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The implementation, performance validation, and testing of a gas-leak optical sensor based on mid-IR quartz-enhanced photoacoustic (QEPAS) spectroscopic technique is reported. A QEPAS sensor was integrated in a vacuum-sealed test station for mechatronic components. The laser source for the sensor is a quantum cascade laser emitting at 10.56 µm, resonant with a strong absorption band of sulfur hexafluoride (SFsub6/sub), which was selected as a leak tracer. The minimum detectable concentration of the QEPAS sensor is 2.7 parts per billion with an integration time of 1 s, corresponding to a sensitivity of leak flows in the 10sup-9/supmbar∙l/s range, comparable with state-of-the-art leak detection techniques.

A novel method for mid-IR laser beam mode cleaning employing hollow core waveguide as a modal filter element is reported. The influence of the input laser beam quality on fiber optical losses and output beam profile using a hollow core waveguide with 200 μm-bore size was investigated. Our results demonstrate that even when using a laser with a poor spatial profile, there will exist a minimum fiber length that allows transmission of only the Gaussian-like fundamental waveguide mode from the fiber, filtering out all the higher order modes. This essentially single mode output is preserved also when the waveguide is bent to a radius of curvature of 7.5 cm, which demonstrates that laser mode filtering can be realized even if a curved light path is required.

A quartz enhanced photoacoustic spectroscopy (QEPAS) sensor, employing an erbium-doped fiber amplified laser source and a custom quartz tuning fork (QTF) with its two prongs spaced ∼800 μm apart, is reported. The sensor employs an acoustic micro-resonator (AmR) which is assembled in an “on-beam” QEPAS configuration. Both length and vertical position of the AmR are optimized in terms of signal-to-noise ratio, significantly improving the QEPAS detection sensitivity by a factor of ∼40, compared to the case of a sensor using a bare custom QTF. The fiber-amplifier-enhanced QEPAS sensor is applied to H2S trace gas detection, reaching a sensitivity of ∼890 ppb at 1 s integration time, similar to those obtained with a power-enhanced QEPAS sensor equipped with a standard QTF, but with the advantages of easy optical alignment, simple installation, and long-term stability.

We report on a quartz-enhanced photoacoustic (QEPAS) gas sensing system for hydrogen sulphide (H2S) detection. The system architecture is based on a custom quartz tuning fork (QTF) optoacoustic transducer with a novel geometry and a quantum cascade laser (QCL) emitting 1.1 mW at a frequency of 2.913 THz. The QTF operated on the first flexion resonance frequency of 2871 Hz, with a quality factor Q = 17,900 at 20 Torr. The tuning range of the available QCL allowed the excitation of a H2S rotational absorption line with a line-strength as small as S = 1.13.10-22 cm/mol. The measured detection sensitivity is 30 ppm in 3 seconds and 13 ppm for a 30 seconds integration time, which corresponds to a minimum detectable absorption coefficient ?min = 2.3.10-7 cm-1 and a normalized noise-equivalent absorption NNEA = 4.4.10-10 W.cm-1.Hz-1/2, several times lower than the values previously reported for near-IR and mid-IR H2S QEPAS sensors.

In a previous paper ( A. Sampaolo and R. Binetti (1986) . Regul. Toxicol. Pharmacol. 6, 129–154) an original pragmatic approach for priority setting and risk assessment among existing chemicals was presented. The experience subsequently acquired in the use of the method suggested some improvements, relevant for an easier application. The priority coefficient R has been redefined so that now it can range between zero (all data available) and 1 (no data available). In order to make the final figures more comparable with each other, the indexes are now expressed as percentage with reference to the maximum score obtainable if all parameters of the equation used had reached the maximum score. New general priority index and general risk index formulas, both for personal direct and environmental exposure, are also defined. Another modification concerns the application of the method to specific aspects, such as irreversible effects, immediate and delayed toxic effects, and so on. The method has then been validated on 80 known substances obtaining the respective ranking according to the specific kind of risk.

An original pragmatic approach is presented for priority setting among existing chemicals, as a result of a study requested by the European Community Commission. On the basis of a set of physicochemical, toxicological, and ecotoxicological properties considered as fundamental for a first evaluation of the substance, the proposed system develops a score for each property in both the presence and absence of the datum. The properties in this first set are considered additive. Other information on the substance (quantity on the market, plurality of possible exposures, bioconcentration, environmental diffusion, persistence and size of risk population) is included in a second set of multiplier parameters. Through this simple and self-improving system it is possible to work out, using different final formulas, different lists for different needs. Using 30 well-known substances as examples with which to check the system, it is possible to see how these substances distribute themselves as to priority for environmental or for direct personal exposure.

Development of trace gas sensors based on mid-infrared interband cascade lasers and quantum cascade lasers as well as their applications will be reported. The sensor technology will use both laser absorption and quartz enhanced photoacoustic spectroscopy.

A finite element model for Quartz Enhanced Photoacoustic Spectroscopy (QEPAS) was developed and applied to study the dependence of performance on sensor dimensions. This model can be a design tool for optimizing tuning fork dimensions.

A Quartz-enhanced photoacoustic sensor using a custom quartz tuning fork with large prong spacing was realized and optimized by improving its spectrophone design and performance. The impact of two-tube and single-tube on-beam micro-resonator (mR) geometries on the detected signal and signal-to-noise ratio (SNR) was investigated. Experimental studies demonstrated that a single-tube configuration offers a shorter mR and a higher SNR than two-tube configuration.

Manufacturing and deployment of high performance mid-IR devices for demanding sensing applications requires a high degree of control over development and manufacturing of components that can be packaged in a ruggedized, reliable and compact instrument.

Quartz tuning forks (QTFs) have been successfully employed as piezoelectric optoacoustic transducers in a quartz enhanced photoacoustic spectroscopic sensor (QEPAS) for sensitive and selective gas sensing applications. Stainless tubes are acoustically coupled with the QTF and act as an acoustic micro-resonator (AmR) to confine the acoustic waves and enhance the QEPAS signal [1]. Very recently, we realized custom QTFs with fundamental resonance frequency of a few KHz [2] in order to approach the typical energy relaxation time of gases, while maintaining a high resonator quality factor. However, lowering the fundamental resonance frequency reduces also the overtone frequencies, leading to their use in QEPAS.

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.

Multi-gas detection represents a suitable solution in many applications, such as environmental and atmospheric monitoring, chemical reaction and industrial process control, safety and security, oilgas and biomedicine. Among optical techniques, Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) has been demonstrated to be a leading-edge technology for addressing multi-gas detection, thanks to the modularity, ruggedness, portability and real time operation of the QEPAS sensors. The detection module consists in a spectrophone, mounted in a vacuum-tight cell and detecting sound waves generated via photoacoustic excitation within the gas sample. As a result, the sound detection is wavelength-independent and the volume of the absorption cell is basically determined by the spectrophone dimensions, typically in the order of few cubic centimeters. In this review paper, the implementation of the QEPAS technique for multi-gas detection will be discussed for three main areas of applications: i) multi-gas trace sensing by exploiting non-interfering absorption features; ii) multi-gas detection dealing with overlapping absorption bands; iii) multi-gas detection in fluctuating backgrounds. The fundamental role of the analysis and statistical tools will be also discussed in detail in relation with the specific applications. This overview on QEPAS technique, highlighting merits and drawbacks, aims at providing ready-to-use guidelines for multi-gas detection in a wide range of applications and operating conditions.

A dual-gas sensor based on the combination of a quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor and an electronic hygrometer was realized for the simultaneous detection of methane (CH4) and water vapor (H2O) in air. The QEPAS sensor employed an interband cascade laser operating at 3.34 &mu, m capable of targeting a CH4 absorption line at 2988.8 cm&minus, 1 and a water line at 2988.6 cm&minus, 1. Water vapor was measured with both the electronic hygrometer and the QEPAS sensor for comparison. The measurement accuracy provided by the hygrometer enabled the adjustment of methane QEPAS signal with respect to the water vapor concentration to retrieve the actual CH4 concentration. The sensor was tested by performing prolonged measurements of CH4 and H2O over 60 h to demonstrate the effectiveness of this approach for environmental monitoring applications.

A study of the front-end electronics for quartz tuning forks (QTFs) employed as optoacoustic transducers in quartz-enhanced photoacoustic spectroscopy (QEPAS) sensing is reported. Voltage amplifier-based electronics is proposed as an alternative to the transimpedance amplifier commonly employed in QEPAS experiments. The possibility to use differential input/output configurations with respect to a single-ended configuration has also been investigated. Four different architectures have been realized and tested: a single-ended transimpedance amplifier, a differential output transimpedance amplifier, a differential input voltage amplifier and a fully differential voltage amplifier. All of these amplifiers were implemented in a QEPAS sensor operating in the mid-IR spectral range. Water vapor in ambient air has been selected as the target gas species for the amplifiers&rsquo, testing and validation. The signal-to-noise ratio (SNR) measured for the different configurations has been used to compare the performances of the proposed architectures. We demonstrated that the fully differential voltage amplifier allows for a nearly doubled SNR with respect to the typically used single-ended transimpedance amplifier.