Quartz Enhanced Photoacoustic Spectroscopy Based on a Custom Quartz Tuning Fork

Quartz-enhanced photoacoustic sensing is a promising method for low-concentration trace-gas monitoring due to the resonant signal enhancement provided by a high-Q quartz tuning fork. However, quartz-enhanced photoacoustic spectroscopy (QEPAS) is associated with a relatively slow acoustic decay, which results in a reduced spectral resolution and signal-to-noise ratio as the wavelength tuning rate is increased. In this work, we investigate the influence of wavelength scan rate on the spectral resolution and signal-to-noise ratio of QEPAS sensors. We demonstrate the acquisition of photoacoustic spectra from 3.1 μm to 3.6 μm using a tunable mid-infrared optical parametric oscillator. The spectra are attained using wavelength scan rates differing by more than two orders of magnitude (from 0.3 nm s−1 to 96 nm s−1). With this variation in scan rate, the spectral resolution is found to change from 2.5 cm−1 to 9 cm−1. The investigated gas samples are methane (in nitrogen) and a gas mixture consisting of methane, water, and ethanol. For the gas mixture, the reduced spectral resolution at fast scan rates significantly complicates the quantification of constituent gas concentrations.

“…The NNEA represents the minimum detectable absorption in units of inverse length, normalized to the laser power and the electrical measurement bandwidth. It is defined as follows [ 24 ]:

where P is the average optical power in watts;

is the target gas absorption in

;

is the detection bandwidth in Hz; and SNR is the signal-to-noise ratio, taken as the ratio of the signal value with the root-mean-square noise within the same bandwidth.…”

Section: Experimental Setupmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Intrinsic Spectral Resolution Limitations of QEPAS Sensors for Fast and Broad Wavelength Tuning

Christensen¹,

Høgstedt²,

Friis³

et al. 2020

“…The OptA sensor achieved a detection limit for BC of 15.7 µg/m 3 and an NNEA of 7.39 × 10 −9 W cm −1 Hz −1/2 , which is comparable to other QEPAS systems that usually achieve an NNEA in the order of 10 −8 to 10 −10 W cm −1 Hz −1/2 [47]. However, the BC detection limit could not be properly characterized due to several factors.…”

Section: Discussionmentioning

confidence: 82%

“…Finally, we exposed the sensor to exhaust which has been filtered using both a HEPA filter and an AC filter in series (Figure 4d), aiming to remove all absorbing species. The burner was set to Normal Operation in all cases, and the SNR of the OptA signal was calculated [47,48]. Figure 4b shows the spectrum of only NO 2 , as most particles were captured by the HEPA filter.…”

Section: Resultsmentioning

confidence: 99%

A Low-Cost Optoacoustic Sensor for Environmental Monitoring

Stylogiannis

Kousias

Kontses

et al. 2021

Attention to Black Carbon (BC) has been rising due to its effects on human health as well its contribution to climate change. Measurements of BC are challenging, as currently used devices are either expensive or impractical for continuous monitoring. Here, we propose an optoacoustic sensor to address this problem. The sensor utilizes a novel ellipsoidal design for refocusing the optoacoustic signal with minimal acoustic energy losses. To reduce the cost of the system, without sacrificing accuracy, an overdriven laser diode and a Quartz Tuning Fork are used as the light source and the sound detector, respectively. The prototype was able to detect BC particles and to accurately monitor changes in concentration in real time and with very good agreement with a reference instrument. The response of the sensor was linearly dependent on the BC particles concentration with a normalized noise equivalent absorption coefficient (NNEA) for soot equal to 7.39 × 10−9 W cm−1 Hz−1/2. Finally, the prototype was able to perform NO2 measurements, demonstrating its ability to accurately monitor both particulate and gaseous pollutants. The proposed sensor has the potential to offer a significant economic impact for BC environmental measurements and source appointment technologies.

“…J. Li demonstrated the use of 75 kHz QTF in the detection of broadband absorbers [33]. M. Duquesnoy demonstrated a custom QTF with a reduced fundamental frequency of 21.2 kHz and a large prong spacing of 2 mm [34]. Recently, a custom QTF with prong spacing up to 1.5 mm and a Q value of ~15,000 were employed for ppb-level ethylene detection by V. Spagnolo [35].…”

Section: Introductionmentioning

confidence: 99%

Application of Micro Quartz Tuning Fork in Trace Gas Sensing by Use of Quartz-Enhanced Photoacoustic Spectroscopy

Lin

Huang

Kan

et al. 2019

A novel quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor based on a micro quartz tuning fork (QTF) is reported. As a photoacoustic transducer, a novel micro QTF was 3.7 times smaller than the usually used standard QTF, resulting in a gas sampling volume of ~0.1 mm3. As a proof of concept, water vapor in the air was detected by using 1.39 μm distributed feedback (DFB) laser. A detailed analysis of the performance of a QEPAS sensor based on the micro QTF was performed by detecting atmosphere H2O. The laser focus position and the laser modulation depth were optimized to improve the QEPAS excitation efficiency. A pair of acoustic micro resonators (AmRs) was assembled with the micro QTF in an on-beam configuration to enhance the photoacoustic signal. The AmRs geometry was optimized to amplify the acoustic resonance. With a 1 s integration time, a normalized noise equivalent absorption coefficient (NNEA) of 1.97 × 10−8 W·cm−1·Hz−1/2 was achieved when detecting H2O at less than 1 atm.