C and δ¹⁸O under the frequency locking condition with a long stability time of 766 s. This study demonstrated the high potential of a novel human breath diagnostic sensor for medical diagnostic with high accuracy, precision, and sensitivity and without frequently repeated calibration.

Direct Comb Vernier Spectroscopy for Fractional Isotopic Ratio Determinations

Accurate isotopic composition analysis of the greenhouse-gasses emitted in the atmosphere is an important step to mitigate global climate warnings. Optical frequency comb-based spectroscopic techniques have shown ideal performance to accomplish the simultaneous monitoring of the different isotope substituted species of such gases. The capabilities of one such technique, namely, direct comb Vernier spectroscopy, to determine the fractional isotopic ratio composition are discussed. This technique combines interferometric filtering of the comb source in a Fabry-Perot that contains the sample gas, with a high resolution dispersion spectrometer to resolve the spectral content of each interacting frequency inside of the Fabry-Perot. Following this methodology, simultaneous spectra of ro-vibrational transitions of 12C16O2 and 13C16O2 molecules are recorded and analyzed with an accurate fitting procedure. Fractional isotopic ratio 13C/12C at 3% of precision is measured for a sample of CO2 gas, showing the potentialities of the technique for all isotopic-related applications of this important pollutant.

Room-temperature Fe:ZnSe laser tunable in the spectral range of 3.7-5.3 µm applied for intracavity absorption spectroscopy of CO2 isotopes, CO and N2O

We demonstrate an intracavity absorption spectroscopy system based on a broadband single-crystal pulsed Fe:ZnSe laser. The laser operates at room-temperature and is continuously tunable in the spectral range of 3.76-5.29 µm. The long-wavelength emission up to 5.29 µm is a record achievement for Fe:ZnSe lasers, to the best of our knowledge. The developed laser system is applied for measurements of gaseous absorption inside the laser resonator. We demonstrate sensitive detection of (i) CO₂ isotopes in the atmosphere and in human breath, (ii) CO in breath (after cigarette smoking) and in the smoke of a smoldering paper, and (iii) N₂O in a gas flow. The achieved detection limits are: 0.1 ppm for ¹²CO₂ and ¹³CO₂, 3 ppm for CO, and 1 ppm for N₂O. The sensitivity of the current system is primarily limited by the short pump-pulse duration of 40 ns. Possibilities for sensitivity enhancement by up to a factor of 10⁷ are discussed.

Real-Time Monitoring of 13C- and 18O-Isotopes of Human Breath CO2 Using a Mid-Infrared Hollow Waveguide Gas Sensor

Real-time measuring of CO₂ isotopes (¹³CO₂, ¹²CO₂, and ¹⁸OC¹⁶O) in exhaled breath using a mid-infrared hollow waveguide gas sensor incorporating a 2.73 μm distributed feedback laser was proposed and demonstrated for the first time based on calibration-free wavelength modulation spectroscopy. The measurement precisions for δ¹³C and δ¹⁸O were, respectively, 0.26 and 0.57‰ for an integration time of 131 s by Allan variance analysis. These measurement precisions achieved in the present work were at least 3.5 times better than those reported using direct absorption spectroscopy and 1.3 times better than those obtained by calibration-needed wavelength modulation absorption spectroscopy. Continuous measurement of three isotopes in the breathing cycle was performed. Alveolar gas from the expirogram was identified, and the ¹³C/¹²C and ¹⁸O/¹⁶O ratios were found to be almost constant during the alveolar plateau, which enables optimization of breath sampling and provides accurate information on metabolic processes. The ¹³C/¹²C and ¹⁸O/¹⁶O isotope ratios at the alveolar plateau of five breath cycles were averaged, yielding δ¹³C and δ¹⁸O values of (-24.3 ± 3.4) and (-30.7 ± 2.6) ‰, respectively. This study demonstrates the feasibility of real-time analysis of ¹³C- and ¹⁸O-isotopes of human breath CO₂ in clinical applications and shows its potential for diagnosing respiratory-related diseases with high sensitivity, selectivity, and specificity.

Laser-Based Trace Gas Detection inside Hollow-Core Fibers: A Review

Thanks to the guidance of an optical wave in air, hollow-core fibers may serve as sampling cells in an optical spectroscopic system. This paper reviews applications of hollow-core optical fibers to laser-based gas sensing. Three types of hollow-core fibers are discussed: Hollow capillary waveguides, photonic band-gap fibers, and negative curvature fibers. Their advantages and drawbacks when used for laser-based trace gas detection are analyzed. Various examples of experimental sensing systems demonstrated in the literature over the past 20 years are discussed.