Quartz-enhanced Photoacoustic Spectroscopy System Research Articles

Near-infrared quartz-enhanced photoacoustic spectroscopy system for ppb-level methane detection

In this paper we present a quartz-enhanced photoacoustic spectroscopy (QEPAS) of methane near 1651 nm. QEPAS is a high-sensitivity gas sensing method that relies on detecting acoustic waves generated by gas molecules. The sensor setup consists of a bismuth-doped fiber amplifier (BDFA) operating at 1651 nm that is used to enhance the amplitude of the QEPAS signal and increase the detection sensitivity. With the BDFA delivering ∼250 mW of optical power to the sample, the minimum detection limit of ∼11 ppb was achieved for the integration time of 150 s.

Transient tracer gas measurements: Development and evaluation of a fast-response SF6 measuring system based on quartz-enhanced photoacoustic spectroscopy.

This study aims to develop a fast-response sulfur hexafluoride (SF6 ) measuring system, and evaluate its performance in tracer gas measurements for studying transient airborne contaminant transport. The new system is based on a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor using a quantum cascade laser. Transient SF6 tracer gas measurements were carried out in an environmental chamber with an instantaneous source using both the QEPAS system and a traditional commercial instrument. Real-time SF6 concentrations, peak SF6 concentrations and average SF6 concentrations for one room time constant under two air change rates obtained by the two instruments were compared. The results show that the QEPAS system, which features a 0.4 s data acquisition interval, can provide detailed real-time SF6 concentrations even when the concentration is changing rapidly. The QEPAS system successfully captured the peak SF6 concentrations for all the studies cases, while commercial instrument failed in most studied cases. In most of the cases, the two instruments obtained similar average SF6 concentrations for one room time constant. However, when the concentration was in rapid change, the two systems would report significantly different results. The QEPAS system can be potentially applied in transient tracer gas measurements under complex scenarios.

Fiber-Coupled Quartz-Enhanced Photoacoustic Spectroscopy System for Methane and Ethane Monitoring in the Near-Infrared Spectral Range.

We report on a fiber-coupled, quartz-enhanced photoacoustic spectroscopy (QEPAS) near-IR sensor for sequential detection of methane (CH4 or C1) and ethane (C2H6 or C2) in air. With the aim of developing a lightweight, compact, low-power-consumption sensor suitable for unmanned aerial vehicles (UAVs)-empowered environmental monitoring, an all-fiber configuration was designed and realized. Two laser diodes emitting at 1653.7 nm and 1684 nm for CH4 and C2H6 detection, respectively, were fiber-combined and fiber-coupled to the collimator port of the acoustic detection module. No cross talk between methane and ethane QEPAS signal was observed, and the related peak signals were well resolved. The QEPAS sensor was calibrated using gas samples generated from certified concentrations of 1% CH4 in N2 and 1% C2H6 in N2. At a lock-in integration time of 100 ms, minimum detection limits of 0.76 ppm and 34 ppm for methane and ethane were achieved, respectively. The relaxation rate of CH4 in standard air has been investigated considering the effects of H2O, N2 and O2 molecules. No influence on the CH4 QEPAS signal is expected when the water vapor concentration level present in air varies in the range 0.6–3%.

Improving the Signal to Noise Ratio of QTF Preamplifiers Dedicated for QEPAS Applications

The signal-to-noise ratio (SNR) is a major factor that limits the detection sensitivity of quartz-enhanced photoacoustic spectroscopy (QEPAS) sensors. The higher the electrical signal level compared to the noise amplitude is the lower the concentration of gases that can be detected. For this reason the preamplifier circuits used in QEPAS should be optimized for low-frequency narrow-band applications. Moreover, special care should be taken when choosing a particular operational amplifier in either a transimpedance or voltage (differential) configuration. It turns out that depending on the preamp topology different operational amplifier parameters should be carefully considered when a high SNR of the whole QEPAS system is required. In this article we analyzed the influence of the crucial parameters of low-noise operational preamplifiers used in QEPAS applications and show the resulting limitations of transimpedance and voltage configurations.

Quartz-enhanced photoacoustic spectroscopy for hydrocarbon trace gas detection and petroleum exploration

Quartz-enhanced photoacoustic spectroscopy represents a valuable candidate for natural gas analysis in the oil & gas field. The core element of this spectroscopy technique is a quartz tuning forks, employed as high quality-factor optoacoustic transducers, capable of operating in a wide range of temperatures and pressures. The robustness and compactness of these sensors, together with the possibility to avoid the use of optical detectors, open the way to the development of a new generation of small-sized gas spectrometers to be potentially employed downhole for source rock characterization and estimation of oil & gas properties. We report here on the realization of a shoe-box sized quartz-enhanced photoacoustic spectroscopy system capable of: i) selective detection of methane and ethane in the parts-per-billion range, and propane in the parts-per-million range, by employing a single interband cascade laser emitting at 3.345 µm; ii) selective detection of 12CH4 and 13CH4 isotopes at the parts-per-billion sensitivity level by employing a quantum cascade laser operating around 7.73 µm.

Trace Gas Detection System Based on All-Optical Quartz-Enhanced Photoacoustic Spectroscopy.

An all-optical quartz-enhanced photoacoustic spectroscopy system (QEPAS) with quadrature point stabilization for trace gas detection was reported. The extrinsic interferometry-based optical fiber Fabry-Perot sensor with quadrature point self-stabilization for detection of quartz prong vibration was used to replace the conventional one. The optimal coefficient of the modulation depth was ∼2.2 theoretically and experimentally, corresponding to the modulation depth of ∼0.1795 cm-1 at an acetylene (C2H2) absorption line of 6534.36 cm-1. Furthermore, the enhancement of QEPAS signal was obtained by using different microresonators. The minimum detectable limit of ∼580 parts per billion by volume (ppbv) was obtained. A normalized noise equivalent absorption coefficient for C2H2 of 2.95 × 10-7 cm-1·W·Hz-1/2 was obtained. The detection sensitivity was enhanced by a factor of ∼2.1 in comparison to the conventional QEPAS system. The linear correlation coefficient of the QEPAS signal response to the C2H2 concentration was 0.998 within the range from 10 parts per million by volume (ppmv) to 500 ppmv. Finally, the long-term stability of the QEPAS system was evaluated using Allan deviation analysis, and the ultimate detection limit of ∼130 ppbv was reached for an optimum averaging time of ∼108 s at atmospheric pressure and ambient temperature.

Fiber evanescent wave quartz-enhanced photoacoustic spectroscopy

In a conventional system of quartz-enhanced photoacoustic spectroscopy (QEPAS), the size of block-like optical collimation focusing lens group is difficult to reduce, and the structural stability is poor, which makes it hard to adapt itself to some special conditions, such as narrow space and vibrating circumstance. Based on this situation, in this research the fiber evanescent wave technique is combined with QEPAS. Therefore, trace gas detection for acetylene (C2H2) based on an all-fiber structural QEPAS system is developed. To obtain the characteristics of fiber evanescent wave, the optical distribution of micro structural fiber is simulated and the evanescent wave power ratio is calculated based on the COMSOL Multiphysics software. In order to increase the QEPAS 2f signal amplitude, the optical path between fiber taper and quartz tuning fork (QTF) and the laser wavelength modulation depth are optimized. In addition, two kinds of QTFs with different resonant frequencies are optimized. Finally, a QTF with a lower resonant frequency of 30.720 kHz is adopted as the acoustic wave transducer, and a minimum detection limit (MDL) of 6.2510-4 (volume fraction) is obtained with a laser wavelength modulation depth of 0.24 cm-1. To investigate the evanescent wave power of micro structural fiber, the fiber taper diameter is measured by a scanning electron microscope. Subsequently, by combining the diameter of fiber taper with the theoretical calculation results, we determine an evanescent wave power of 455.9 W, and the normalization of noise equivalent absorption (NNEA) which indicates the sensor sensitivity is 4.1810-7 cm-1WHz-1/2.

Modeling and implementation of a fiber-based quartz-enhanced photoacoustic spectroscopy system

A compact fiber-based quartz-enhanced photoacoustic spectroscopy system requiring no extra optical lens has been developed for the detection of acetylene. The system is based on a common fiber or an axicon fiber to conduct the light. Experimental results show comparable detection sensitivity to that of tapered fiber-based photoacoustic spectroscopy systems. The system is easy to adjust and has low insertion loss and is thus well suited for small portable sensor applications in the future. A numerical model based on actual beam profiles was furthermore developed as part of the study to investigate the optimal positions of the fibers and to verify the experimental results.

Detection of nitric oxide in air with a 52 μm distributed-feedback quantum cascade laser using quartz-enhanced photoacoustic spectroscopy

We demonstrate the detection of nitric oxide in both air and nitrogen atmospheres with a 5.2 μm distributed-feedback quantum cascade laser in a quartz-enhanced photoacoustic spectroscopy system. The photoacoustic signal generated by nitric oxide is shown to be several times larger in air than in nitrogen due to the faster vibrational-translational energy relaxation process induced by the presence of oxygen. A sensitivity of 2.5 parts-per-million by volume is achieved in air at atmospheric pressure.