Applications of quartz tuning forks in spectroscopic gas sensing

Abstract

A recently introduced approach to photoacoustic detection of trace gases utilizing a quartz tuning fork ͑TF͒ as a resonant acoustic transducer is described in detail. Advantages of the technique called quartz-enhanced photoacoustic spectroscopy ͑QEPAS͒ compared to conventional resonant photoacoustic spectroscopy include QEPAS sensor immunity to environmental acoustic noise, a simple absorption detection module design, and its capability to analyze gas samples ϳ1 mm 3 in volume. Noise sources and the TF properties as a function of the sampled gas pressure, temperature and chemical composition are analyzed. Previously published results for QEPAS based chemical gas sensing are summarized. The achieved sensitivity of 5.4ϫ 10 −9 cm −1 W/ ͱ Hz is compared to recent published results of photoacoustic gas sensing by other research groups. An experimental study of the long-term stability of a QEPAS-based ammonia sensor is presented. The results of this study indicate that the sensor exhibits very low drift, which allows data averaging over Ͼ3 h of continuous concentration measurements. Architecture and practical implementation of autonomous QEPAS-sensor controller electronics is described. Future developments of QEPAS technique are outlined.

Performance evaluation of a near infrared QEPAS based ethylene sensor

“…The total length of each of the two μR tubes is 5.1 mm (close to λ/2 for a sound wave of 32.8 kHz in air or N 2 ). This is approximately twice longer than have been used in earlier sensor versions [2,3] and was experimentally found to produce higher photoacoustic signals at >500 Torr gas pressure. The gaps between each of the μR tubes and a QTF surface were in the 30-50 μm range.…”

“…Therefore, the selected optimum C 2 H 4 absorption line allows calibrating both 2f and 3f zero levels of the CEU electronics by tuning the laser current to 118.2 mA. Second, a negative peak (3) at 138.0 mA (6176.90 cm −1 ) is almost as strong as the positive target peak (2). Therefore, for better zero level drift compensation, the laser can be alternately tuned to either the peak at 6177.14 cm −1 or the valley at 6176.90 cm −1 with the C 2 H 4 concentration being subsequently derived from the difference of the measured signals.…”

“…The architecture of the ethylene sensor used in this work was described in previous publications [2,6] and was applied to the detection of several gases, including CO 2 , H 2 O, H 2 S, NH 3 , CH 4 , HCN, and C 2 H 2 in the NIR spectral region. The current ethylene sensor schematic is shown in Fig.…”

“…Since the discovery of QEPAS several years ago [1], it has been demonstrated with several simple molecules using a single rovibrational line in fundamental mid-IR absorption bands [2,3] (e.g., CO, N 2 O, H 2 CO, C 2 H 6 ) or in near-IR combination bands (e.g., NH 3 [4], CO 2 [5], HCN [6], H 2 S [7]). More recently, QEPAS has also been implemented with larger molecules with broad, unresolved absorption spectra, such as acetone and freons [8][9][10].…”

Near-infrared resonant photoacoustic gas measurement using simultaneous dual-frequency excitation

“…Sigrist ( ) Laser Spectroscopy and Sensing Laboratory, Institute of Quantum Electronics, ETH Zurich, Schafmattstr. 16, 8093 Zurich, Switzerland e-mail: sigrist@iqe.phys.ethz.ch Fax: +41- for photoacoustic cells have been proposed: acoustically resonant [4][5][6][7][8][9][10], multipass [11,12], intracavity [13,14], parallel [15], "windowless" [16], differential [17] and quartz enhanced [18]. In most known photoacoustic sensing schemes, the absolute values of the microphone signal are measured and used to derive the concentration of the gas of interest ( [3] and references therein).…”