All-fiber multifunction differential absorption CO2 lidar integrating single-photon and coherent detection

This study demonstrates a differential absorption lidar (DIAL) for CO2 that integrates both single-photon direct detection and coherent detection. Based on all-fiber 1572 nm wavelength devices, this compact lidar achieves detection of CO2 concentration, wind field, and single photon aerosol backscattering signal. First, by comparing DIAL with VAISALA-GMP343, the concentration deviation between the two devices is less than 5 ppm, proving the accuracy of the DIAL. Second, through the scanning detection experiment in Chaohu Lake, Hefei, not only the CO2 concentration between single-photon detection and coherent detection but also the wind field was obtained, proving the multifunctionality and stability of the DIAL. Benefiting from the advantages of combined the two detection methods, single photon detection offers 3-km CO2 and aerosol backscattering signals; coherent detection offers a 360-m shorter blind zone and wind field. This DIAL can achieve monitoring of CO2 flux and sudden emissions, which can effectively compensate for the shortages of in-situ sensors and spaceborne systems.

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.

Methane detection with a near-infrared heterodyne phase-sensitive dispersion spectrometer at a stronger frequency modulation using direct injection-current dithering

Heterodyne phase-sensitive dispersion spectrometer (HPSDS) retrieves the concentration of gas samples by measuring the refractive index fluctuations near the molecular resonance. Compared to previous HPSDS studies focusing on pure intensity modulation, it is attractive to investigate the performance of HPSDS sensor based on a distributed feedback (DFB) laser under conditions where frequency modulation is much higher than intensity modulation. In this work, we report the implementation of a near-infrared HPSDS for methane detection based on the direct modulation of a DFB laser. The performance of our HPSDS is assessed using the characteristic absorption peak of methane near 1653.7 nm. Long-time measurements show that our HPSDS has a detection limit (MDL) of 1.22 ppm at standard atmospheric pressure and room temperature. In the same experimental conditions, we have experimentally compared HPSDS to wavelength modulation spectroscopy (WMS) to evaluate the dynamical range, long-term stability, and precision limits of the two methods.

Open-Path Atmospheric Transmission of Diode-Pumped Alkali Lasers in Maritime and Desert Environments

A tunable diode laser absorption spectroscopy (TDLAS) device has been developed to study long-path atmospheric transmission near diode pumped alkali laser (DPAL) emission wavelengths. By employing a single aperture and retro reflector in a mono-static configuration, the noise associated with atmospheric and platform jitter were reduced by a factor of ∼30 and the open-air path length was extended to 4.4 km and over a very broad spectral range, up to 120 cm^-1. Water vapor absorption lines near the rubidium (Rb) and cesium (Cs) variants of the DPAL near 795 and 894 nm, oxygen lines near the potassium (K) DPAL near 770 nm, and water vapor absorption in the vicinity of the neodymium-doped yttrium aluminum garnet (Nd:YAG) laser 1.064 μm and chemical oxygen iodine laser (COIL) 1.3 μm lines were studied. The detection limit for path absorbance increases from ΔA = 0.0017 at 100 m path length to 0.085 for the 4.4 km path. Comparison with meteorological instruments for maritime and desert environments yields agreement for the 2.032 km path to within 1.5% for temperature, 4.5% for pressure, and 5.1% for concentration, while agreements for the 4.4 km path are within 1.4% for temperature, 7.7% for pressure, and 23.5% for concentration. An intra cavity output spectroscopy (ICOS) device was also used as a spectral reference to verify location of atmospheric lines. Implications of TDLAS collection system design on signal-to-noise (S/N) are discussed as well as the effect of path turbulence on baseline noise and inform the selection of the DPAL variant least affected by molecular absorption.

Quantum-Enhanced Absorption Spectroscopy with Bright Squeezed Frequency Combs

Absorption spectroscopy is a widely used technique that permits the detection and characterization of gas species at low concentrations. We propose a sensing strategy combining the advantages of frequency modulation spectroscopy with the reduced noise properties accessible by squeezing the probe state. A homodyne detection scheme allows the simultaneous measurement of the absorption at multiple frequencies and is robust against dispersion across the absorption profile. We predict a significant enhancement of the signal-to-noise ratio that scales exponentially with the squeezing factor. An order of magnitude improvement beyond the standard quantum limit is possible with state-of-the-art squeezing levels facilitating high precision gas sensing.

Highly sensitive HF detection based on absorption enhanced light-induced thermoelastic spectroscopy with a quartz tuning fork of receive and shallow neural network fitting

Due to its advantages of non-contact measurement and high sensitivity, light-induced thermoelastic spectroscopy (LITES) is one of the most promising methods for corrosive gas detection. In this manuscript, a highly sensitive hydrogen fluoride (HF) sensor based on LITES technique is reported for the first time. With simple structure and strong robustness, a shallow neural network (SNN) fitting algorithm is introduced into the field of spectroscopy data processing to achieve denoising. This algorithm provides an end-to-end approach that takes in the raw input data without any pre-processing and extracts features automatically. A continuous wave (CW) distributed feedback diode (DFB) laser with an emission wavelength of 1.27 µm was used as the excitation source. A Herriott multi-pass cell (MPC) with an optical length of 10.1 m was selected to enhance the laser absorption. A quartz tuning fork (QTF) with resonance frequency of 32,767.52 Hz was adopted as the thermoelastic detector. An Allan variance analysis was performed to demonstrate the system stability. When the integration time was 110 s, the minimum detection limit (MDL) was found to be 71 ppb. After the SNN fitting algorithm was used, the signal-to-noise ratio (SNR) of the HF-LITES sensor was improved by a factor of 2.0, which verified the effectiveness of this fitting algorithm for spectroscopy data processing.

Tomographic Absorption Spectroscopy for H2O Transport in a Laminar Jet with Inverse Concentration Gradient

We report a tomographic absorption spectroscopy (TAS) study of water vapor transport in a laminar jet issuing into the ambient. The jet was generated using compressed dry air that was straightened by a honeycomb and a smooth contraction nozzle. A TAS scheme using the water vapor in the ambient as absorbing species and the absorption line near 1368.598 nm was proposed to study the H₂O transport in the laminar jet with an inverse concentration gradient. One-dimensional tomography was conducted at various heights above the nozzle, and the results were validated by the predictions from computational fluid dynamics (CFD) simulations. Particularly, the variations in the concentration gradient in the shear layer at different heights were captured. The 2D distribution of water concentration in the dry laminar jet was obtained experimentally. The present study shows that TAS has great potential in the research of mass transfer and scalar field of gaseous flows.

Electro-optically tunable single-frequency lasing from neodymium-doped lithium niobate microresonators

Tunable light sources are a key enabling technology for many applications such as ranging, spectroscopy, optical coherence tomography, digital imaging and interferometry. For miniaturized laser devices, whispering gallery resonator lasers are a well-suited platform, offering low thresholds and small linewidths, however, many realizations suffer from the lack of reliable tuning. Rare-earth ion-doped lithium niobate offers a way to solve this issue. Here we present a single-frequency laser based on a neodymium-doped lithium niobate whispering gallery mode resonator that is tuned via the linear electro-optic effect. Using a special geometry, we suppress higher-order transverse modes and hence ensure single-mode operation. With an applied voltage of just 68 V, we achieve a tuning range of 3.5 GHz. The lasing frequency can also be modulated with a triangular control signal. The freely running system provides a frequency and power stability of better than Δ ν=20MHz and 6 %, respectively, for a 30-minute period. This concept is suitable for full integration with existing photonic platforms based on lithium niobate.

The Trace Gas Monitoring Method Based on Diode Laser Wavelength-Modulation Spectroscopy Technology for the Detection of Clinical Blood Infection

It is important to monitor and evaluate the growth of microorganisms in order to accurately judge the situation of blood microbial infection. In this paper, diode laser wavelength modulation spectroscopy (DLWMS) technology is used to design a set of low-cost, high sensitivity, fast dynamic responses and a non-invasive trace gas measurement system, which can quickly and accurately assess the concentration of carbon dioxide (CO2) produced by blood microbial reproduction. The measurement principle and spectral processing algorithm of DLWMS are introduced first. The automatic and rapid detection of CO2 is realized through a self-designed optical system. By using the system to detect blood infection, the accuracy of the technology was verified. Therefore, it also indicates that DLWMS CO2 monitoring is a highly sensitive, fast-response and non-invasive technology, which can accurately and quickly determine blood infection and meet the clinical application requirements of human septicemia, bacteremia and other diseases.

Simultaneous measurement of H2O concentration and effective absorption optical path length under unknown optical path length condition based on a single spectral line

Quantitative gas measurement under the condition of unknown optical path length is a challenge in laser absorption spectroscopy technology field. In this paper, we proposed a tunable diode laser absorption spectroscopy line shape analysis (TDLAS-LSA) method for simultaneous measurement of water vapor concentration and effective optical path length (EOPL) under unknown optical path conditions. A single H₂O absorption line near 1383.9 nm (7226.02 cm^-1) was selected, and its line strength, self-broadening coefficient and temperature-dependence coefficient were measured experimentally to improve the HITRAN databases. The Lorentz broadening and line area were accurately extracted by Hartmann-Tran profile (HTP) fitting, and the gas concentration and EOPL were calculated based on the spectral line shape analysis method. Eight concentrations of water vapor in the range of 146 ppm ∼ 4.39% were measured experimentally, and the maximum average deviation between the TDLAS-LSA method and the commercial sensor was less than 7.1%. Comparing the EOPL with mechanical measurement, the maximum deviation of multiple measurements is less than 5.7%. The results showed that the TDLAS-LSA method can effectively perform gas sensing under unknown optical path conditions, and has great application potential in low-cost, in-situ and multi-parameter simultaneous measurement.

Compact QEPAS humidity sensor in SF6 buffer gas for high-voltage gas power systems

In SF₆ insulated high-voltage gas power systems, H₂O is the most problematic impurity which not only decreases insulation performance but also creates an acidic atmosphere that promotes corrosion. Corrosion damages electrical equipment and leads to leaks, which pose serious safety hazards to people and the environment. A QEPAS-based sensor system for the sub-ppm level H₂O detection in SF₆ buffer gas was developed by use of a near-infrared commercial DFB diode laser. Since the specific physical constants of SF₆ are strongly different from that of N₂ or air, the resonant frequency and Q-factor of the bare quartz tuning fork (QTF) had changed to 32,763 Hz and 4173, respectively. The optimal vertical detection position was 1.2 mm far from the QTF opening. After the experimental optimization of acoustic micro-resonator (AmR) parameters, gas pressures, and modulation depths, a detection limit of 0.49 ppm was achieved for an averaging time of 1 s, which provided a powerful prevention tool for the safety monitoring in power systems.

A Compact Fiber-Coupled NIR/MIR Laser Absorption Instrument for the Simultaneous Measurement of Gas-Phase Temperature and CO, CO2, and H2O Concentration

A fiber-coupled, compact, remotely operated laser absorption instrument is developed for CO, CO₂, and H₂O measurements in reactive flows at the elevated temperatures and pressures expected in gas turbine combustor test rigs with target pressures from 1-25 bar and temperatures of up to 2000 K. The optical engineering for solutions of the significant challenges from the ambient acoustic noise (~120 dB) and ambient test rig temperatures (60 °C) are discussed in detail. The sensor delivers wavelength-multiplexed light in a single optical fiber from a set of solid-state lasers ranging from diodes in the near-infrared (~1300 nm) to quantum cascade lasers in the mid-infrared (~4900 nm). Wavelength-multiplexing systems using a single optical fiber have not previously spanned such a wide range of laser wavelengths. Gas temperature is inferred from the ratio of two water vapor transitions. Here, the design of the sensor, the optical engineering required for simultaneous fiber delivery of a wide range of laser wavelengths on a single optical line-of-sight, the engineering required for sensor survival in the harsh ambient environment, and laboratory testing of sensor performance in the exhaust gas of a flat flame burner are presented.

Multi-species hydrocarbon measurement using TDLAS with a wide scanning range DFG laser

Tunable diode laser absorption spectroscopy (TDLAS) is a widely used diagnostic technique due to its high sensitivity, fast response, low cost, and other merits. Hydrocarbon detection is a field of great interest in the application of tunable diode lasers as hydrocarbons are fundamental molecules in many industrial processes. Many tunable diode lasers are only suitable for single species detection due to the short scanning range and in real situations. However, different hydrocarbon species tend to exist simultaneously. Here we present a laser system based on the difference-frequency generation (DFG) method for simultaneous hydrocarbon mixtures detection. The direct absorption spectra of different hydrocarbons covering various groups (e.g., alkane, olefin, and aromatic) were measured. The measurements of the concentration dependence of absorbance for each molecule were carried out. The R² values were larger than 0.997, which demonstrated the system can measure hydrocarbons covering different molecular classes accurately. The mixture components were identified using the independent component analysis and quantitative analysis was performed using the classical least-squares method. Future studies will focus on the validation of the system in actual processes.

Multi-species hydrocarbon measurement using TDLAS with a wide scanning range DFG laser

Tunable diode laser absorption spectroscopy (TDLAS) is a widely used diagnostic technique due to its high sensitivity, fast response, low cost, and other merits. Hydrocarbon detection is a field of great interest in the application of tunable diode lasers as hydrocarbons are fundamental molecules in many industrial processes. Many tunable diode lasers are only suitable for single species detection due to the short scanning range and in real situations. However, different hydrocarbon species tend to exist simultaneously. Here we present a laser system based on the difference-frequency generation (DFG) method for simultaneous hydrocarbon mixtures detection. The direct absorption spectra of different hydrocarbons covering various groups (e.g., alkane, olefin, and aromatic) were measured. The measurements of the concentration dependence of absorbance for each molecule were carried out. The R² values were larger than 0.997, which demonstrated the system can measure hydrocarbons covering different molecular classes accurately. The mixture components were identified using the independent component analysis and quantitative analysis was performed using the classical least-squares method. Future studies will focus on the validation of the system in actual processes.

A Short Review of Cavity-Enhanced Raman Spectroscopy for Gas Analysis

The market of gas sensors is mainly governed by electrochemical, semiconductor, and non-dispersive infrared absorption (NDIR)-based optical sensors. Despite offering a wide range of detectable gases, unknown gas mixtures can be challenging to these sensor types, as appropriate combinations of sensors need to be chosen beforehand, also reducing cross-talk between them. As an optical alternative, Raman spectroscopy can be used, as, in principle, no prior knowledge is needed, covering nearly all gas compounds. Yet, it has the disadvantage of a low quantum yield through a low scattering cross section for gases. There have been various efforts to circumvent this issue by enhancing the Raman yield through different methods. For gases, in particular, cavity-enhanced Raman spectroscopy shows promising results. Here, cavities can be used to enhance the laser beam power, allowing higher laser beam-analyte interaction lengths, while also providing the opportunity to utilize lower cost equipment. In this work, we review cavity-enhanced Raman spectroscopy, particularly the general research interest into this topic, common setups, and already achieved resolutions.

Herriott cell spot imaging increases the performance of tunable laser spectrometers

With the availability of high-power (milliwatts) single-mode tunable laser sources that operate at room temperature across the infrared (IR) region, tunable laser spectrometers have seen an explosion of growth in applications that include commercial, Earth and planetary science, and medical and industrial sensing. While the laser sources themselves have shown steady improvement, the detection architecture of using a single-element detector at one end of a multipass cell has remained unchanged over the last few decades. We present here an innovative new approach using a detector array coupled to an IR-transmissive mirror to image all or part of the multipass spot pattern of the far mirror and record spectra for each pixel. This novel approach offers improved sensitivity, increased dynamic range, laser power normalization, contaminant subtraction, resilience to misalignment, and reduces the instrument power requirement by avoiding the need for "fringe-wash" heaters. With many tens of pixels representing each spot during the laser spectral scan, intensity and optical fringe amplitude and phase information are recorded. This allows selection and manipulation (e.g., co-addition, subtraction) of the pixel output spectra to minimize optical interference fringes thereby increasing sensitivity. We demonstrate a factor of ∼20 sensitivity improvement over traditional single-element detection. Dynamic range increase of a factor of ∼100 is also demonstrated through spot selection representing different pathlengths. Additionally, subtracting the spectrum of the first spot from that of the higher pass normalizes the laser power and removes the contribution of contaminant gas and fringes in the fore-optics region. These initial results show that this imaging method is particularly advantageous for multi-channel laser spectrometers, and, once the image field is analyzed, pixel selection can be used to minimize data rate and volume collection requirements. This technique could be beneficial to enhanced-cavity detection schemes.

Ultrathin two-dimensional Fe-doped cobaltous oxide as a piezoelectric enhancement mechanism in quartz crystal tuning fork (QCTF) photodetectors

An innovative ultrathin two-dimensional (2D) Fe-doped cobaltous oxide (Fe-CoO) coated quartz crystal tuning fork (QCTF) was introduced for the purpose of developing a low-cost photoelectric detector with a simple configuration. The enhancement mechanism of the piezoelectric signal in the ultrathin 2D Fe-CoO-coated QCTF detector is assumed to be the synergetic photocarrier transfer and photothermal effect of ultrathin 2D Fe-CoO. The ultrathin 2D nanosheet structure of Fe-CoO with a large specific surface area can efficiently absorb and convert light into heat in the QCTF, and the photocarrier transfer from the Fe-CoO nanosheet to the electrode of the QCTF contributes to the enhancement in electricity given the shortened diffusion distance of carriers to the surfaces of the 2D nanosheet. Finite element modeling was adopted to simulate the thermoelastic expansion and mechanical resonance of the QCTF with 2D Fe-CoO coating to support experimental results and analyses. Moreover, the effects of 2D Fe-CoO on the performance of QCTF-based photoelectric detectors were investigated. This Letter demonstrates that ultrathin 2D materials have great potential in applications such as costly and tiny QCTF detectors, light sensing, biomedical imaging, and spectroscopy.

Calibration of Quartz-Enhanced Photoacoustic Sensors for Real-Life Adaptation

We report on the use of quartz-enhanced photoacoustic spectroscopy for continuous carbon-dioxide measurements in humid air over a period of six days. The presence of water molecules alters the relaxation rate of the target molecules and thus the amplitude of the photoacoustic signal. Prior to the measurements, the photoacoustic sensor system was pre-calibrated using CO₂ mole fractions in the range of 0–10⁻³ (0–1000 ppm) and at different relative humidities between 0% and 45%, while assuming a model hypothesis that allowed the photoacoustic signal to be perturbed linearly by H₂O content. This calibration technique was compared against an alternative learning-based method, where sensor data from the first two days of the six-day period were used for self-calibration. A commercial non-dispersive infrared sensor was used as a CO₂ reference sensor and provided the benchmark for the two calibration procedures. In our case, the self-calibrated method proved to be both more accurate and precise.

Multiple reflections enhanced fiber-optic photoacoustic sensor for gas micro-leakage

A multiple reflections-enhanced fiber-optic photoacoustic (PA) gas sensor for gas micro-leakage is introduced. Multiple reflections of the excitation laser occur on the inner surface of a reflective ring to enhance the PA signal. The PA signal is obtained by measuring the deflection of the gold-coated poly (phenylene sulfide) (PPS) diaphragm with a Fabry-Perot interferometer (FPI). The second harmonic wavelength modulation spectrum (2f-WMS) technology can essentially eliminate the fundamental frequency noise generated by the wavelength-independent absorption of the reflective ring. Experimental results show that the PA signal can be effectively enhanced 11.7 times by the multiple reflections optical path compare with the double-pass optical path. The minimum detection limit of the system is achieved to be 23.6 ppb. The designed PA gas sensor is suited for remote detection of gas micro-leakage.

Laser wavelength metrology with low-finesse etalons and Bayer filters

We present a wavelength meter with picometer-scale resolution based on etaloning effects of inexpensive glass slides and the built-in color filters of a consumer grade CMOS camera. After calibrating the device to a commercial meter, we tested the device's calibration stability using two tunable visible lasers for a period of over 16 days. The wavelength error over that entire period has a standard deviation of 5.29 parts per million (ppm) about a most probable error of 0.90 ppm. Within 24 hours of calibration, this improves to 0.04 ppm with a standard deviation of 3.94 ppm.

Advanced Fiber-Coupled Diode Laser Sensor for Calibration-Free 1f-WMS Determination of an Absorption Line Intensity

A new scheme for a calibration-free diode laser absorption spectroscopy (DLAS) sensor for measuring the parameters of harsh zones is proposed. The key element of the scheme is a micro-prism retroreflector (MPRR). The MPRR facilitates an increase in the mechanical stability of the sensor and a decrease in the background thermal radiation in the hot areas of a tested zone. Reduction in the broadband thermal emission allowed the application of a differential logarithmic conversion (LC) technique for elimination of the residual amplitude modulation and other sources of non-selective attenuation of the probing laser beam. LC allows the use of a 1f-wavelength modulation spectroscopy (WMS) detection scheme. Combination of LC and a 1f-WMS algorithm provided a new modification of calibration-free DLAS, which could be particularly useful for probing harsh zones with pronounced strong turbulence and high levels of acoustic and electrical noise. The influence of the experimental parameters and characteristics of the main electronic components of the recording and processing system on the accuracy of the integral line intensity determination is investigated theoretically and experimentally. The proposed optical scheme of a DLAS sensor and algorithm for the data processing allowed the integral intensity of an absorption line to be obtained. The potential for the scheme was exemplified with a single water vapor absorption line at 7185.6 cm^-1. Simultaneous detection of several absorption lines and data processing using the developed algorithm provides the final goal of a DLAS sensor-determination of temperature and partial pressure of a test molecule in a probed gas volume. The developed scheme allows the spatial multiplexing of the radiation of different diode lasers (DLs), which can be used if various test molecules are to be detected, or absorption lines of a test molecule are detected over different wavelength intervals.

Cepstral analysis for baseline-insensitive absorption spectroscopy using light sources with pronounced intensity variations

This paper presents a data-processing technique that improves the accuracy and precision of absorption-spectroscopy measurements by isolating the molecular absorbance signal from errors in the baseline light intensity (I_o) using cepstral analysis. Recently, cepstral analysis has been used with traditional absorption spectrometers to create a modified form of the time-domain molecular free-induction decay (m-FID) signal, which can be analyzed independently from I_o. However, independent analysis of the molecular signature is not possible when the baseline intensity and molecular response do not separate well in the time domain, which is typical when using injection-current-tuned lasers [e.g., tunable diode and quantum cascade lasers (QCLs)] and other light sources with pronounced intensity tuning. In contrast, the method presented here is applicable to virtually all light sources since it determines gas properties by least-squares fitting a simulated m-FID signal (comprising an estimated I_o and simulated absorbance spectrum) to the measured m-FID signal in the time domain. This method is insensitive to errors in the estimated I_o, which vary slowly with optical frequency and, therefore, decay rapidly in the time domain. The benefits provided by this method are demonstrated via scanned-wavelength direct-absorption-spectroscopy measurements acquired with a distributed-feedback (DFB) QCL. The wavelength of a DFB QCL was scanned across the CO P(0,20) and P(1,14) absorption transitions at 1 kHz to measure the gas temperature and concentration of CO. Measurements were acquired in a gas cell and in a laminar ethylene-air diffusion flame at 1 atm. The measured spectra were processed using the new m-FID-based method and two traditional methods, which rely on inferring (instead of rejecting) the baseline error within the spectral-fitting routine. The m-FID-based method demonstrated superior accuracy in all cases and a measurement precision that was ≈1.5 to 10 times smaller than that provided using traditional methods.

Instrumentation for quantitative analysis of volatile compounds emission at elevated temperatures. Part 1: Design and implementation

A novel suite of instrumentation for the characterisation of materials held inside an air-tight tube furnace operated up to 250 °C has been developed. Real-time detection of released gases (volatile organic compounds (VOCs), CO₂, NO, NO₂, SO₂, CO and O₂) was achieved combining commercial off-the-shelf (COTS) gas sensors and sorbent tubes for further qualitative and semi-quantitative analysis by gas chromatography-mass spectrometry coupled to thermal desorption (TD-GC-MS). The test system was designed to provide a controlled flow (1000 cm³ min^-1) of hydrocarbon free air through the furnace. The furnace temperature ramp was set at a rate of 5 °C min^-1 with 10 min dwell points at 70 °C, 150 °C, 200 °C and 250 °C to allow time for stabilisation and further headspace sampling onto sorbent tubes. Experimental design of the instrumentation is described here and an example data set upon exposure to a gas sample is presented.

InGaAs Membrane Waveguide: A Promising Platform for Monolithic Integrated Mid-Infrared Optical Gas Sensor

Mid-infrared (mid-IR) absorption spectroscopy based on integrated photonic circuits has shown great promise in trace-gas sensing applications in which the mid-IR radiation directly interacts with the targeted analyte. In this paper, considering monolithic integrated circuits with quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), the InGaAs-InP platform is chosen to fabricate passive waveguide gas sensing devices. Fully suspended InGaAs waveguide devices with holey photonic crystal waveguides (HPCWs) and subwavelength grating cladding waveguides (SWWs) are designed and fabricated for mid-infrared sensing at λ = 6.15 μm in the low-index contrast InGaAs-InP platform. We experimentally detect 5 ppm ammonia with a 1 mm long suspended HPCW and separately with a 3 mm long suspended SWW, with propagation losses of 39.1 and 4.1 dB/cm, respectively. Furthermore, based on the Beer-Lambert infrared absorption law and the experimental results of discrete components, we estimated the minimum detectable gas concentration of 84 ppb from a QCL/QCD integrated SWW sensor. To the best of our knowledge, this is the first demonstration of suspended InGaAs membrane waveguides in the InGaAs-InP platform at such a long wavelength with gas sensing results. Also, this result emphasizes the advantage of SWWs to reduce the total transmission loss and the size of the fully integrated device's footprint by virtue of its low propagation loss and TM mode compatibility in comparison to HPCWs. This study enables the possibility of monolithic integration of quantum cascade devices with TM polarized characteristics and passive waveguide sensing devices for on-chip mid-IR absorption spectroscopy.

Hydrogen Sensor Based on Tunable Diode Laser Absorption Spectroscopy

A laser-based hydrogen (H₂) sensor using wavelength modulation spectroscopy (WMS) was developed for the contactless measurement of molecular hydrogen. The sensor uses a distributed feedback (DFB) laser to target the H₂ quadrupole absorption line at 2121.8 nm. The H₂ absorption line exhibited weak collisional broadening and strong collisional narrowing effects. Both effects were investigated by comparing measurements of the absorption linewidth with detailed models using different line profiles including collisional narrowing effects. The collisional broadening and narrowing parameters were determined for pure hydrogen as well as for hydrogen in nitrogen and air. The performance of the sensor was evaluated and the sensor applicability for H₂ measurement in a range of 0–10 %v of H₂ was demonstrated. A precision of 0.02 %v was achieved with 1 m of absorption pathlength (0.02 %v∙m) and 1 s of integration time. For the optimum averaging time of 20 s, precision of 0.005 %v∙m was achieved. A good linear relationship between H₂ concentration and sensor response was observed. A simple and robust transmitter–receiver configuration of the sensor allows in situ installation in harsh industrial environments.

Gas Detection Using Portable Deep-UV Absorption Spectrophotometry: A Review

Several gas molecules of environmental and domestic significance exhibit a strong deep-UV absorption. Therefore, a sensitive and a selective gas detector based on this unique molecular property (i.e., absorption at a specific wavelength) can be developed using deep-UV absorption spectrophotometry. UV absorption spectrometry provides a highly sensitive, reliable, self-referenced, and selective approach for gas sensing. This review article addresses the recent progress in the application of deep-UV absorption for gas sensing owing to its inherent features and tremendous potentials. Applications, advancements, and challenges related to UV emission sources, gas cells, and UV photodetectors are assessed and compared. We present the relevant theoretical aspects and challenges associated with the development of portable sensitive spectrophotometer. Finally, the applications of UV absorption spectrometry for ozone, NO₂, SO₂, and aromatic organic compounds during the last decades are discussed and compared. A portable UV absorption spectrophotometer can be developed by using LEDs, hollow core waveguides (HCW), and UV photodetectors (i.e., photodiodes). LED provides a portable UV emission source with low power input, low-intensity drifts, low cost, and ease of alignment. It is a quasi-chromatic UV source and covers the absorption band of molecules without optical filters for absorbance measurement of a target analyte. HCWs can be applied as a miniature gas cell for guiding UV radiation for measurement of low gas concentrations. Photodiodes, on the other hand, offer a portable UV photodetector with excellent spectral selectivity with visible rejection, minimal dark current, linearity, and resistance against UV-aging.

Time-resolved continuous-filtering Vernier spectroscopy of H2O and OH radical in a flame

We use broadband near-infrared continuous-filtering Vernier spectroscopy (CF-VS) for time-resolved detection of H₂O and OH radical in a premixed CH₄/air flat flame. The CF-VS spectrometer is based on a femtosecond Er:fiber laser, an external cavity that contains the flame, and a detection system comprising a rotating diffraction grating and photodetectors. Spectra of H₂O and OH radical around 1570 nm are continuously recorded with 6.6 GHz spectral resolution, 4.0 × 10^-7 cm^-1 absorption sensitivity, and 25 ms time resolution, while the fuel-air equivalence ratio is periodically modulated with a square wave. The concentrations of the two analytes are retrieved with percent level precision by a fit of a Vernier model to each spectrum spanning 13 nm. The temporal profiles of both concentrations in each modulation cycle are repeatable and the steady-state concentration levels are in good agreement with predictions based on one-dimensional simulations of a static flat flame. The robust CF-VS spectrometer opens up for quantitative monitoring of multiple products of time-varying combustion processes with relatively simple data acquisition procedures.

Laser Absorption Sensing Systems: Challenges, Modeling, and Design Optimization

Laser absorption spectroscopy (LAS) is a promising diagnostic method capable of providing high-bandwidth, species-specific sensing, and highly quantitative measurements. This review aims at providing general guidelines from the perspective of LAS sensor system design for realizing quantitative species diagnostics in combustion-related environments. A brief overview of representative detection limits and bandwidths achieved in different measurement scenarios is first provided to understand measurement needs and identify design targets. Different measurement schemes including direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS), and their variations are discussed and compared in terms of advantages and limitations. Based on the analysis of the major sources of noise including electronic, optical, and environmental noises, strategies of noise reduction and design optimization are categorized and compared. This addresses various means of laser control parameter optimization and data processing algorithms such as baseline extraction, in situ laser characterization, and wavelet analysis. There is still a large gap between the current sensor capabilities and the demands of combustion and engine diagnostic research. This calls for a profound understanding of the underlying fundamentals of a LAS sensing system in terms of optics, spectroscopy, and signal processing.

Near-Infrared C2H2 Detection System Based on Single Optical Path Time Division Multiplexing Differential Modulation Technique and Multi-Reflection Chamber

A time division multiplexing differential modulation technique is proposed to address the interference problem caused by the fluctuation of laser light intensity in the single optical path detection system. Simultaneously, a multi-reflection chamber is designed and manufactured to further improve the system’s precision with an optical path length of 80 m. A near-infrared C2H2 detection system was developed. The absorption peak of the acetylene (C2H2) molecule near 1520 nm was selected as the absorption line. A laser driver is developed, and a lock-in amplifier is used to extract the second harmonic (2f) signal. A good linear relationship existed between C2H2 concentration and the 2f signal, and the correlation coefficient was 0.9997. In the detection range of 10–100 ppmv, the minimum detection limit was 0.3 ppmv, and the precision was 2%. At 50 ppmv, C2H2 and continuous detection for 10 h, the data average was 50.03 ppmv, and the fluctuation was less than ±1.2%. The Allan variance method was adopted to evaluate the long-term characteristic of the system. At 1 s of integration time, the Allan deviation was 0.3 ppmv. When the integration time reached 362 s, the Allan deviation was 0.0018 ppmv, which indicates the good stability of the detection system.

Simultaneous and interference-free measurements of temperature and C2H4 concentration using a single tunable diode laser at 1.62 µm

In the tunable diode laser absorption spectroscopy-based diagnostics, the absorption of the measured target species may be influenced by the interference absorption from other vapor-phase species and the extinction from particles and liquid droplets, especially at high temperatures and pressures. Here, we report the first application (to our knowledge) of a differential absorption diagnostic for interference-free, simultaneous measurement of temperature and ethylene concentration using a single distributed-feedback diode laser near 1.62 μm. According to the detailed study of the C₂H₄ spectra in this region, two wavelength pairs are chosen to measure the temperature based on six selection criteria. C₂H₄ concentration is measured by one of the selected wavelength pairs with higher differential absorption. To validate the developed system, experiments are performed in a well-controlled heated static cell at a range of temperatures (300-900 K) and pressures (1-6 atm). The measurement accuracies for temperature and ethylene concentration are 1.83% and 1.65%, respectively, over the considered ranges. The precision, stability, and detection limit are also analyzed to validate the system's performance. This system can potentially be applied in a variety of combustion applications.

Off-resonant photoacoustic spectroscopy for analysis of multicomponent gas mixtures at high concentrations using broadband vibrational overtones of individual gas species

The broadband photoacoustic spectroscopy (PAS) technique is proposed and demonstrated for measurement of CH₄, CO₂, and H₂O vapor in the 1.6 to 2.0 μm wavelength region. The wide spectrum of a supercontinuum light source is used to cover broadband absorption bands of multiple gas species. This sensor works in the off-resonant frequency of the designed photoacoustic cell and exhibits a wide concentration measurement range of parts per billion by volume (ppb-v) to 100%. The PAS sensor is further tested in real time by measuring the concentration of CO₂, CH₄, and H₂O vapor in biogas plants.

Influence of Line Pair Selection on Flame Tomography Using Infrared Absorption Spectroscopy

We report the influence of absorption line selection on the tomographic results for high-temperature flames by numerical and experimental methods. Different combinations of infrared H₂O absorption transitions are utilized with the Tikhonov-regularized Abel inversion to reconstruct the radial distribution of temperature and H₂O concentration in a flat flame. It is shown that besides using the mathematical algorithm such as regularization, selecting a line pair with a large Δ E″ (>1390 cm^-1) also reduces the reconstruction uncertainty at 300-2000 K. In this study, a proper selection of absorption line pairs reduces the reconstruction uncertainty by 25% at the same level of noise. The line pair of H₂O transitions at 4029.524 cm^-1 and 4030.729 cm^-1 is recommended for the tomography of high-temperature flames at 1000-3000 K, whereas the line pair of 7185.597 cm^-1 and 7444.352 cm^-1 can be used at 300-1000 K.

Laser frequency locking and intensity normalization in wavelength modulation spectroscopy for sensitive gas sensing

A novel method for laser frequency locking and intensity normalization in wavelength modulation spectroscopy (WMS)-based gas sensor system is reported. The center spacing between two second harmonic peaks demodulated from the rising and falling edges of a scanning triangular wave (for wavelength scan) is employed as a frequency locking reference. Amplitude of the directly acquired sine signal (for wavelength modulation) in the spectral region far away from the absorption feature is employed as an intensity normalization reference. A 50 ppm CH₄:N₂ sample sealed in a multi-pass cell at 1 atm was employed as the target analyte for demonstration. The frequency locking significantly improves measurement accuracy, and the introduced intensity normalization method realized a ~3 times SNR improvement as compared to the commonly used 1f normalization method under frequency locking conditions. A minimum measurement precision of ~2.5 ppbv was achieved with a normalized noise equivalent absorption coefficient of 1.8 × 10^-9 cm^-1Hz^-1/2.

Towards Gas Discrimination and Mapping in Emergency Response Scenarios Using a Mobile Robot with an Electronic Nose

Emergency personnel, such as firefighters, bomb technicians, and urban search and rescue specialists, can be exposed to a variety of extreme hazards during the response to natural and human-made disasters. In many of these scenarios, a risk factor is the presence of hazardous airborne chemicals. The recent and rapid advances in robotics and sensor technologies allow emergency responders to deal with such hazards from relatively safe distances. Mobile robots with gas-sensing capabilities allow to convey useful information such as the possible source positions of different chemicals in the emergency area. However, common gas sampling procedures for laboratory use are not applicable due to the complexity of the environment and the need for fast deployment and analysis. In addition, conventional gas identification approaches, based on supervised learning, cannot handle situations when the number and identities of the present chemicals are unknown. For the purpose of emergency response, all the information concluded from the gas detection events during the robot exploration should be delivered in real time. To address these challenges, we developed an online gas-sensing system using an electronic nose. Our system can automatically perform unsupervised learning and update the discrimination model as the robot is exploring a given environment. The online gas discrimination results are further integrated with geometrical information to derive a multi-compound gas spatial distribution map. The proposed system is deployed on a robot built to operate in harsh environments for supporting fire brigades, and is validated in several different real-world experiments of discriminating and mapping multiple chemical compounds in an indoor open environment. Our results show that the proposed system achieves high accuracy in gas discrimination in an online, unsupervised, and computationally efficient manner. The subsequently created gas distribution maps accurately indicate the presence of different chemicals in the environment, which is of practical significance for emergency response.

Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review

The vast majority of gaseous chemical substances exhibit fundamental rovibrational absorption bands in the mid-infrared spectral region (2.5–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection of trace gases. We reviewed primarily mid-infrared tunable laser-based broadband absorption spectroscopy for trace gas detection, focusing on 2008–2018. The scope of this paper is to discuss recent developments of system configuration, tunable lasers, detectors, broadband spectroscopic techniques, and their applications for sensitive, selective, and quantitative trace gas detection.

Simple calibrated nonlinear excitation regime two-line atomic fluorescence thermometry

Nonlinear excitation regime two-line atomic fluorescence (NTLAF) is a promising two-dimensional (2D) thermometry technique for turbulent sooty flames. However, the complexity of calibrating three system parameters and expensive instruments restricts the application of the current NTLAF technique. Here we propose a simple and cheap NTLAF measurement approach based on a one-parameter model and tunable diode laser absorption spectroscopy (TDLAS) calibration. Using this methodology, only one system parameter, instead of three as in traditional NTLAF, is to be calibrated by path-averaged temperature acquired by the TDLAS technique. As a demonstration, instantaneous 2D thermometry data of a homemade burner were acquired using this approach, with measurement uncertainty of ∼4.5% and deviation from both reference TDLAS results and Raleigh scattering measurement results less than 50 K, typically within 20 K. This approach offers a novel simplified NTLAF solution for noncontact, in-suit, high-resolution 2D temperature measurement and is expected to greatly improve the compatibility of the NTLAF technique in scientific research and engineering applications.

Determination of the Maximum Temperature in a Non-Uniform Hot Zone by Line-of-Site Absorption Spectroscopy with a Single Diode Laser

A new algorithm for the estimation of the maximum temperature in a non-uniform hot zone by a sensor based on absorption spectrometry with a diode laser is developed. The algorithm is based on the fitting of the absorption spectrum with a test molecule in a non-uniform zone by linear combination of two single temperature spectra simulated using spectroscopic databases. The proposed algorithm allows one to better estimate the maximum temperature of a non-uniform zone and can be useful if only the maximum temperature rather than a precise temperature profile is of primary interest. The efficiency and specificity of the algorithm are demonstrated in numerical experiments and experimentally proven using an optical cell with two sections. Temperatures and water vapor concentrations could be independently regulated in both sections. The best fitting was found using a correlation technique. A distributed feedback (DFB) diode laser in the spectral range around 1.343 µm was used in the experiments. Because of the significant differences between the temperature dependences of the experimental and theoretical absorption spectra in the temperature range 300–1200 K, a database was constructed using experimentally detected single temperature spectra. Using the developed algorithm the maximum temperature in the two-section cell was estimated with accuracy better than 30 K.

Development of a 3-dimensional tissue lung phantom of a preterm infant for optical measurements of oxygen-Laser-detector position considerations

There is a need to further improve the clinical care of our most vulnerable patients-preterm infants. Novel diagnostic and treatment tools facilitate such advances. Here, we evaluate a potential percutaneous optical monitoring tool to assess the oxygen and water vapor content in the lungs of preterm babies. The aim is to prepare for further clinical studies by gaining a detailed understanding of how the measured light intensity and gas absorption signal behave for different possible geometries of light delivery and receiver. Such an experimental evaluation is conducted for the first time utilizing a specially developed 3-dimensional-printed optical phantom based on a geometry model obtained from computer tomography images of the thorax (chest) of a 1700-g premature infant. The measurements yield reliable signals for source-detector distances up to about 50 mm, with stronger gas absorption signals at long separations and positions related to the lower part of the lung, consistent with a larger relative volume of this. The limitations of this study include the omission of scattering tissue within the lungs and that similar optical properties are used for the wavelengths employed for the 2 gases, yielding no indication on the optimal wavelength pair to use.

Ppbv-Level Ethane Detection Using Quartz-Enhanced Photoacoustic Spectroscopy with a Continuous-Wave, Room Temperature Interband Cascade Laser

A ppbv-level quartz-enhanced photoacoustic spectroscopy (QEPAS)-based ethane (C₂H₆) sensor was demonstrated by using a 3.3 μm continuous-wave (CW), distributed feedback (DFB) interband cascade laser (ICL). The ICL was employed for targeting a strong C₂H₆ absorption line located at 2996.88 cm⁻¹ in its fundamental absorption band. Wavelength modulation spectroscopy (WMS) combined with the second harmonic (2f) detection technique was utilized to increase the signal-to-noise ratio (SNR) and simplify data acquisition and processing. Gas pressure and laser frequency modulation depth were optimized to be 100 Torr and 0.106 cm⁻¹, respectively, for maximizing the 2f signal amplitude. Performance of the QEPAS sensor was evaluated using specially prepared C₂H₆ samples. A detection limit of 11 parts per billion in volume (ppbv) was obtained with a 1-s integration time based on an Allan-Werle variance analysis, and the detection precision can be further improved to ~1.5 ppbv by increasing the integration time up to 230 s.

Novel Semi-Parametric Algorithm for Interference-Immune Tunable Absorption Spectroscopy Gas Sensing

One of the most common limits to gas sensor performance is the presence of unwanted interference fringes arising, for example, from multiple reflections between surfaces in the optical path. Additionally, since the amplitude and the frequency of these interferences depend on the distance and alignment of the optical elements, they are affected by temperature changes and mechanical disturbances, giving rise to a drift of the signal. In this work, we present a novel semi-parametric algorithm that allows the extraction of a signal, like the spectroscopic absorption line of a gas molecule, from a background containing arbitrary disturbances, without having to make any assumption on the functional form of these disturbances. The algorithm is applied first to simulated data and then to oxygen absorption measurements in the presence of strong fringes.To the best of the authors' knowledge, the algorithm enables an unprecedented accuracy particularly if the fringes have a free spectral range and amplitude comparable to those of the signal to be detected. The described method presents the advantage of being based purely on post processing, and to be of extremely straightforward implementation if the functional form of the Fourier transform of the signal is known. Therefore, it has the potential to enable interference-immune absorption spectroscopy. Finally, its relevance goes beyond absorption spectroscopy for gas sensing, since it can be applied to any kind of spectroscopic data.

Fiber-ring laser-based intracavity photoacoustic spectroscopy for trace gas sensing

We demonstrated a novel trace gas sensing method based on fiber-ring laser intracavity photoacoustic spectroscopy. This spectroscopic technique is a merging of photoacoustic spectroscopy (PAS) with a fiber-ring cavity for sensitive and all-fiber gas detection. A transmission-type PAS gas cell (resonant frequency f₀=2.68  kHz) was placed inside the fiber-ring laser to fully utilize the intracavity laser power. The PAS signal was excited by modulating the laser wavelength at f₀/2 using a custom-made fiber Bragg grating-based modulator. We used this spectroscopic technique to detect acetylene (C₂H₂) at 1531.6 nm as a proof of principle. With a low Q-factor (4.9) of the PAS cell, our sensor achieved a good linear response (R²=0.996) to C₂H₂ concentration and a minimum detection limit of 390 ppbv at 2-s response time.

Sensitivity enhancement and fringe reduction in tunable diode laser spectroscopy using hemispherical diffusers

The use of diffuse, highly reflective optical components, in particular, a hemispherical BaSO₄ diffuser, at the point of light injection into non-transparent or turbid media was evaluated as a means to increase the measurement sensitivity of spectroscopic absorption measurements. By performing the light injection from, e.g., an optical fiber through a component designed to make the light diffuse and to reflect (and thereby re-inject) light scattered from the sample, the total amount of light delivered into the sample is increased. Further, the occurrence of possible interference fringes is strongly reduced.

SEALDH-II-An Autonomous, Holistically Controlled, First Principles TDLAS Hygrometer for Field and Airborne Applications: Design-Setup-Accuracy/Stability Stress Test

Instrument operation in harsh environments often significantly impacts the trust level of measurement data. While commercial instrument manufacturers clearly define the deployment conditions to achieve trustworthy data in typical standard applications, it is frequently unavoidable in scientific field applications to operate instruments outside these commercial standard application specifications. Scientific instrumentation, however, is employing cutting-edge technology and often highly optimized but also lacks long-term field tests to assess the field vs. laboratory performance. Recently, we developed the Selective Extractive Laser Diode Hygrometer (SEALDH-II), which addresses field and especially airborne applications as well as metrological laboratory validations. SEALDH-II targets reducing deviations between airborne hygrometers (currently up to 20% between the most advanced hygrometers) with a new holistic, internal control and validation concept, which guarantees the transfer of the laboratory performance into a field scenario by capturing more than 80 instrument internal "housekeeping" data to nearly perfectly control SEALDH-II's health status. SEALDH-II uses a calibration-free, first principles based, direct Tuneable Diode Laser Absorption Spectroscopy (dTDLAS) approach, to cover the entire atmospheric humidity measurement range from about 3 to 40,000 ppmv with a calculated maximum uncertainty of 4.3% ± 3 ppmv. This is achieved not only by innovations in internal instrument monitoring and design, but also by active control algorithms such as a high resolution spectral stabilization. This paper describes the setup, working principles, and instrument stabilization, as well as its precision validation and long-term stress tests in an environmental chamber over an environmental temperature and humidity range of ΔT = 50 K and ΔRH = 80% RH, respectively.

Optical Gas Sensing of Ammonia and Amines Based on Protonated Porphyrin/TiO₂ Composite Thin Films

Open porous and transparent microcolumnar structures of TiO₂ prepared by physical vapour deposition in glancing angle configuration (GLAD-PVD) have been used as host matrices for two different fluorescent cationic porphyrins, 5-(N-methyl 4-pyridyl)-10,15,20-triphenyl porphine chloride (MMPyP) and meso-tetra (N-methyl 4-pyridyl) porphine tetrachloride (TMPyP). The porphyrins have been anchored by electrostatic interactions to the microcolumns by self-assembly through the dip-coating method. These porphyrin/TiO₂ composites have been used as gas sensors for ammonia and amines through previous protonation of the porphyrin with HCl followed by subsequent exposure to the basic analyte. UV–vis absorption, emission, and time-resolved spectroscopies have been used to confirm the protonation–deprotonation of the two porphyrins and to follow their spectral changes in the presence of the analytes. The monocationic porphyrin has been found to be more sensible (up to 10 times) than its tetracationic counterpart. This result has been attributed to the different anchoring arrangements of the two porphyrins to the TiO₂ surface and their different states of aggregation within the film. Finally, there was an observed decrease of the emission fluorescence intensity in consecutive cycles of exposure and recovery due to the formation of ammonium chloride inside the film.

Laboratory investigation of three distinct emissions monitors for hydrochloric acid

The measurement of hydrochloric acid (HCl) on a continuous basis in coal-fired plants is expected to become more important if HCl standards become implemented as part of the Federal Mercury and Air Toxics Standards (MATS) standards that are under consideration. For this study, the operational performance of three methods/instruments, including tunable diode laser absorption spectroscopy (TDLAS), cavity ring down spectroscopy (CRDS), and Fourier transform infrared (FTIR) spectroscopy, were evaluated over a range of real-world operating environments. Evaluations were done over an HCl concentration range of 0-25 ppmv and temperatures of 25, 100, and 185 °C. The average differences with respect to temperature were 3.0% for the TDL for values over 2.0 ppmv and 6.9% of all concentrations, 3.3% for the CRDS, and 4.5% for the FTIR. Interference tests for H₂O, SO₂, and CO, CO₂, and NO for a range of concentrations typical of flue gases from coal-fired power plants did not show any strong interferences. The possible exception was an interference from H₂O with the FTIR. The instrument average precision over the entire range was 4.4% for the TDL with better precision seen for concentrations levels of 2.0 ppmv and above, 2.5% for the CRDS, and 3.5% for the FTIR. The minimum detection limits were all on the order of 0.25 ppmv, or less, utilizing the TDL values with a 5-m path. Zero drift was found to be 1.48% for the TDL, 0.88% for the CRDS, and 1.28% for the FTIR.

IMPLICATIONS

This study provides an evaluation of the operational performance of three methods/instruments, including TDL absorption spectroscopy (TDLAS), cavity ring down spectroscopy (CRDS), and FTIR spectroscopy, for the measurement of hydrochloric acid (HCl) over a range of real-world operating environments. The results showed good instrument accuracy as a function of temperature and no strong interferences for flue gases typical to coal-fired power plants. The results show that these instruments would be viable for the measurement of HCl in coal-fired plants if HCl standards become implemented as part of the Federal Mercury and Air Toxics Standards (MATS) standards that are under consideration.

Multi-mode absorption spectroscopy using a quantum cascade laser for simultaneous detection of NO and H2O

Detection of multiple transitions in NO and H2O using multi-mode absorption spectroscopy, MUMAS, with a quantum cascade laser, QCL, operating at 5.3 μm at scan rates up to 10 kHz is reported. The linewidth of longitudinal modes of the QCL is derived from pressure-dependent fits to experimental MUMAS data. Variations in the spectral structure of the broadband, multi-mode, output of the commercially available QCL employed are analysed to provide accurate fits of modelled MUMAS signatures to the experimental data.

Comparison of Tunable Diode Laser Absorption Spectroscopy and Isothermal Micro-calorimetry for Non-invasive Detection of Microbial Growth in Media Fills

Two methods were investigated for non-invasive microbial growth-detection in intact glass vials as possible techniques for automated inspection of media-filled units. Tunable diode laser absorption spectroscopy (TDLAS) was used to determine microbially induced changes in O2 and CO2 concentrations within the vial headspaces. Isothermal microcalorimetry (IMC) allowed the detection of metabolic heat production. Bacillus subtilis and Streptococcus salivarius were chosen as test organisms. Parameters as robustness, sensitivity, comparability and time to detection (TtD) were evaluated to assess method adequacy. Both methods robustly detected growth of the tested microorganisms within less than 76 hours using an initial inoculum of <10CFU. TDLA turned out to be less sensitive than TDLA and IMC, as some false negative results were observed. Compared to the visual media-fill examination of spiked samples, the investigated techniques were slightly slower regarding TtD. Although IMC showed shorter TtD than TDLAS the latter is proposed for automating the media-fill inspection, as larger throughput can be achieved. For routine use either TDLA or a combination of TDLA and TDLA should be considered. IMC may be helpful for replacing the sterility assessment of commercial drug products before release.

Broadband on-chip near-infrared spectroscopy based on a plasmonic grating filter array

We demonstrate an ultra-compact, broadband on-chip near-infrared (NIR) spectroscopy system based on a narrow-band plasmonic filter array. The entire filter array, consisting of 28 individual subwavelength metallic gratings, was monolithically integrated in a thin gold film on a quartz substrate, covering a 270 nm spectra from 1510 nm to 1780 nm. In order to achieve a high spectral resolution, extremely narrow slits are created for the gratings with a polymer waveguide layer on top, generating narrow-band guided-mode resonances through coupling with the surface-plasmon resonances of the metallic gratings. Experimental results show that the transmission bands of the filter array have full width at half-maximum of only 7 nm-13 nm, which is sufficient for NIR spectroscopy. The NIR absorption spectroscopy of xylene using the on-chip plasmonic filter array matches very well with the results from conventional Fourier transform infrared spectroscopy, which proves the great potential for NIR sensing applications.