Quartz-enhanced Photoacoustic Spectroscopy Research Articles

Non-contact quartz-enhanced photoacoustic spectroscopy

Non-contact quartz-enhanced photoacoustic spectroscopy (NC-QEPAS) was proposed and developed for trace gas analysis. The NC-QEPAS aims at solving the problem that the quartz tuning fork (QTF) must be immersed in the gases for photoacoustic wave transducing, which limits its application for corrosive and dusty gas sensing. In this work, the QTF was isolated from the gas, realizing “non-contact” detection. An elastic parylene film was synthesized and then patched to the slit of a QEPAS gas cell. With an optimized coupling effect, the parylene film shows a resonance enhancement with the QTF and acoustic micro-resonator, realizing non-contact photoacoustic detection of gas. The NC-QEPAS not only increases the photoacoustic signal amplitude but also decreases the background noise. Compared to traditional contact QEPAS with QTF immersed in the gas, the NC-QEPAS shows a signal-to-noise enhancement factor of 13. A normalized noise equivalent absorption coefficient of 8.8 × 10−9 cm−1 W Hz−1/2 was achieved. Allan deviation shows good long-term stability of the NC-QEPAS sensor. With an integration time of 1000 s, the developed QEPAS sensor shows a detection limit of 0.4 ppm. The detection limit can be further improved with longer integration time.

Signal-to-Noise Ratio Analysis for the Voltage-Mode Read-Out of Quartz Tuning Forks in QEPAS Applications

Quartz tuning forks (QTFs) are employed as sensitive elements for gas sensing applications implementing quartz-enhanced photoacoustic spectroscopy. Therefore, proper design of the QTF read-out electronics is required to optimize the signal-to-noise ratio (SNR), and in turn, the minimum detection limit of the gas concentration. In this work, we present a theoretical study of the SNR trend in a voltage-mode read-out of QTFs, mainly focusing on the effects of (i) the noise contributions of both the QTF-equivalent resistor and the input bias resistor RL of the preamplifier, (ii) the operating frequency, and (iii) the bandwidth (BW) of the lock-in amplifier low-pass filter. A MATLAB model for the main noise contributions was retrieved and then validated by means of SPICE simulations. When the bandwidth of the lock-in filter is sufficiently narrow (BW = 0.5 Hz), the SNR values do not strongly depend on both the operating frequency and RL values. On the other hand, when a wider low-pass filter bandwidth is employed (BW = 5 Hz), a sharp SNR peak close to the QTF parallel-resonant frequency is found for large values of RL (RL > 2 MΩ), whereas for small values of RL (RL < 2 MΩ), the SNR exhibits a peak around the QTF series-resonant frequency.

Quartz enhanced photoacoustic spectroscopy gas sensing exploiting beat frequency approach

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 as consequence of infrared modulated light absorption. The generated electric signal depends on the gas concentration. An accurate and reliable QEPAS measurement requires the QTF characterization, in terms of resonance frequency and quality factor. Furthermore, tens of seconds are required to complete the tuning range scan of the laser employed to detect the selected gas. 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 166 ppb of NO, and a highly accurate measurement of the QTF resonance frequency and quality factor were demonstrated using BF-QEPAS.

Quartz tuning fork based photoacoustic spectroscopy and sensing

Quartz crystal tuning forks (QTFs) resonators are key components for timing and frequency measurements, due to their high stability, high-quality factors and low power consumptions. Thanks to their piezoelectric properties, QTFs are employed as sensitive element in many fields and systems such as atomic force microscopy, near-field and microwave microscopy, and mass/viscosity sensor. Since 2002, QTFs are also widely used as a sharply resonant acoustic transducer to detect weak photoacoustic excitation for Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS). Among most sensitive optical techniques, QEPAS was demonstrated as the leading-edge technology for addressing these application requirements, providing also modularity, ruggedness, portability and allowing the use of extremely small volumes. QEPAS technique does not require an optical detector, it is wavelength independent, it is immune to environmental noise and can operate in a wide range of temperature and pressure. These factors, together with its proven reliability and ruggedness, represent the main distinct advantages with respect to other laser-based techniques for environmental monitoring and in situ detection. Starting from the basic physical principles governing the QTF physics, I will review the main results achieved by exploiting custom QTFs for QEPAS sensing and as photodetector in LITES setup, with a main focus on real-world applications.

Simultaneous Detection of Gas Concentration and Light Intensity Based on Dual-Quartz-Enhanced Photoacoustic-Photothermal Spectroscopy

This research proposes a method for the simultaneous acquisition of the second harmonic (2f) signal of quartz-enhanced photoacoustic spectroscopy (QEPAS) and the first harmonic (1f) signal of quartz-enhanced photothermal spectroscopy (QEPTS) based on the dual-quartz-enhanced photoacoustic–photothermal spectroscopy. The laser beam is first wavelength-modulated by the injection current and then intensity-modulated by an acoustic-optic modulator. The frequency of the wavelength modulation is half of the QTF1 resonant frequency, and the frequency of the intensity modulation is equal to the QTF2 resonant frequency. A modulated laser beam traveled through the two arms of the QTF1 and converged on the root of the QTF2. The 2f photoacoustic and 1f photothermal signals are concurrently obtained using the frequency division multiplexing technology and lock-in amplifiers, which allows the simultaneous detection of the gas concentration and laser light intensity. CH4 is chosen as the target gas, and the variations of the 2f photoacoustic and 1f photothermal signals are evaluated at various gas concentrations and light intensities. According to the experiments, the amplitude of the 1f photothermal signal has a good linear connection with light intensity (R2 = 0.998), which can be utilized to accurately revise the 2f photoacoustic signal while light intensity fluctuates. Over a wide range of concentrations, the normalized 2f photoacoustic signals exhibit an excellent linear response (R2 = 0.996). According to the Allan deviation analysis, the minimum detection limit for CH4 is 0.39 ppm when the integration time is 430 s. Compared with the light intensity correction using a photodetector for the QEPAS system, this approach offers a novel and effective light intensity correction method for concentration measurements employing 2f analysis. It also has the advantages of low cost and compact volume, especially for mid-infrared and terahertz systems.

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.

On the measurement of local vapor concentration around sessile water droplet with high spatiotemporal resolution

Droplet evaporation is a seemingly simple heat and mass transfer process, while of which high-fidelity characterization of the vapor field remains a long-standing challenge. In this brief communication, we report the first experimental observation of the local vapor concentration of the sessile water droplet with unprecedently high spatiotemporal resolution. The experimental measurement was enabled by a tailored quartz-enhanced photoacoustic spectroscopy (QPEAS) system. In this system, a near-infrared, continuous-wave, distributed feedback laser at 1392 nm was utilized to excite the photoacoustic signal of water vapor above the sessile droplet. The photoacoustic signal between the prongs of a tiny quartz tuning fork (QTF) was detected and further interpreted as the vapor concentration information. The feasibility of QEPAS for droplet evaporation characterization was carefully verified. As a demonstration, we measured the time histories of water vapor concentration above the evaporating droplet in both open and closed chambers at the spatial resolution of ∼30 micrometers and temporal resolution of 0.1 ms. Interestingly, we found the vapor concentration above the evaporating droplet in open ambient featured distinct fluctuations indiscernible to the conventional photography method. Such fluctuation has never been reported before and further corroborated the insufficiency of quasi-steady approximation for droplet evaporation. This work not only extends the application of QEPAS to more complicated physical processes, but also advances the state of the art in the experimental characterization of sessile water droplet evaporation, which opens up a new possibility to unravel the intricate mechanism of the vapor-mediated droplet motions.

Quartz-Enhanced Photoacoustic Spectroscopy for One-Health

We report on the development of Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) technology to detect 8 different air pollutants, namely CH4, NO2, CO2, N2O, CO, NO, SO2 and NH3, with the same acoustic detection module and interchangeable laser sources, to prove the modularity of the technique as well as the adaptability to different lasers. For each gas species, the fine structure of the infrared absorption bands has been simulated by using HITRAN database. Each gas species was detected with an ultimate detection limit well below their typical natural abundance in air even with a signal integration time as low as 0.1 s.

Compact GC-QEPAS for On-Site Analysis of Chemical Threats.

This paper reports on a compact, portable, and selective chemical sensor for hazardous vapors at trace levels, which is under development and validation within the EU project H2020 "RISEN". Starting from the prototype developed for a previous EU project, here, we implemented an updated two-stage purge and trap vapor pre-concentration system, a more compact MEMS- based fast gas-chromatographic separation module (Compact-GC), a new miniaturized quartz-enhanced photoacoustic spectroscopy (QEPAS) detector, and a new compact laser source. The system provides two-dimensional selectivity combining GC retention time and QEPAS spectral information and was specifically designed to be rugged, portable, suitable for on-site analysis of a crime scene, with accurate response in few minutes and in the presence of strong chemical background. The main upgrades of the sensor components and functional modules will be presented in detail, and test results with VOCs, simulants of hazardous chemical agents, and drug precursors will be reported and discussed.

Compact QEPAS CO2 sensor system using a quartz tuning fork-embedded and in-plane configuration

A quartz-enhanced photoacoustic spectroscopy (QEPAS) system was developed based on a novel quartz tuning fork-embedded, and in-plane QEPAS technique, while increasing the efficiency of assembly alignment with the aid of 3D printing technology. In this paper, it was verified that the simplified numerical model enabled rapid optimization of the dimensions of the acoustic micro-resonator (AmR), and this was validated by experimental results. The optimized results are as follows: outer diameter = 3.2 mm, inner diameter = 0.5 mm, length = 6.77 mm. The acoustic transducer used a commercial quartz tuning fork with a resonance frequency of 30.69 kHz. After optimizing the laser wavelength modulation amplitude and the dimensions of the AmR, a minimum CO2 detection limit of 1.06 ppm was obtained, corresponding to a normalized noise equivalent absorption of 5.5 × 10−9 cm−1 WHz−1/2.

Simultaneous Monitoring of Atmospheric CH4, N2O, and H2O Using a Single Gas Sensor Based on Mid-IR Quartz-Enhanced Photoacoustic Spectroscopy.

An optical sensor based on external-cavity quantum cascade laser (EC-QCL) was developed for simultaneous triple-species monitoring of CH4, N2O, and H2O vapor using off-beam quartz-enhanced photoacoustic spectroscopy (OB-QEPAS). The EC-QCL wavelength was scanned over three neighboring absorption lines of CH4 (1260.81 cm-1), N2O (1261.06 cm-1), and H2O vapor (1261.58 cm-1) by tuning the grating of the EC-QCL with a piezoelectric actuator. Molecular relaxation effects impacting the generation of the QEPAS signals resulting from light absorption by CH4 and N2O molecules were investigated in the mid-infrared region near 8 μm. A theoretical model was introduced for the mid-infrared region, including the beneficial influence of water vapor. An enhancement of the QEPAS signals by a factor of 3 for CH4 in air and of 20% for N2O in air was observed in humidified samples compared to that in dry samples. The QEPAS measurement was scaled by the calibrated reference spectrometers; detection limits of 98 ppbv for CH4, 12 ppbv for N2O, and 750 ppmv for H2O vapor were obtained with a 1σ signal-to-noise ratio (SNR = 1) in humidified gas mixtures. Real-time Kalman filtering was applied to improve the measurement precision by a factor of approximately 4 while keeping the same temporal resolution, leading to measurement precisions of 60 ppbv for CH4, 10 ppbv for N2O, and 0.07% for H2O in the measurements of 1.99 ppmv CH4 and 312 ppbv N2O humidified with 2.8% H2O vapor, with a 1 s lock-in amplifier time constant and an equivalent bandwidth of 0.1 Hz.

Measurement of methane, nitrous oxide and ammonia in atmosphere with a compact quartz-enhanced photoacoustic sensor

We demonstrated a 19-inches 6U-rack sensing system based on quartz-enhanced photoacoustic spectroscopy for simultaneous detection of methane, nitrous oxide, and ammonia in atmosphere. The system embeds two detection modules, each one equipped with a quantum cascade laser as light source and a spectrophone composed of a T-shaped quartz tuning fork coupled with acoustic resonator tubes. The sensing system results compact, lightweight, and portable, with a computer interface for an easy end-user operation. The calibration was performed in laboratory environment with certified concentrations. Because of the NH3 large and permanent dipole moment, ammonia QEPAS signal was used to model and characterize adsorption/desorption processes, for an accurate and reliable sensor calibration. At 0.1 s signal integration time, detection limits of 28 ppb, 9 ppb, and 6 ppb were obtained for CH4, N2O, and NH3, respectively. Since these detection limits were well below the natural abundance of the three analytes in atmosphere, the sensor was employed to monitor CH4, N2O, and NH3 concentrations in laboratory air.

Light intensity correction for quartz-enhanced photoacoustic spectroscopy using photothermal baseline

A convenient method of light intensity correction for quartz-enhanced photoacoustic spectroscopy (QEPAS) using photothermal baseline is demonstrated. The laser beam passes through the prongs of the quartz tuning fork (QTF) and then focused on the root of the prongs. First harmonic (1f) analysis is utilized to process the simultaneously induced photoacoustic and photothermal signals. The optical path length for photothermal spectroscopy is minimized to millimeter level, yielding negligible gas absorption. The demodulated 1f signal can be regarded as the superposition of the photoacoustic signal and the non-absorption photothermal baseline. A good linear relationship (R2 = 0.999) is observed between amplitude of photothermal baseline and light intensity. QEPAS signal normalized by photothermal baseline shows a good immunity to light intensity variation. An excellent linear response between normalized QEPAS signal and gas concentration is achieved. According to the Allan deviation analysis, the minimum detection limit for CH4 is 0.31 ppm at an integration time of 1,200 s. With this strategy, the precise gas concentration and accurate light intensity of a QEPAS system can be simultaneously obtained with only a single QTF. Compared with the light intensity correction using a photodetector or a power meter, this method entails a low cost and small footprint. It is promising to mitigate the influence from light intensity drift in long-term field measurement of QEPAS systems.

Additively Manufactured Detection Module with Integrated Tuning Fork for Enhanced Photo-Acoustic Spectroscopy.

Starting from Quartz-Enhanced Photo-Acoustic Spectroscopy (QEPAS), we have explored the potential of a tightly linked method of gas/vapor sensing, from now on referred to as Tuning-Fork-Enhanced Photo-Acoustic Spectroscopy (TFEPAS). TFEPAS utilizes a non-piezoelectric metal or dielectric tuning fork to transduce the photoacoustic excitation and an optical interferometric readout to measure the amplitude of the tuning fork vibration. In particular, we have devised a solution based on Additive Manufacturing (AM) for the Absorption Detection Module (ADM). The novelty of our solution is that the ADM is entirely built monolithically by Micro-Metal Laser Sintering (MMLS) or other AM techniques to achieve easier and more cost-effective customization, extreme miniaturization of internal volumes, automatic alignment of the tuning fork with the acoustic micro-resonators, and operation at high temperature. This paper reports on preliminary experimental results achieved with ammonia at parts-per-million concentration in nitrogen to demonstrate the feasibility of the proposed solution. Prospectively, the proposed TFEPAS solution appears particularly suited for hyphenation to micro-Gas Chromatography and for the analysis of complex solid and liquid traces samples, including compounds with low volatility such as illicit drugs, explosives, and persistent chemical warfare agents.

Infrared dual-gas CH4/C2H2 sensor system based on dual-channel off-beam quartz-enhanced photoacoustic spectroscopy and time-division multiplexing technique

Highly sensitive and stable measurement of methane (CH4) and acetylene (C2H2) based on a novel dual-channel off-beam quartz-enhanced photoacoustic spectroscopy and time-division multiplexing technique was realized by a compact 3D-printed gas cell with a size of 3 × 2 × 1 cm3. Two near-infrared distributed feedback diode lasers were employed to target the CH4 absorption line at 6046.9 cm−1 and the C2H2 absorption line at 6521.2 cm−1, respectively. Second-harmonic wavelength modulation spectroscopy method was used for photoacoustic signal recovery. A minimum detection level of ∼ 7.63 parts-per-million in volume (ppmv) for CH4 and a level of ∼ 17.47 ppmv for C2H2 were achieved with a 1 s lock-in integration time, leading to a normalized noise equivalent absorption (NNEA) coefficient of 7.24 × 10-8 cm−1·W·Hz−1 and 3.73 × 10-8 cm−1·W·Hz−1 for CH4 and C2H2, respectively. Allan-Werle deviation analysis was employed to evaluate the stability and the minimum detection limit (MDL) of the developed photoacoustic CH4/C2H2 dual-gas photoacoustic sensor. Owing to the high stability of the developed sensor system, an MDL of ∼ 0.73 ppmv and an MDL of ∼ 1.60 ppmv with a 100 s averaging time were achieved for CH4 and C2H2, respectively.

Theoretical analysis and experimental optimization of an elliptical acoustic resonator in quartz-enhanced photoacoustic spectroscopy

We report an elliptical resonator suitable for quartz-enhanced photoacoustic spectroscopy. The theoretical model of the elliptical resonator is first constructed and demonstrates the existence of the eigenmodes in the elliptical resonator. Then the sound field modal distributions for the different eigenmodes in the elliptical resonator are simulated by means of the finite element method. The geometric parameters of the elliptical resonator are experimentally optimized. And the performance of the elliptical resonator is assessed using a high-power multimode laser diode for the NO2 detection. Due to the high matching efficiency of the spatial profile between the elliptical resonator and the laser beam, the elliptical resonator can achieve a gain factor of 19.2 higher than 5.6 from the circular resonator.

Dual-comb quartz-enhanced photoacoustic spectroscopy

Photoacoustic spectroscopy (PAS) using two optical combs is a new-born technique, offering appealing features, including broad optical bandwidths, high resolutions, fast acquisition speeds, and wavelength-independent photoacoustic detection, for chemical sensing. However, its further application to, e.g., trace detection, is jeopardized due to the fundamentally and technically limited sensitivity and specificity. Here, we take a different route to comb-enabled PAS with acoustically enhanced sensitivity and nonlinear spectral hole-burning defined resolution. We demonstrate dual-comb quartz-enhanced PAS with two near-infrared electro-optic combs and a quartz tuning fork. Comb-line-resolved multiplexed spectra are acquired for acetylene with a single-pass detection limit at the parts-per-billion level. The technique is further extended to the mid-infrared (for methane), enabling improved sensitivity. More importantly, we measure nonlinear dual-comb photoacoustic spectra for the 12C2H2 ν1 + ν3 band P(17) transition with sub-Doppler pressure-broadening dominated homogeneous linewidths (e.g., 45.8 MHz), hence opening up new opportunities for Doppler-free photoacoustic gas sensing.

Multi-gas quartz-enhanced photoacoustic sensor for environmental monitoring exploiting a Vernier effect-based quantum cascade laser

We report on a gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) able to detect multiple gas species for environmental monitoring applications, by exploiting a Vernier effect-based quantum cascade laser as the excitation source. The device emission spectrum consists of ten separated emission clusters covering the range from 2100 up to 2250 cm−1. Four clusters were selected to detect the absorption features of carbon monoxide (CO), nitrous oxide (N2O), carbon dioxide (CO2), and water vapor (H2O), respectively. The sensor was calibrated with certified concentrations of CO, N2O and CO2 in a wet nitrogen matrix. The H2O absorption feature was used to monitor the water vapor within the gas line during the calibration. Minimum detection limits of 6 ppb, 7 ppb, and 70 ppm were achieved for CO, N2O and CO2, respectively, at 100 ms of integration time. As proof of concept, the QEPAS sensor was tested by continuously sampling indoor laboratory air and monitoring the analytes concentrations.

On-site GC-QEPAS analysis on the scene of crime: The RISEN project

The capability to provide timely forensic analytical information is of paramount importance in criminal investigations involving chemical substances of toxicological interest. Miniaturized Gas Chromatography (GC) systems are suitable sensors to be deployed on the scene of crime, allowing to analyse specimens there, providing timely information to protect security and safety of specialists on-site and saving the time needed to collect, transport and analyse them in a forensic laboratory. An innovative hand-portable point sensor has been recently developed using a fully Micro-Electro-Mechanical-System (MEMS)-based compact-GC platform of the size of a few cm3 coupled to a miniaturized detector based on InfraRed Quartz Enhanced Photo Acoustic Spectroscopy (QEPAS). Sensing is based on the piezoelectric transduction of the photo-acoustic signal that is generated when laser radiation is absorbed by molecules in vapour phase in the QEPAS cell. Signal intensity is proportional to laser intensity, to the absorption cross section of the substance, and to its concentration. QEPAS spectra are very similar to the spectra obtained by classical IR absorption spectroscopy (IRAS), but, unlike IRAS cells, QEPAS cells can be designed as very cheap components of miniature size. The GC-QEPAS prototype is built upon a patented compact-GC architecture integrating three silicon micro-machined chips for sample pre-concentration, injection and GC separation and a QEPAS sensor based on a new ultra-wide tunability Quantum Cascade Laser (QCL) source, designed and assembled into a package matching the needs of hand-portability by the operator. The system was equipped with a compact air sampler, which also contains a first stage of pre-concentration, to allow vapour analysis on-site. Preliminary tests were carried out in a 60 litres glass box, where vaporizing solutions (between 1 and 100 microlitres) using a syringe of target substances. Sampling time was less than 60 seconds. The GC-QEPAS sensor was used to successfully analyse ppm and sub-ppm amounts of two nerve agent simulants: dimethyl methyl phosphonate (DMMP) and dipropylene glycol methyl ether (DPGME). Methyl salicylate (a blister agent simulant) and sulfur mustard (a real blister agent sample) were also tested. Other substances of interest in forensic toxicology where tested, including precursors of drugs of abuse. The sensor showed also its capability of analyzing amphetamine types stimulants (ATS) and drug precursors both as pure compounds and as mixtures. LoDs measured were in the order of about 10 ng for many precursors, and ten times larger for target compounds containing an amino group. Ethanol, propanol, gasoline and paint vapours at much higher concentrations, up to saturation vapor pressure, compared to the target chemical substances of forensic interest were also analysed to study the selectivity of the system in a real scenario. The miniature size of the column has allowed the separation of substances in cycle times of 2–3 minutes, to be compared with chemical substances eluting after retention times in the order of tens of minutes with traditional GC systems commonly used in forensic laboratories for casework. The total absorption chromatograms and the photoacoustic absorption spectra related to chromatographic peaks of both pure compounds and mixture will be shown, to demonstrate how the approach developed in the RISEN project will be able to support an improved approach to crime scene activities in the near future.

Position effect of laser beam waist in quartz-enhanced photoacoustic spectroscopy

The position effect of laser beam waist with respect to quartz tuning fork (QTF) on the signal of quartz enhanced photoacoustic spectroscopy (QEPAS) was investigated. A laser beam was collimated by a fiber-coupled Grin collimator (FGC) which was assembled to a QEPAS system. The experimental results showed that the maximum signal amplitude occurs at different distances from the FGCs to the QTF. The relationship between the position of the beam waist and the signal amplitude was analyzed. The influence of the beam waist position in the in-plane QEPAS and the on-beam QEPAS is discussed in detail.