Abstract
We report on a quartz-enhanced photoacoustic (QEPAS) sensor for methanol (CH3OH) detection employing a novel quartz tuning fork (QTF), specifically designed to enhance the QEPAS sensing performance in the terahertz (THz) spectral range. A discussion of the QTF properties in terms of resonance frequency, quality factor and acousto-electric transduction efficiency as a function of prong sizes and spacing between the QTF prongs is presented. The QTF was employed in a QEPAS sensor system using a 3.93 THz quantum cascade laser as the excitation source in resonance with a CH3OH rotational absorption line located at 131.054 cm−1. A minimum detection limit of 160 ppb in 30 s integration time, corresponding to a normalized noise equivalent absorption NNEA = 3.75 × 10−11 cm−1W/Hz½, was achieved, representing a nearly one-order-of-magnitude improvement with respect to previous reports.
Highlights
Spectroscopic techniques for trace gas sensing and monitoring have shown great potential for non-invasive chemical analysis requiring high sensitivity and selectivity
In this this work work we we reported reported on onaaQEPAS
With design, the prong width and length were reduced with respect to the novel geometry
Summary
Spectroscopic techniques for trace gas sensing and monitoring have shown great potential for non-invasive chemical analysis requiring high sensitivity and selectivity. Wavelength modulation spectroscopy, employing liquid-He-cooled cw-operating QCLs combined with low-noise bolometer detectors, is promising in terms of THz-sensing performances [9,10] This approach would allow reaching high sensitivity and selectivity, but suffers from complexity and the disadvantage of using cryogenic cooling systems for both generation and detection of THz radiation. The factor Q K can be used as the figure of merit to compare the sensing performance of the two QTFs in a QEPAS sensor, if all other sensor parameters (e.g., laser power, absorption strength, gas pressure and sound conversion efficiency) are kept constant. A comparison of the performances between the two THz QEPAS sensor systems shows a sensitivity enhancement of approximately one order of magnitude when using the N-QTF
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