Parts-per-billion detection of carbon monoxide: A comparison between quartz-enhanced photoacoustic and photothermal spectroscopy

Photoacoustic methane detection inside a MEMS microphone

An innovative laser based photoacoustic (PA) gas sensing concept with intrinsic miniaturization potential was developed and investigated for methane trace gas detection. An interband cascade laser (ICL) with an optical power of 8.5 mW targets a methane (CH₄) absorption line feature around 3057.7 cm^-1 (or 3270 nm). The ICL was focused into the sound port of a MEMS microphone, where the PA signal was generated and detected using a wavelength modulation concept (2f-WMS-PAS). The MEMS microphone was successfully implemented as an intrinsically miniaturized PA cell being gas sensing volume, acoustic resonator and sound transducer at once. Frequencies between 2 kHz and 100 kHz were investigated and used for methane detection. A sensitive and resonant methane detection at 41.8 kHz was investigated by concentration variations between 0 and 10 ppm CH₄ in N₂. A limit of detection ( 3 σ -LOD) of 329 ppb was estimated. The long term stability of this sensor was investigated by the measurement of methane in ambient air. A noise equivalent concentration (NEC) of 14 ppb (parts per billion) at an average time of 10 s was estimated. This value corresponds to a normalized noise equivalent absorption (NNEA) of 2 ⋅ 1 0 - 8 W cm^-1 Hz^-1/2. Using the MEMS microphone directly as PA cell offers the possibility for an extremely miniaturized, highly sensitive and very cost-efficient photoacoustic trace gas sensor.

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.

Selective Mid-IR Metamaterial-Based Gas Sensor System: Proof of Concept and Performances Tests

In this paper, we propose a highly selective and efficient gas detection system based on a narrow-band IR metasurface emitter integrated with a resistive heater. In order to develop the sensor for the detection of specific gases, both the microheater and metasurface structures have been optimized in terms of geometry and materials. Devices with different metamaterial structures and geometries for the heater have been tested. Our prototype showed that the modification of the spectral response of metasurface-based structures is easily achieved by adapting the geometrical parameters of the plasmonic micro-/nanostructures in the metasurface. The advantage of this system is the on-chip integration of a thermal source with broad IR radiation with the metasurface structure, obtaining a compact selective radiation source. From the experimental data, narrow emission peaks (FWHM as low as 0.15 μm), corresponding to the CO₂, CH₄, and CO absorption bands, with a radiant power of a few mW were obtained. It has been shown that, by changing the bias voltage, a shift of a few tens of nm around the central emission wavelength can be obtained, allowing fine optimization for gas detection applications.

Sensitivity enhanced NIR photoacoustic CO detection with SF6 promoting vibrational to translational relaxation process

A challenge for slowly relaxing carbon monoxide (CO) molecules detection using photoacoustic spectroscopy (PAS) is to promote the vibration-translation (V-T) relaxation process. Addressing this challenge, a sensitivity enhanced photoacoustic CO sensor with sulfur hexafluoride (SF₆) as the promotor is investigated and demonstrated. A 1568 nm near-infrared (NIR) laser diode and a customized optical amplifier are used as the excitation source to generate the photoacoustic signal. A differential photoacoustic cell is simulated and designed to obtain identical laminar flow distribution in the resonant cell to suppress the flow noise. The modulation frequency and added SF₆ volume ratio are optimized experimentally to achieve optimal sensitivity. Feasibility and performance of the CO sensor with a small amount of SF₆ as promotor is discussed and evaluated, obtaining a ~ 2 times improvement of signal value compared to the one with pure N₂ background and resulting in a minimum detection limit of 467.5 ppb for CO detection.

Compact and portable quartz-enhanced photoacoustic spectroscopy sensor for carbon monoxide environmental monitoring in urban areas

We report on the realization, calibration, and test outdoor of a 19-inches rack 3-units sized Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) trace gas sensor designed for real-time carbon monoxide monitoring in ambient air. Since CO acts as a slow energy relaxer when excited in the mid-infrared spectral region, its QEPAS signal is affected by the presence of relaxation promoters, such as water vapor, or quenchers like molecular oxygen. We analyzed in detail all the CO relaxation processes with typical collisional partners in an ambient air matrix and used this information to evaluate oxygen and humidity-related effects, allowing the real CO concentration to be retrieved. The sensor was tested outdoor in a trafficked urban area for several hours providing results comparable with the daily averages reported by the local air inspection agency, with spikes in CO concentration correlated to the passages of heavy-duty vehicles.

Integrated near-infrared QEPAS sensor based on a 28 kHz quartz tuning fork for online monitoring of CO2 in the greenhouse

In this paper, a highly sensitive and integrated near-infrared CO₂ sensor was developed based on quartz-enhanced photoacoustic spectroscopy (QEPAS). Unlike traditional QEPAS, a novel pilot line manufactured quartz tuning fork (QTF) with a resonance frequency f ₀ of 28 kHz was employed as an acoustic wave transducer. A near-infrared DFB laser diode emitting at 2004 nm was employed as the excitation light source for CO₂ detection. An integrated near-infrared QEPAS module was designed and manufactured. The QTF, acoustic micro resonator (AmR), gas cell, and laser fiber are integrated, resulting in a super compact acoustic detection module (ADM). Compared to a traditional 32 kHz QTF, the QEPAS signal amplitude increased by > 2 times by the integrated QEPAS module based on a 28 kHz QTF. At atmospheric pressure, a 5.4 ppm detection limit at a CO₂ absorption line of 4991.25 cm^-1 was achieved with an integration time of 1 s. The long-term performance and stability of the CO₂ sensor system were investigated using Allan variance analysis. Finally, the minimum detection limit (MDL) was improved to 0.7 ppm when the integration time was 125 s. A portable CO₂ sensor system based on QEPAS was developed for 24 h continuous monitoring of CO₂ in the greenhouse located in Guangzhou city. The CO₂ concentration variations were clearly observed during day and night. Photosynthesis and respiration plants can be further researched by the portable CO₂ sensor system.

Quartz crystal tuning fork based 2f/1f wavelength modulation spectroscopy

A compact gas sensing system based on the quartz crystal tuning fork (QCTF) and 2f/1f wavelength modulation spectroscopy (2f/1f-WMS) technique was reported for the first time. An ingenious laser modulation strategy and frequency division multiplexing demodulation algorithm were developed for realizing a single QCTF to detect the first harmonic and second harmonic signals simultaneously. The influence of laser power change and excitation position the QCTF based 2f/1f-WMS technique was first investigated in detail. The results show that, compared with the traditional QCTF-2f method, the reported QCTF based 2f/1f technique has better immunity. To further evaluate this sensing technique, real-time monitoring of ambient water vapor (H₂O) was made, the results show that the developed QCTF based 2f/1f-WMS technique can effectively improve the long-term stability and has super anti-interference ability to various environmental disturbance factors, such as light beam jitter, airflow fluctuation, and mechanical vibration, which proves it has great potential in practical field applications, especially for harsh environment.

On the Limit of Detection in Infrared Spectroscopic Imaging

Infrared (IR) spectroscopic imaging instruments' performance can be characterized and optimized by an analysis of their limit of detection (LOD). Here we report a systematic analysis of the LOD for Fourier transform IR (FT-IR) and discrete frequency IR (DFIR) imaging spectrometers. In addition to traditional measurements of sample and blank data, we propose a decision theory perspective to pose the determination of LOD as a binary classification problem under different assumptions of noise uniformity and correlation. We also examine three spectral analysis approaches, namely, absorbance at a single frequency, average of absorbance over selected frequencies and total spectral distance - to suit instruments that acquire discrete or contiguous spectral bandwidths. The analysis is validated by refining the fabrication of a bovine serum albumin protein microarray to provide eight uniform spots from ∼2.8 nL of solution for each concentration over a wide range (0.05-10 mg/mL). Using scanning parameters that are typical for each instrument, we estimate a LOD of 0.16 mg/mL and 0.12 mg/mL for widefield and line scanning FT-IR imaging systems, respectively, using the spectral distance approach, and 0.22 mg/mL and 0.15 mg/mL using an optimal set of discrete frequencies. As expected, averaging and the use of post-processing techniques such as minimum noise fraction transformation results in LODs as low as ∼0.075 mg/mL that correspond to a spotted protein mass of ∼112 fg/pixel. We emphasize that these measurements were conducted at typical imaging parameters for each instrument and can be improved using the usual trading rules of IR spectroscopy. This systematic analysis and methodology for determining the LOD can allow for quantitative measures of confidence in imaging an analyte's concentration and a basis for further improving IR imaging technology.

All-optical dynamic analysis of the photothermal and photoacoustic response of a microcantilever by laser Doppler vibrometry

Light absorption induced thermoelastic and photoacoustic excitation, combined with laser Doppler vibrometry, was utilized to analyze the dynamic mechanical behavior of a microcantilever. The measured frequency response, modal shapes, and acoustic coupling effects were interpreted in the framework of a simple Bernouilli-Euler model and quantitative 3D finite element method (FEM) analysis. Three opto-mechanical generation mechanisms, each initiated by modulated optical absorption and heating, were identified both by an analytical and finite element model. In decreasing order of importance, optically induced cantilever bending is found to be caused by: (i) differences in photoacoustically induced pressure oscillations in the air adjacent to the illuminated and dark side of the cantilever, resulting from heat transfer from the illuminated cantilever to the nearby air, acting as a volume velocity piston, and (ii) thermoelastic stresses accompanying temperature and thermal expansion gradients in the cantilever, (iii) photoacoustically induced pressure oscillations in the air adjacent to the illuminated cantilever holder and frame.

Near-infrared laser photoacoustic gas sensor for simultaneous detection of CO and H2S

A ppb-level H₂S and CO photoacoustic spectroscopy (PAS) gas sensor was developed by using a two-stage commercial optical fiber amplifier with a full output power of 10 W. Two near-infrared diode lasers with the central wavenumbers of 6320.6 cm^-1 and 6377.4 cm^-1 were employed as the excitation laser source. A time-division multiplexing method was used to simultaneously detect CO and H₂S with an optical switch. A dual-resonator structural photoacoustic cell (PAC) was theoretically simulated and designed with a finite element analysis. A µV level background noise was achieved with the differential and symmetrical PAC. The performance of the multi-component sensor was evaluated after the optimization of frequency, pressure and modulation depth. The minimum detection limits of 31.7 ppb and 342.7 ppb were obtained for H₂S and CO at atmospheric pressure.

Ppt level carbon monoxide detection based on light-induced thermoelastic spectroscopy exploring custom quartz tuning forks and a mid-infrared QCL

In this paper, we report on an ultra-highly sensitive light-induced thermoelastic spectroscopy (LITES)-based carbon monoxide (CO) sensor exploiting custom quartz tuning forks (QTFs) as a photodetector, a multi-pass cell and a mid-infrared quantum cascade laser (QCL) for the first time. The QCL emitting at 4.58 µm with output power of 145 mW was employed as exciting source and the multi-pass cell was employed to increase the gas absorption pathlength. To reduce the noise level, wavelength modulation spectroscopy (WMS) and second harmonic demodulation techniques were exploited. Three QTFs including two custom QTFs (#1 and #2) with different geometries and a commercial standard QTF (#3) were tested as photodetector in the gas sensor. When the integration time of the system was set at 200 ms, minimum detection limits (MDLs) of 750 part-per-trillion (ppt), 4.6 part-per-billion (ppb) and 5.8 ppb were achieved employing QTF #1 #2, and #3, respectively. A full sensor calibration was achieved using the most sensitive QTF#1, demonstrating an excellent linear response with CO concentration.