Detection of volatile organic compounds (VOCs) with particular attention to the BTEX group consisting of benzene, toluene, ethylbenzene, and xylene has risen as an important task in fields as environmental monitoring and breath analysis. Sensors based on optical detection techniques may represent an alternative to traditional approaches. However, optical sensing has been limited in terms of measurements selectivity as BTEX point out absorption features around 3.3 μm, where strong interferences from hydrocarbons occur. This issue can be eliminated operating at longer wavelength in the region from 13 to 15 μm, where BTEX show distinct and isolated absorption features. However, the investigations in this region have been limited by the lack of suitable laser sources as well as by the low performances of commercial detectors. In this work, a new approach to benzene detection is proposed, employing a long wavelength InAs-based QCL as light source and a quartz tuning fork as detector for TDLAS measurements, in light-induced thermoelastic spectroscopy (LITES) detection scheme. Analyte detection in the range of hundreds of part-per-billion with a shoe-box size design is demonstrated.
In the last few decades, quartz has been the chosen material to fabricate piezoelectric resonators such as MEMS, racetrack resonators and tuning forks (TFs), which have been widely employed for a variety of sensing applications in several different fields, spanning from environmental monitoring to oil and gas industry. However, throughout the last decade, lithium niobate (LiN) started being employed in the integrated photonics field to build acousto-optical, electro-optical and nonlinear optical devices. In particular, 128° y-cut LiN has a higher density (4.64 g/cm3), Young modulus (145 GPa), and its piezoelectric coefficients are overall one order of magnitude greater compared to z-cut quartz. In this work, a custom lithium niobate tuning fork (LiNTF) is employed for the first time as a piezoelectric transducer in a photoacoustic spectroscopy-based apparatus devoted to gas sensing. The LiNTF was obtained from a 128° y-cut LN wafer and exhibits a resonance frequency f0 = 39196.6 Hz and a quality factor Q = 5900 at atmospheric pressure. For this proof of concept, a water vapor absorption line falling at 7181.14 cm-1 (1.392 μm) was targeted, achieving a signal to noise ratio (SNR) of 400 for a standard air sample having a 1.2% concentration of water vapor at atmospheric pressure and 100 ms of lock-in integration time. An Allan – Werle analysis showed a one order of magnitude improvement in the SNR when increasing the integration time up to 20 s. These preliminary results mark a first step towards the realization of LiNTF-based devices integrated on LiN platforms for gas sensing applications.
Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a highly sensitive optical technique, suitable for real-time and in situ trace gas detection. In QEPAS, Quartz tuning forks (QTF) are employed as piezoelectric transducers of sound waves, induced by gas non-radiative energy relaxation following an infrared modulated light absorption. The generated electric signal depends on the gas concentration. An accurate and reliable QEPAS measurement requires: i) the QTF characterization, in terms of resonance frequency and quality factor and ii) the tuning range scan of the laser employed to detect the selected gas. These two operations could take several minutes. Beat frequency QEPAS (BF-QEPAS) is an alternative approach to standard QEPAS. In BF-QEPAS, a fast scan of the laser tuning range is employed to generate an acoustic pulse. Gas concentration, QTF resonance frequency, and quality factor can be measured acquiring and analyzing the transient response of the QTF to the acoustic pulse. In this work, a custom T-shaped QTF was employed to detect nitrogen monoxide (NO), targeting its absorption feature at 1900.07 cm-1 with an interband cascade laser. A minimum detection limit as low as 180 ppb of NO at an integration time of 5 ms was achieved, and a highly accurate measurement of the QTF resonance frequency and quality factor were demonstrated using BF-QEPAS. Finally, the possibility to fully scan the laser tuning range in less than 15 s was proved.
Simultaneous detection of different gas species represents an indispensable asset for several applications, such as instantaneous quantification of isotope concentration ratios, self-calibrating sensors, and monitoring of the temporal evolution of a chemical reaction. In this research work, a dual-gas quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for a real-time analysis and in a continuous flow monitoring of one reactant and one product of a gas-phase chemical reaction involving nitrogen dioxide (NO2) and water vapor (i.e., H2O) – as reaction reactants – and nitrogen monoxide (NO) – as one of the reaction products – was realized. The QEPAS sensor implemented a spectrophone composed of a pair of metallic acoustic resonator tubes applied at both antinode points of a custom quartz tuning fork (QTF). In this configuration, two different quantum cascade lasers (QCLs) were used, having an emission wavelength centered at 5.26 μm – resonant with a nitrogen monoxide absorption feature located at 1,900.075 cm-1 – and at 6.25 μm – resonant with a nitrogen dioxide absorption feature located at 1,601.77 cm-1 –, respectively. The chemical reaction was studied by injecting in the gas line a certified concentration of 5,000 parts-per-million (ppm) of NO2:N2 and monitoring the QEPAS signals at four different total gas flow values, i.e., 10, 20, 30 and 50 standard cubic centimeters per minute (SCCM), respectively.
Hydrogen sulfide (H2S) is a harmful gas whose emissions are associated to natural gas leaks. Laser-based spectroscopy techniques have been demonstrated to be well suited for real time gas monitoring, providing high sensitivity and selectivity. Quartz enhanced photoacoustic spectroscopy (QEPAS) helped in developing of compact sensors. In the QEPAS sensor here presented, a distribute feedback diode laser emitting around 2.6 μm and a T-shaped quartz tuning fork coupled with acoustic resonator tubes were used to detect H2S in methane-based matrix. The realized QEPAS sensor is a ready-to use solution for H2S leaks monitoring in presence of methane (CH4) at the percent scale.
Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) is a sensitive trace gas detection technique employing Quartz Tuning Forks (QTF) as sensitive elements. Usually, a transimpedance amplifier is used as QTF front-end electronic, with a 10 MΩ feedback resistor. With a low-pass filter time constant of 100 ms, the thermal noise of the QTF intrinsic resistance (~100 kΩ) represents the main noise contribution. For real-time applications, shorter time constants need to be employed. In this work, we studied different amplifier structures with the aim to reduce the QTF thermal noise and improve the signalto- noise ratio when the QTF is used in QEPAS sensors.
Here we report on the application of multivariate analysis on optical sensors for gas detection based on Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technique, focused on the analysis of complex gas mixtures. In real-world applications the effects of spectral and non-spectral interference occurring within the gas samples cannot be neglected in order to increase sensors selectivity and accuracy. In this work, Partial Least Squares Regression (PLSR) is selected as regression technique and tested on different gas samples for different applications. PLSR is able to retrieve analytes concentrations filtering out both: i) spectral contributions of analytes characterized by strongly overlapping features; ii) correlation effects due to the interaction among the sample’s components, i.e., matrix effects characterizing the photoacoustic detection.
In petrochemical industry, downstream operations can be optimized detecting hydrocarbons in the gas phase. Laser-based spectroscopic sensors are well suited for this application, since they proved to be highly selective, sensitive and robust. Quartz enhanced photoacoustic spectroscopy (QEPAS) is an optical technique, offering also a high level of compactness. In this work, a QEPAS sensor implementing a custom T-shaped quartz tuning fork and an interband cascade laser emitting at 3367 nm was employed to detect methane, ethane and propane in natural gas-like mixtures. The partial least square regression analysis was implemented to measure the hydrocarbons concentrations in the acquired signals.
In this work, we study the non-radiative energy relaxation rate of methane isotopologues in the mid-infrared spectral range, at about 7.719 µm. We exploited quartz-enhanced photoacoustic spectroscopy to measure the photoacoustic signal at different pressures, exploiting several custom quartz tuning forks operating in the range from 3 to 16 kHz. For each isotopologue the relaxation time in a matrix of water vapor and nitrogen was retrieved, to evaluate the effect of water vapor as an energy relation promoter. Moreover, mixtures composed of 12CH4 and 13CH4, with concentration ratios different from the natural abundance, in a matrix of water vapor and nitrogen were analyzed.
We report on a sensor for methane (C1) and ethane (C2) detection employing a quartz tuning fork as a photodetector for tunable diode laser spectroscopy (TDLAS). In this configuration, the QTF is immersed in the gas mixture under investigation within a vacuum-tight cell. Concentrations of methane and ethane in nitrogen-based mixtures ranged from traces up to percent. An interband cascade laser emitting at 3.345 μm was used as light source. Natural gas-like mixtures were generated in 1:10 nitrogen dilution, and gas mixtures composition was retrieved with an accuracy >98%. Decreasing the target gases concentration, minimum detection limits of 770 ppb and 75 ppb for C1 and C2, respectively, were measured at 10 s integration time.
We report on a compact, lightweight, and portable sensor for simultaneous detection of methane, nitrous oxide, and ammonia in atmosphere. The sensor embeds two detection modules based on quartz-enhanced photoacoustic spectroscopy, with two quantum cascade lasers as light sources and two spectrophones with custom-made quartz tuning forks. Dedicated electronic boards were designed to control the laser sources, to stabilize the gas flow and to perform data analysis, with a computer interface for an easy end-user operation. The sensor was calibrated in laboratory environment with certified concentrations. Detection limits well below the natural abundance in standard air were achieved.
Here we report on the multi-gas detection of carbon monoxide (CO), nitrous oxide (N2O), carbon dioxide (CO2), and water vapor (H2O) by using a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor exploiting a Vernier effect-based quantum cascade laser as excitation source. The device emission wavelengths ranged from 2100 cm-1 to 2250 cm-1. The achieved minimum detection limits were 6 ppb, 7 ppb, and 71 ppm fr CO, N2O, and CO2, respectively, at 100 ms of integration time. Finally, QEPAS sensor performances were tested retrieving the concentrations of the target gases within laboratory air.
We report on the detection of the isotopologues 12CH4 and 13CH4 by employing a quartz-enhanced photoacoustic spectroscopy (QEPAS)-based sensor. By properly selecting the exciting light source and the working conditions, two absorption lines, having a negligible cross-section ratio temperature coefficient of -6.7‰/°C and a cross section ratio of ~ 0.06 for a natural abundance of each isotope, can be targeted. The QEPAS signal of the two isotopologues was acquired for mixtures in nitrogen of methane in natural abundance in a wide range of concentrations (0.02%-20%) showing a non-linear trend with high concentrations and a constant ratio comparable with the cross-section.
We report on a highly sensitive and selective optical sensor for detection of carbon monoxide (CO) in a sulfur hexafluoride (SF6) gas matrix by using quartz-enhanced photoacoustic spectroscopy (QEPAS) technique. The sensor uses a mid-infrared quantum cascade laser with central wavelength at 4.61 μm as light source and a spectrophone consisting of a novel 8 kHz T-shaped quartz tuning fork with grooved prongs coupled with a pair of resonator tubes for photoacoustic detection. A minimum detection limit of 10 ppb at 10 s of signal integration time was achieved.
The main limitations of tunable diode laser absorption spectroscopy (TDLAS) sensors are represented by the high cost, limited detection bandwidth and low adaptability of photodetectors to work in harsh environments. In this work we present an extensive study on quartz tuning forks (QTFs) used as photodetectors, exploiting the opto-thermo-elastic energy conversion arising from the laser radiation-QTF interaction. The role of the strain field, accumulation time and working pressure of the quartz resonator in this Light-Induced Thermo-Elastic Spectroscopy (LITES) approach was then evaluated for a whole set of tuning forks. Once identified the most performant resonator, this QTF was implemented in a TDLAS setup and it was combined with laser diodes, interband- and quantum-cascade laser sources emitting from 1 μm to 10.5 μm and targeting different gas spacies. The detection limits achieved for the QTF were comparable or even lower down to one order of magnitude with respect to market-available photodetectors.
We report on a comparison between the piezoelectric and interferometric readouts of vibrations in quartz tuning forks (QTFs) when employed as sound wave transducers in quartz-enhanced photoacoustic trace gas sensors. We demonstrate the possibility to properly design the QTF geometry to enhance interferometric readout signal with respect to the piezoelectric one and vice versa. When resonator tubes are acoustically coupled with the QTFs, signal-to-noise ratio enhancements are observed for both readout approaches. These results open the way to the implementation of optical readout of QTF vibrations in applications where external electromagnetic field could distort the piezoelectric signal.
Many applications such as toxic gas detection or H2S monitoring in natural gas require operation in the THz spectral region, where gas species show distinct spectral “fingerprints” that can be easily discriminated by the gas matrix background absorption features.
So far, continuous-wave THz quantum cascade lasers employed in quartz-enhanced photoacoustic (QEPAS) sensors required liquid helium-cooling systems. In this work, we demonstrated the first liquid nitrogen-cooled THz QEPAS sensor for H2S detection operated in pulsed mode and mounting a spectrophone based on a quartz tuning fork with 1.5 mm prong spacing. A sensitivity level in the part-per-billion concentration range was achieved.
Gas mixtures analysis is a challenging task because of the demand for sensitive and highly selective detection techniques. Partial least squares regression (PLSR) is a statistical method developed as generalization of standard multilinear regression (MLR), widely employed in multivariate analysis for relating two data matrices even with noisy and strongly correlated experimental data. In this work, PLSR is proposed as a novel approach for the analysis of gas mixtures spectra acquired with quartz-enhanced photoacoustic spectroscopy (QEPAS). Results obtained analyzing CO/N2O and CH4/C2H2/N2O gas mixtures are presented. A comparison with standard MLR approach highlights a prediction errors reduction up to 5 times.
We report on a quartz-enhanced photoacoustic (QEPAS) sensor employing a monolithic distributed-feedback quantum cascade laser array operating in a pulsed mode as a light source. The array consists of 32 quantum cascade lasers emitting in a spectral range from 1190 cm-1 to 1340 cm-1, which covers two absorption branches of nitrous oxide (N2O) and several absorption features of (CH4). The versatility of the QEPAS technique combined with the rapid wavelength tuning provided by the ultra-compact, low-power consuming laser source allowed the detection of N2O and CH4 with detection sensitivities below a part-per-million at atmospheric pressure.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.