Off-beam quartz-enhanced photoacoustic spectroscopy-based sensor for hydrogen sulfide trace gas detection using a mode-hop-free external cavity quantum cascade laser

Appl. Phys. B (2017) 123:141 DOI 10.1007/s00340-017-6717-8 Off‑beam quartz‑enhanced photoacoustic spectroscopy‑based sensor for hydrogen sulfide trace gas detection using a mode‑hop‑free external cavity quantum cascade laser Marek Helman1 · Harald Moser2 · Alina Dudkowiak1 · Bernhard Lendl2 Received: 30 November 2016 / Accepted: 30 March 2017 © The Author(s) 2017. This article is an open access publication Abstract Hydrogen sulfide (H2S) trace gas detection based on off-beam quartz-enhanced photoacoustic spectroscopy using a continuous wave (CW), mode-hop-free external cavity (EC) quantum cascade laser tunable from 1310 to 1210 cm−1 was performed. A 1σ minimum detection limit of 492 parts per billion by volume (ppbv) using a 1 s lock-in time constant was obtained by targeting the line centered at 1234.58 cm−1. This value corresponds to a normalized noise equivalent absorption coefficient for H2S of 3.05 × 10−9 W cm−1 Hz−1/2. 1 Introduction The sensitive and selective detection of gaseous sulfur species with the emphasis on hydrogen sulfide (H2S) down to trace concentrations is of essential importance across a wide range of applications including production control and environmental monitoring purposes in the field of petrochemical, paper, and pulp or biotechnological processes. The wide occurrence of H2S in these processes, and due to its negative impact on process stability and product quality, the concentration of H2S needs to be tightly monitored [1]. Furthermore, personal safety considerations and legal concentration limits also necessitate the accurate determination of H 2S levels. The occupational exposure limit * Alina Dudkowiak 1 Faculty of Technical Physics, Poznan University of Technology, Piotrowo 3, 60‑965 Poznan, Poland 2 Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9/164, 1060 Vienna, Austria recommended by the European Agency for Safety and Health at Work (EU-OSHA) is 5 ppmv [2]. The permissible exposure limit value for H2S is 10 ppmv, the Immediately Dangerous to Life and Health (IDLH) level is 300 ppmv and lethal concentrations are in the range of 2000 ppmv [2]. In practice, concentrations ranging from sub-ppmv levels at low pressures to several per cents at atmospheric conditions need to be monitored. Despite a variety of online monitoring options for gaseous H2S, its reliable quantitative and selective determination still remains challenging in the field of chemical sensors [3–5]. In the field of laser spectroscopy, the constant improvement of quantum cascade lasers (QCLs) has led to their application as reliable sources of coherent light ranging from the mid-infrared (MIR) to the terahertz spectral region for sensitive detection of molecular species on their fundamental vibrational, respectively, rotational bands [6–9]. Due to their tailorable emission wavelength, high output power, compactness, narrow spectral linewidth, and wavelength tuneability, QCLs are optimal choices for spectroscopic applications. In addition, optical resonator designs are constantly improved over the years with the distributed feedback (DFB) [10] and the external cavity (EC) [11] approach being the most prominent ones. A general aim with respect to the ongoing development of QCLs for sensing applications is to reduce the line width of the emitted radiation to a minimum while achieving a spectral coverage as large as possible. So far, EC-QCLs offer the largest tuning range which, depending on the employed gain medium, may cover up to several hundreds of wavenumbers. The external cavity design facilitates broadband spectral tuning by an external diffraction grating, while the selection of the emission wavelength takes place by changing the grating angle relative to the QCL chip. 13 141 Page 2 of 8 Technically viable setups for H2S trace gas measurements in the MIR spectral region using QCLs are often chosen to be based on absorbance measurements in multireflection Herriott cells [12] as the effective optical interaction pathlength will be increased up to several tens of meters. Recently, the first application of ring-cavitysurface-emitting quantum-cascade lasers (RCSE-QCLs) for sensitive H 2S gas measurements was reported [13]. Together with phase-sensitive detection techniques, such as wavelength modulation spectroscopy (WMS) [14, 15], the generally dominating 1/f electronic noise can be drastically minimized, and in general, high detection sensitivities of H2S can be achieved, enabling on-line monitoring of H2S and CH4 content in a petrochemical process stream [16]. Complementary to the direct absorption approach, photoacoustic spectroscopy (PAS) detects pressure waves which are caused by a thermal expansion of the gas due to nonradiative relaxation of molecules after absorption of modulated laser radiation. Conventional PAS utilizes broadband condenser or electret microphones as acoustic transducer [17]. A variant of the classical approach, called quartzenhanced photoacoustic spectroscopy (QEPAS), employs a quartz tuning fork (QTF) as a sharply resonant acoustic transducer to detect weak photoacoustic excitations [18]. The QTF is an mm-sized piezoelectric element, which converts a deformation caused by acoustic waves into charges that can be measured by its electrodes. Commercial readily available QTFs, which are designed as frequency standard for smartphones, watches, and clocks, are low-cost oscillators with a resonance frequency of 32,768 Hz and a quality that can exceed 100,000 in vacuum [19]. Only the antisymmetric QTF vibration, i.e., when the two prongs bend in opposite directions, is piezo-electric active [20]. Thus, in a typical QEPAS arrangement, the laser beam is transmitted through the gap of the QTF formed by the two tines to probe strong photoacoustic signals. A significant enhancement of the detected QEPAS signal can be achieved when an acoustic resonator is added to the QTF. In case of the typical on beam configuration, a tube is added in front and behind the QTF, exploiting a longitudinal acoustic resonance [20]. By this means, a 30-fold increase in the SNR compared to the bare tuning fork can be achieved [21]. In principal, the noise of a QEPAS sensor is determined by the thermal noise of the QTF. However, additional noise can be introduced by unintended illumination of the transducer. The gap formed by the two tines is only 300 μm, which makes it often difficult to focus the beam through this configuration. An alternative resonator focuses the excitation beam with some lateral displacement in parallel to the two QTF tines through an acoustic resonator consisting of only a single tube [22]. This configuration is referred to as offbeam QEPAS. The resonator has a small aperture in the 13 M. Helman et al. middle to enable generated pressure waves to exit and to couple with the QTF, which is positioned closely to the aperture. The benefit of this off-beam configuration is that the laser (...truncated)