Hollow Waveguide Gas Research Articles

Simultaneous measurement of 13C-,18O-, and 17O- isotopes of CO2 using a compact mid-infrared hollow waveguide gas sensor

A compact gas sensor has been developed for the first time employing a quantum cascade laser (QCL) with a central wavelength of 4.32 μm, combined with direct absorption spectroscopy technology and a hollow waveguide. The sensor could measure the concentrations of four isotopic molecules (16O12C16O, 18O12C16O, 16O13C16O, and 17O12C16O) in CO2 gas simultaneously, thereby obtaining the relative isotope ratios of δ13C, δ18O, and δ17O. By adjusting the incident light path, the anomalous absorption peaks in the system have been effectively suppressed, and a compact feedback function generator has been integrated to ensure control of the laser output wavelength to eliminate the loss of measurement precision caused by wavelength fluctuations. Under the two conditions of free-running and wavelength-controlled, a standard CO2 sample with a concentration of 5 % was measured continuously. The results showed that under wavelength-controlled conditions, the precision of the four isotopic molecule concentration measurements was improved by a minimum of 1.8 times, and the measurement precision could be further improved after filtering. According to the Allan variance analysis, the detection precision for δ13C, δ18O, and δ17O were 0.7 ‰, 0.62 ‰, and 1.58 ‰, respectively, at the optimal integration time of 98 seconds. This study has the potential for respiratory diagnosis to detect simultaneously the values of δ13C, δ18O, and δ17O in exhaled samples.

Long-wave infrared pulsed external-cavity QCL spectrometer using a hollow waveguide gas cell

A spectrometer built using an external cavity pulsed quantum cascade laser is described. The spectrometer has a tuning range from 10 – 13 µm (1,000 – 769 cm−1) and is designed to target volatile organic compounds (VOCs) which often exhibit water-free molecular absorption within the region. The spectrometer utilizes a hollow silica waveguide gas cell which has an internal volume of a few millilitres, a fast response time (∼1 s), and is advantageous when only low sample volumes, similar to the cell volume, are available. Propane is used as a test gas because it is easy to handle, and its spectral profile is comparable to VOCs of interest. Its absorption in the region is primarily within the ν21 band which spans from 10.55 – 11.16 µm (948 – 896 cm−1). Spectral measurements at a range of concentrations show good linearity and an Allan deviation of absorbance values recorded over a 100-minute period indicates a minimum detectable absorbance of 3.5×10−5 at an integration time of 75 s.

Reagent-less and sub-minute quantification of sulfite in food samples using substrate-integrated hollow waveguide gas sensors coupled to deep-UV LED

The increasing consumption of processed foods demands the usage of chemical preservatives to ensure freshness and extended shelf life. For this purpose, sodium sulfite and its derivatives have been widely used in a variety of food products to inhibit microbial spoilage and for mitigating oxidative decay. However, the excessive consumption of sulfite may cause health problems, thus requiring rapid and accurate analytical methods for the rapid identification of threshold levels. Conventionally, sulfite is volatilized from food samples by acidification followed by trapping of the gaseous SO2 and determination using a suitable analytical technique. Herein, we propose a yet unprecedented reagent-less approach via direct absorbance measurements of gaseous SO2 at 280 nm after sample acidification. The detection system combines a deep-UV LED and a SiC photodiode with a substrate-integrated hollow waveguide (iHWG) gas cell. Absorbance measurements were performed using a log-ratio amplifier circuitry, resulting in noise levels <0.7 mAU. This innovative concept enabled the determination of sulfite in beverages in the range of 25–1000 mg L−1 with suitable linearity (r2 > 0.99) and an analysis time <30 s. The limit of detection (LOD) was calculated at 14.3 mg L−1 (3σ) with an iHWG providing an optical path length of 75 mm. As a proof of concept, this innovative analytical platform was employed for sulfite quantification in concentrated grape juice, coconut water and beer, with suitable accuracy in terms of recovery (83–117%) and favorable comparison with the official Monier-Williams method. Given the inherent modularity and adaptability of the device concept, we anticipate the application of the proposed analytical platform for the in-situ studies addressing sulfite and other volatilized preservatives in a wide variety of food products with tailorable detectability.

A novel mid-infrared hollow waveguide gas sensor for measuring water vapor isotope ratios in the atmosphere

A novel mid-infrared hollow waveguide gas sensor was developed to target H218O, H216O, H217O, and HDO absorption lines at 3662.9196, 3663.04522, 3663.32128, and 3663.84202 cm−1, respectively, based on wavelength modulation spectroscopy using a 2.73 µm distributed feedback diode laser. A hollow waveguide fiber with a 5-m length and 1-mm inner diameter was used for gas absorption. A dew point generator with liquid water with known water–isotope ratios was used to calibrate the sensor. Detection limits of 35.18 ppbv, 4.69 ppmv, 60.53 ppbv, and 3.88 ppbv were obtained for H218O, H216O, H217O, and HDO, respectively, in a 96-s integration time, which resulted in isotopic ratio measurement precision of 0.85‰, 0.57‰, and 10.48‰ for δ18O, δ17O, and δD, respectively. Field measurements of H218O, H216O, and H217O concentration were conducted on the Nanchang Hangkong University campus to evaluate sensor performance.

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.

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.

Influence of Light Coupling Configuration and Alignment on the Stability of HWG-Based Gas Sensor System for Real-Time Detection of Exhaled Carbon Dioxide

A mid-infrared tunable diode laser absorption spectroscopy (TDLAS) gas sensor based on hollow waveguide (HWG) gas cell for real-time exhaled carbon dioxide (eCO2) detection is reported. A $2.73~\mu \text{m}$ distributed feedback (DFB) laser was used to target a strong CO2 absorption line, and wavelength modulation spectroscopy (WMS) with the second harmonic (WMS- $2f$ ) was used to retrieve the CO2 concentration with high sensitivity. The influence of different parameters, including coupling configuration of HWG, laser-to-HWG and HWG-to-detector coupling alignment on the stability of the HWG sensor is systematically studied. The HWG eCO2 sensor showed a fast response time of 2.7s, detection limit of 17 ppmv, and measurement precision of 20.9 ppmv with a 0.54 s temporal resolution. The eCO2 concentrations changed in breath cycles were measured in real time. The Allan variance indicated that the detection limit can reach 1.7 ppmv, corresponding to a detection sensitivity of $1.3\times 10^{\mathbf {-8}}$ cm−1 Hz−1/2, as the integration time increases to 26 s. This work demonstrates the performance characteristics and merits of HWG eCO2 sensor for exhaled breath analysis and potential detection for other exhaled gases.

Online monitoring of carbon dioxide and oxygen in exhaled mouse breath via substrate-integrated hollow waveguide Fourier-transform infrared-luminescence spectroscopy

Exhaled breath offers monitoring bio markers, as well as diagnosing diseases and therapeutic interventions. In addition, vital functions may be non-invasively monitored online. Animal models are frequently used in research for determining novel therapeutic approaches and/or for investigating biological pathways. The exhaled carbon dioxide concentration, exhaled and inhaled oxygen concentration, and the subsequent respiratory quotient (RQ) offer insight into metabolic activity. One may adapt breath sampling systems and equipment designed for human applications to large animal studies. However, such adaptations are usually impossible for small animals due to their minuscule breath volume. Here, we present a system for the online monitoring of exhaled breath in a ‘mouse intensive care unit’ (MICU) based on a modified Fourier-transform infrared spectrometer equipped with a substrate-integrated hollow waveguide gas cell, and a luminescence-based oxygen flow-through sensor integrated into the respiratory equipment of the MICU. Thereby, per-minute resolution of O2 consumption and CO2 production was obtained, and the 95% confidence range of the determined RQ was ±0.04 or approximately ±5% of the nominal value. Changes in the RQ value caused by intervention in either the metabolic or respiratory system may therefore reliably be detected.

Sensitivity improvement of midinfrared gas sensing system using single-wavelength quantum cascade laser and hollow-waveguide gas cell

The configuration of a gas analysis system using the combination of a single-wavelength quantum cascade laser and a hollow-optical-fiber gas cell was optimized to improve measurement sensitivity. On the basis of theoretical calculation, a 6-m looped fiber gas cell was introduced into the system to more sensitively measure the concentration of nitric monoxide (NO) gas. High-precision measurement is achieved by precisely scanning the wavelength of the laser around the sharp absorption peak of NO gas. Concentrations of NO gas as low as 100 ppb were successfully measured.

Fiber-Coupled Substrate-Integrated Hollow Waveguides: An Innovative Approach to Mid-infrared Remote Gas Sensors.

In this study, an innovative approach based on fiberoptically coupled substrate-integrated hollow waveguide (iHWG) gas cells for the analysis of low sample volumes suitable for remote broad- and narrow-band mid-infrared (MIR; 2.5-20 μm) sensing applications is reported. The feasibility of remotely addressing iHWG gas cells, configured in a double-pass geometry via a reflector, by direct coupling to a 7-around-1 mid-infrared fiber bundle is demonstrated, facilitating low-level hydrocarbon gas analysis. For comparison studies, two iHWGs with substrate dimensions of 50 × 50 × 12 mm (L × W × H) and geometric channel lengths of 138 and 58.5 mm, serving as miniature light-guiding gas cells, were fiber-coupled to a Fourier transform infrared spectrometer enabling broadband MIR sensing. In addition to the fundamental feasibility of this concept, the achievable sensitivity toward several gaseous hydrocarbons and the reproducibility of assembling the fiber-iHWG interface were investigated.

Low-volume, fast response-time hollow silica waveguide gas cells for mid-IR spectroscopy.

Hollow silica waveguides (HSWs) are used to produce long path length, low-volume gas cells, and are demonstrated with quantum cascade laser spectroscopy. Absorption measurements are made using the intrapulse technique, which allows measurements to be made across a single laser pulse. Simultaneous laser light and gas coupling is achieved through the modification of commercially available gas fittings with low dead volume. Three HSW gas cell configurations with different path lengths and internal diameters are analyzed and compared with a 30 m path length astigmatic Herriott cell. Limit of detection measurements are made for the gas cells using methane at a wavelength 7.82 μm. The lowest limit of detection was provided by HSW with a bore diameter of 1000 μm and a path length of 5 m and was measured to be 0.26 ppm, with a noise equivalent absorbance of 4.1×10-4. The long-term stability of the HSW and Herriott cells is compared through analysis of the Allan-Werle variance of data collected over a 24 h period. The response times of the HSW and Herriott cells are measured to be 0.8 s and 36 s, respectively.

Detection of methyl mercaptan with a 3393-nm distributed feedback interband cascade laser

Attention has been focused recently on the harmful effects and malodor of methyl mercaptan (CH3SH), so it is desired to detect CH3SH in situ, sensitively, and selectively. We detected methyl mercaptan via tunable laser absorption spectroscopy (TLAS) with a room-temperature distributed feedback interband cascade laser emitting around 3393 nm and a hollow waveguide gas cell with 5 m length. The fundamental characteristic fingerprint absorptions of CH3SH from 3260 to 3400 nm were examined, and the spectral line 3393.584 nm (corresponding to the ν2 C–H symmetric stretch) was determined to be the optimum for CH3SH detection. The response characteristics of the TLAS system were established by implementing a set of CH3SH concentration gradient experiments with wavelength-scanned direct absorption spectroscopy. The results show that CH3SH TLAS spectra are in excellent agreement with spectra from the Pacific Northwest National Laboratory database; the TLAS response linearity is 0.987, and the detection limit is as low as 25 ppbv (parts per billion by volume, 10−9) with integrated time 1.84 s, corresponding to an absorbance of 1.34 × 10−4 (near the theoretical detection limit). Overall, the TLAS system is a robust method for CH3SH monitoring of industrial waste gas emissions.

Substrate-Integrated Hollow Waveguides: A New Level of Integration in Mid-Infrared Gas Sensing

A new generation of hollow waveguide (HWG) gas cells of unprecedented compact dimensions facilitating low sample volumes suitable for broad- and narrow-band mid-infrared (MIR; 2.5-20 μm) sensing applications is reported: the substrate-integrated hollow waveguide (iHWG). iHWGs are layered structures providing light guiding channels integrated into a solid-state substrate material, which are competitive if not superior in performance to conventional leaky-mode fiber optic silica HWGs having similar optical pathlengths. In particular, the provided flexibility in device and optical design and the wide variety of manufacturing strategies, substrate materials, access to the optical channel, and optical coating options highlight the advantages of iHWGs in terms of robustness, compactness, and cost-effectiveness. Finally, the unmatched modularity of this novel waveguide approach facilitates tailoring iHWGs to almost any kind of gas sensor technology providing adaptability to the specific demands of a wide range of sensing scenarios. Device fabrication is demonstrated for the example of a yin-yang-shaped gold-coated iHWG fabricated within an aluminum substrate with a footprint of only 75 mm × 50 mm × 12 mm (L × W × H), yet providing a nominal optical absorption path length of more than 22 cm. The analytical utility of this device for advanced MIR gas sensing applications is demonstrated for the gaseous constituents butane, carbon dioxide, cyclopropane, isobutylene, and methane.

Improving the performance of hollow waveguide-based infrared gas sensors via tailored chemometrics

The use of chemometrics in order to improve the molecular selectivity of infrared (IR) spectra has been evaluated using classic least squares (CLS), partial least squares (PLS), science-based calibration (SBC), and multivariate curve resolution-alternate least squares (MCR-ALS) techniques for improving the discriminatory and quantitative performance of infrared hollow waveguide gas sensors. Spectra of mixtures of isobutylene, methane, carbon dioxide, butane, and cyclopropane were recorded, analyzed, and validated for optimizing the prediction of associated concentrations. PLS, CLS, and SBC provided equivalent results in the absence of interferences. After addition of the spectral characteristics of water by humidifying the sample mixtures, CLS and SBC results were similar to those obtained by PLS only if the water spectrum was included in the calibration model. In the presence of an unknown interferant, CLS revealed errors up to six times higher than those obtained by PLS. However, SBC provided similar results compared to PLS by adding a measured noise matrix to the model. Using MCR-ALS provided an excellent estimation of the spectra of the unknown interference. Furthermore, this method also provided a qualitative and quantitative estimation of the components of an unknown set of samples. In summary, using the most suitable chemometrics approach could improve the selectivity and quality of the calibration model derived for a sensor system, and may avoid the need to analyze expensive calibration data sets. The results obtained in the present study demonstrated that (1) if all sample components of the system are known, CLS provides a sufficiently accurate solution; (2) the selection between PLS and SBC methods depends on whether it is easier to measure a calibration data set or a noise matrix; and (3) MCR-ALS appears to be the most suitable method for detecting interferences within a sample. However, the latter approach requires the most extensive calculations and may thus result in limited temporal resolution, if the concentration of a component should be continuously monitored.

Multivariate determination of 13CO2/12CO2 ratios in exhaled mouse breath with mid-infrared hollow waveguide gas sensors

The (12)CO2/(13)CO2 isotope ratio is a well-known marker in breath for a variety of biochemical processes and enables monitoring, e.g., of the glucose metabolism during sepsis. Using animal models-here, at a mouse intensive care unit-the simultaneous determination of (12)CO2 and (13)CO2 within small volumes of mouse breath was enabled by coupling a novel low-volume hollow waveguide gas cell to a compact Fourier transform infrared spectrometer combined with multivariate data evaluation based on partial least squares regression along with optimized data preprocessing routines.

Breath Analysis with Broadly Tunable Quantum Cascade Lasers

With the availability of broadly tunable external cavity quantum cascade lasers (EC-QCLs), particularly bright mid-infrared (MIR; 3-20 μm) light sources are available offering high spectral brightness along with an analytically relevant spectral tuning range of >2 μm. Accurate isotope ratio determination of (12)CO2 and (13)CO2 in exhaled breath is of critical importance in the field of breath analysis, which may be addressed via measurements in the MIR spectral regime. Here, we combine for the first time an EC-QCL tunable across the (12)CO2/(13)CO2 spectral band with a miniaturized hollow waveguide gas cell for quantitatively determining the (12)CO2/(13)CO2 ratio within the exhaled breath of mice. Due to partially overlapping spectral features, these studies are augmented by appropriate multivariate data evaluation and calibration techniques based on partial least-squares regression along with optimized data preprocessing. Highly accurate determinations of the isotope ratio within breath samples collected from a mouse intensive care unit validated via hyphenated gas chromatography-mass spectrometry confirm the viability of IR-HWG-EC-QCL sensing techniques for isotope-selective exhaled breath analysis.

Toward the quantification of the 13CO2/12CO2 ratio in exhaled mouse breath with mid-infrared hollow waveguide gas sensors

Mouse sepsis models are used to gain insight into the complex processes involved with patients suffering from glucose metabolism disorders. Measuring the expiratory release of (13)CO(2) after administering stable labeled (13)C(6)-glucose enables assessment of the in vivo integrity and functionality of key metabolic processes. In the present study, we demonstrate that Fourier transform infrared spectroscopy operating in the mid-infrared spectral regime (2-20 μm) combined with hollow waveguide gas sensing modules simultaneously serving as a miniaturized gas cell and as a waveguide are capable of quantitatively monitoring (13)CO(2) enrichment levels in low volume mouse breath samples.

Near-Infrared Hollow Waveguide Gas Sensors

The development of a hollow core waveguide (HWG) gas sensor in combination with a fast and compact near-infrared (NIR) spectrometer is presented. The spectrometer operates in the spectral range of 1200-1400 nm and may thus be applied for the detection of gas-phase analytes providing NIR absorptions in that spectral window such as, e.g., methane. Since mid-infrared spectroscopy in combination with HWGs has already been successfully demonstrated for probing hydrocarbons in the gas phase, the present study investigates the achievable sensitivity in the NIR spectral regime. Methane has been selected as an exemplary analyte due to the fact that it shows strong absorption features in the mid-infrared (mid-IR) fingerprint area, but also overtone bands in the NIR. Since the HWG simultaneously serves as a miniaturized absorption gas cell and as an optical waveguide for NIR radiation, a compact yet optical and cost-efficient sensor device was established providing an interesting alternative in target sensing for mid-IR devices. The achieved limit of detection (LOD) was 5.7% (vol./vol.) methane for a 9.5 cm long HWG, 1.6% (vol./vol.) methane for a 39.1 cm long HWG, and 1.3% (vol./vol.) methane for a setup using a 77.4 cm long HWG, which provides the most practical HWG dimensions among the three investigated setups. Limit of quantitation (LOQ) values were calculated at 20.1% (vol./vol.) methane, 8.7% (vol./vol.) methane, and 5.6% (vol./vol.) methane, respectively.

A hollow-waveguide gas correlation radiometer for ultra-precise column measurements of formaldehyde on Mars

We present preliminary results in the development of a miniaturized gas correlation radiometer that implements a hollow-core optical fiber (hollow-waveguide) gas correlation cell. The substantial reduction in mass and volume of the gas correlation cell makes this technology appropriate for an orbital mission—capable of pinpointing sources of trace gases in the Martian atmosphere. Here, we demonstrate a formaldehyde (H2CO) sensor and report a detection limit equivalent to ∼30 ppb in the Martian atmosphere. The relative simplicity of the technique allows it to be expanded to measure a range of atmospheric trace gases of interest on Mars such as methane (CH4), water vapor (H2O), deuterated water vapor (HDO), and methanol (CH3OH). Performance of a formaldehyde instrument in a Mars orbit has been simulated assuming a 3 m long, 1000 µm inner diameter hollow-core fiber gas correlation cell, a 92.8° sun-synchronous orbit from 400 km with a horizontal sampling scale of 10 km × 10 km. Initial results indicate that for 1 s of averaging, a detection limit of 1 ppb is possible.

An approach to the spectral simulation of infrared hollow waveguide gas sensors

Optical simulations enable to model an entire chemical gas sensing platform based on hollow waveguides (HWGs) operating in the mid-infrared spectral regime using a three-dimensional representation of the sensor components and taking the spectral response to virtual analytes into account. Furthermore, a strategy for including the spectral response of dielectrically coated HWGs is demonstrated. Utilizing experimentally obtained spectroscopic data recorded at well-defined conditions, the complex refractive indices of selected target analytes (i.e., methane, butane, and isobutylene) have been derived based on a refined harmonic oscillator model. In turn, these parameters have enabled to directly assign the dielectric functions of these analytes to virtual objects representing the analyte within the modeled sensor setup. In a next step, spectroscopic sensor response functions have been simulated as absorbance spectra across selected wavelength regimes utilizing spectrally resolved ray-tracing techniques and have been compared to experimental data.