A theoretical-experimental framework for the analysis of the dynamic response of a QEPAS tuning fork device immersed in a fluid medium

Quartz-enhanced photoacoustic spectroscopy (QEPAS) is a trace gas sensing technique that employs a designed high-quality factor quartz tuning fork (QTF) as acousto-electric transducer. The first in-plane skew-symmetric flexural mode of the QTF is excited when weak resonant sound waves are generated between the QTF prongs. Thus, the performance of a QEPAS sensor strongly depends on the resonance properties of the QTF, namely the determination of flexural eigenfrequencies and air damping loss. In this work, we present a mixed theoretical-experimental framework to study the dynamic response of a QTF while vibrating in a fluid environment. Due to the system linearity, the dynamic response of the resonator immersed in a fluid medium is obtained by employing a Boundary Element formulation based on an ad hoc calculated Green's function. In particular, the QTF is modelled as constituted by a pair of two Euler-Bernoulli cantilevers partially coupled by a distributed linear spring. As for the forces exerted by the fluid on QTF structure, the fluid inertia and viscosity as well as an additional diffusivity term, whose influence is crucial for the correct evaluation of the system response, have been taken into account. By corroborating the theoretical analysis with the experimental outcomes obtained by means of a vibro-acoustic setup, the fluid response coefficients and the dynamics of the QTF immersed in a fluid environment are fully determined

Atmospheric CH4 and N2O measurements near Greater Houston area landfills using a QCL-based QEPAS sensor system during DISCOVER-AQ 2013

A quartz-enhanced photoacoustic absorption spectroscopy (QEPAS)-based gas sensor was developed for methane (CH4) and nitrous-oxide (N 2O) detection. The QEPAS-based sensor was installed in a mobile laboratory operated by Aerodyne Research, Inc. to perform atmospheric CH 4 and N2O detection around two urban waste-disposal sites located in the northeastern part of the Greater Houston area, during DISCOVER-AQ, a NASA Earth Venture during September 2013. A continuous wave, thermoelectrically cooled, 158 mW distributed feedback quantum cascade laser emitting at 7.83 μm was used as the excitation source in the QEPAS gas sensor system. Compared to typical ambient atmospheric mixing ratios of CH4 and N2O of 1.8 ppmv and 323 ppbv, respectively, significant increases in mixing ratios were observed when the mobile laboratory was circling two waste-disposal sites in Harris County and when waste disposal trucks were encountered. © 2014 Optical Society of America

Hydrogen peroxide detection with quartz-enhanced photoacoustic spectroscopy using a distributed-feedback quantum cascade laser

A quartz-enhanced photoacoustic spectroscopy sensor system was developed for the sensitive detection of hydrogen peroxide (H2O2) using its absorption transitions in the v6 fundamental band at ∼7.73 μm. The recent availability of distributed-feedback quantum cascade lasers provides convenient access to a strong H2O2 absorption line located at 1295.55 cm−1. Sensor calibration was performed by means of a water bubbler that generated titrated average H2O2vapor concentrations. A minimum detection limit of 12 parts per billion (ppb) corresponding to a normalized noise equivalent absorption coefficient of 4.6 × 10−9 cm−1W/Hz1/2 was achieved with an averaging time of 100 s

Quartz-enhanced photoacoustic spectroscopy exploiting low-frequency tuning forks as a tool to measure the vibrational relaxation rate in gas species

We demonstrated that quartz-enhanced photoacoustic spectroscopy (QEPAS) is an efficient tool to measure the vibrational relaxation rate of gas species, employing quartz tuning forks (QTFs) as sound detectors. Based on the dependence of the QTF resonance frequency on the resonator geometry, a wide range of acoustic frequencies with narrow detection bandwidth was probed. By measuring the QEPAS signal of the target analyte as well as the resonance properties of different QTFs as a function of the gas pressure, the relaxation time can be retrieved. This approach has been tested in the near infrared range by measuring the CH4 (nν4) vibrational relaxation rate in a mixture of 1% CH4, 0.15 % H2O in N2, and the H2O (ν1) relaxation rate in a mixture of 0.5 % H2O in N2. Relaxation times of 3.2 ms Torr and 0.25 ms Torr were estimated for CH4 and H2O, respectively, in excellent agreement with values reported in literature

H2S quartz-enhanced photoacoustic spectroscopy sensor employing a liquid-nitrogen-cooled THz quantum cascade laser operating in pulsed mode

In this work, we report on a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for hydrogen sulfide (H2S) detection, exploiting a liquid-nitrogen-cooled THz quantum cascade laser (QCL) operating in pulsed mode. The spectrophone was designed to accommodate a THz QCL beam and consisted of a custom quartz tuning fork with a large prong spacing, coupled with acoustic resonator tubes. The targeted rotational transition falls at 2.87 THz (95.626 cm−1), with a line-strength of 5.53 ∙ 10-20 cm/mol. A THz QCL peak power of 150 mW was measured at a heat sink temperature of 81 K, pulse width of 1 μs and repetition rate of 15.8 kHz. A QEPAS record sensitivity for H2S detection in the THz range of 360 part-per-billion in volume was achieved at a gas pressure of 60 Torr and 10 s integration time

Broadband detection of methane and nitrous oxide using a distributed-feedback quantum cascade laser array and quartz-enhanced photoacoustic sensing

Here we report on the broadband detection of nitrous oxide (N2O) and methane (CH4) mixtures in dry nitrogen by using a quartz-enhanced photoacoustic (QEPAS) sensor exploiting an array of 32 distributed-feedback quantum cascade lasers, within a spectral emission range of 1190−1340 cm−1 as the excitation source. Methane detection down to a minimum detection limit of 200 ppb at 10 s lock-in integration time was achieved. The sensor demonstrated a linear response in the range of 200−1000 ppm. Three different mixtures of N2O and CH4 in nitrogen at atmospheric pressure have been analyzed. The capability of the developed QEPAS sensor to selectively determine the N2O and CH4 concentrations was demonstrated, in spite of significant overlap in their respective absorption spectra in the investigated spectral range

Mode matching of a laser-beam to a compact high finesse bow-tie optical cavity for quartz enhanced photoacoustic gas sensing

We report on the optical characterization of a compact bow-tie cavity composed of two flat mirrors and two concave mirrors, all having a reflectance > 99.99% in the spectral range between 4.8 μm and 5.3 μm, mounted in a stainless-steel enclosure. The cavity was designed for the implementation of an intracavity-quartz enhanced photoacoustic sensor system. The propagation parameters of the intra-cavity beam were determined using the ABCD-matrix method, allowing the analytical formulation of the size of two beam waists occurring inside the cavity. A collimated mid-infrared laser beam was optically coupled and mode matched into the bow-tie cavity via a focusing lens. A cavity finesse of ∼ 2000 was measured at a pressure of 90 Torr inside the cavity, corresponding to an optical power enhancement factor of ∼ 320

Recent advances in quartz enhanced photoacoustic sensing

This review aims to discuss the latest advancements in quartz-enhanced photoacoustic spectroscopy (QEPAS) based trace-gas sensing. Starting from the QEPAS basic physical principles, the most used QEPAS configurations will be described. This is followed by a detailed theoretical analysis and experimental study regarding the influence of quartz tuning forks (QTFs) geometry on their optoacoustic transducer performance. Furthermore, an overview of the latest developments in QEPAS trace-gas sensor technology employing custom QTFs will be reported. Results obtained by exploiting novel micro-resonator configurations, capable of increasing the QEPAS signal-to-noise ratio by more than two orders of magnitude and the utilization of QTF overtone flexural modes for QEPAS based sensing will be presented. A comparison of the QEPAS performance of different spectrophone configurations is reported based upon signal-to-noise ratio. Finally, a novel QEPAS approach allowing simultaneous dual-gas detection will be described

Light-induced thermo-elastic effect in quartz tuning forks exploited as a photodetector in gas absorption spectroscopy

We report on a study of light-induced thermo-elastic effects occurring in quartz tuning forks (QTFs) when exploited as near-infrared light detectors in a tunable diode laser absorption spectroscopy sensor setup. Our analysis showed that when the residual laser beam transmitted by the absorption cell is focused on the QTF surface area where the maximum strain field occurs, the QTF signal-to-noise ratio (SNR) is proportional to the strain itself and to the QTF accumulation time. The SNR was also evaluated when the pressure surrounding the QTF was lowered from 700 Torr to 5 Torr, resulting in an enhancement factor of v4 at the lowest pressure. At 5 torr, the QTF employed as light detector showed an SNR v6.5 times higher than that obtained by using a commercially available amplified photodetector