Abstract
In this paper the performances of two spectrophones for quartz-enhanced photoacoustic spectroscopy (QEPAS)-based ethane gas sensing were tested and compared. Each spectrophone contains a quartz tuning fork (QTF) acoustically coupled with a pair of micro-resonator tubes and having a fundamental mode resonance frequency of 32.7 kHz (standard QTF) and 12.4 kHz (custom QTF), respectively. The spectrophones were implemented into a QEPAS acoustic detection module (ADM) together with a preamplifier having a gain bandwidth optimized for the respective QTF resonance frequency. Each ADM was tested for ethane QEPAS sensing, employing a custom pigtailed laser diode emitting at ~1684 nm as the exciting light source. By flowing 1% ethane at atmospheric pressure, a signal-to-noise ratio of 453.2 was measured by implementing the 12.4 kHz QTF-based ADM, ~3.3 times greater than the value obtained using a standard QTF. The minimum ethane concentration detectable using a 100 ms lock-in integration time achieving the 12.4 kHz custom QTF was 22 ppm.
Highlights
The number of technological fields requiring in-situ and real time gas tracing is rapidly increasing and the need for ad hoc sensing approaches for specific applications [1,2]
ADM2 exhibits a higher Q-factor with respect to ADM1 despite a lower fundamental in-planeBoth flexural mode andresonance normalized to the peak are resonance frequency
Measurements were performed using wavelength modulation and 2f-detection: a sinusoidal dither at half of the spectrophone resonance frequency was applied to the laser current driver using a waveform generator and the quartz tuning fork (QTF) signal was demodulated at the spectrophone resonance frequency by a lock-in amplifier
Summary
The number of technological fields requiring in-situ and real time gas tracing is rapidly increasing and the need for ad hoc sensing approaches for specific applications [1,2]. The first QTF implemented in a QEPAS sensor was a standard 32.7 kHz QTF designed for timing applications [22] This QTF has a quality factor of 10,000 in air that increases up to 100,000 in vacuum. On a comparison between spectrophones, onecustom implementing a standard spectrophones haveand been coupled a pair 12.4 of optimized tubes, They have been tested in a QEPAS sensor employing a DFB diode laser emitting were coupled with a pair of optimized resonator tubes, in an “on beam” configuration. An investigation on the optimum working conditions as they were tested in a QEPAS sensor employing a DFB diode laser emitting at 1684 nm as a light source well as an of the ultimate detectiononlimit and long-term stability has performed. An investigation the optimum working conditions as been well as an analysis of the ultimate detection limit and long-term stability was performed
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