Professor Sandro Mengali and his research team at Consorzio CREO have developed a novel detection method for enhanced photo-acoustic spectroscopy. Their device is constructed using additive manufacturing (AM) and integrates a tuning fork for acoustic detection. Detailed information about their new technique can be found in the latest issue of Sensors.
Professor Mengali’s group refers to its new detection module as Tuning-Fork-Enhanced Photo-Acoustic Spectroscopy (TFEPAS). TFEPAS, built on the foundations of Quartz-Enhanced Photo-Acoustic Spectroscopy (QEPAS), is employed for gas or vapor sensing.
The Use of Gas Sensing
The emission of detrimental gas into the atmosphere from industrial manufacturing has been a growing concern. The negative impact on biodiversity and the environment has forced many governing bodies to take action to monitor the quality of air frequently or in real-time.
Among the many methods employed for gas sensing, QEPAS has grown in prominence in the last few decades. This method has successfully been used to evaluate a variety of gas mixtures and has demonstrated excellent sensitivity and selectivity.
To analyze complex vapor traces, QEPAS has recently been in micro-gas chromatography. However, technical issues have restricted the application of a QEPAS sensor and limited its effectiveness. For example, precise manual positioning and alignment are needed to integrate an absorption sensor within the device.
The quartz tuning fork must be specially designed and micro-machined for particular applications. Using commercial tuning forks also requires removing the welds or working at temperatures below the welds' melting point.
Tuning-Fork-Enhanced Photo-Acoustic Spectroscopy (TFEPAS)
Prof. Mengali’s team set out to overcome the limitations imposed by QEPAS by implementing TFEPAS, which is based on a non-piezoelectric tuning fork and an optical interferometer. Its miniaturized photoacoustic device for TFEPAS uses monolithic integration of the fork, acoustic micro-resonator, and Absorption Detection Module (ADM) into one block utilizing AM techniques.
The detection design devised in the current study integrates the structure of the analysis cell with the tuning fork and the acoustic micro-resonator into a small, monolithic piece of stainless steel. This approach is conducive for AM to test the viability of the TFEPAS solution.
An object is constructed through additive manufacturing (AM) by carefully layering materials to the forms indicated by a digital design. The layers may be liquid, metal, cement, or plastic powder. A 3D object is created by fusing each layer. Micro-Metal Laser Sintering (MMLS) was implemented for this specific detector construction. The viability of designs with sub-millimeter features was explored since the tuning fork and the acoustic micro-resonator require tiny size and high spatial resolution.
On the detection end, to maximize sensitivity and minimize the signal-to-noise ratio, an interferometric readout was used. Interferometric detection has produced results comparable to those of a piezoelectric readout.
The tuning fork vibration is induced using a Quantum Cascade Laser. Photothermal excitation is induced by focusing laser pulses directly on the fork. Using a spectrum analyzer, the component of the photodiode signal at the same QCL modulation frequency is extracted to provide the interferometric readout signal.
The goal of the TFEPAS sensor is to be reliable, transportable, and field-capable. Consequently, a second, more robust configuration was implemented for the interferometric readout where optical fiber coupling replaced free space coupling.
Verifying the Performance of TFEPAS
The performance of the AM TFEPAS device in terms of sensitivity and linearity of the response was verified by a series of tests using ammonia gas at various concentrations. A cylinder with a sample mixture of 100 ppm ammonia in nitrogen was utilized for testing. The results showed that the limit of detection (LOD) is better than 1 ppm from examining the signal-to-noise ratio (S/N).
The AM TFEPAS device's spectral resolution was tested across a wide spectral range using the same gas model of ammonia 100 ppm in nitrogen to assess its capacity to recognize infrared (IR) absorption spectra.
The QCL laser scans several ammonia IR absorption lines during these tests. The Pacific Northwest National Laboratory's (PNNL) database was searched for the reference ammonia IR absorption spectrum and compared it to the TFEPAS measurements. The accuracy of TFEPAS was verified to be consistent with the PNNL data.
The novel design presented in Sensors, where the ADM is entirely constructed monolithically using AM techniques, enables extremely small internal volumes, automatic alignment of the tuning fork with the acoustic micro-resonators, high-temperature operation, and easier and more affordable customization.
To show the viability of the new design, experiments were conducted using ammonia at a parts-per-million concentration in nitrogen. The TFEPAS solution seems exceptionally well suited for hyphenation to micro-Gas Chromatography and analyzing complicated solid and liquid trace materials, including substances with limited volatility.
Viola, Roberto, Nicola Liberatore, and Sandro Mengali. (2022) Additively Manufactured Detection Module with Integrated Tuning Fork for Enhanced Photo-Acoustic Spectroscopy. Sensors 22, no. 19: 7193. www.mdpi.com/1424-8220/22/19/7193/htm