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We report on the detection of the isotopologues 12CH4 and 13CH4 by employing a quartz-enhanced photoacoustic spectroscopy (QEPAS)-based sensor. By properly selecting the exciting light source and the working conditions, two absorption lines, having a negligible cross-section ratio temperature coefficient of -6.7‰/°C and a cross section ratio of ~ 0.06 for a natural abundance of each isotope, can be targeted. The QEPAS signal of the two isotopologues was acquired for mixtures in nitrogen of methane in natural abundance in a wide range of concentrations (0.02%-20%) showing a non-linear trend with high concentrations and a constant ratio comparable with the cross-section.

Gas mixtures analysis is a challenging task because of the demand for sensitive and highly selective detection techniques. Partial least squares regression (PLSR) is a statistical method developed as generalization of standard multilinear regression (MLR), widely employed in multivariate analysis for relating two data matrices even with noisy and strongly correlated experimental data. In this work, PLSR is proposed as a novel approach for the analysis of gas mixtures spectra acquired with quartz-enhanced photoacoustic spectroscopy (QEPAS). Results obtained analyzing CO/N2O and CH4/C2H2/N2O gas mixtures are presented. A comparison with standard MLR approach highlights a prediction errors reduction up to 5 times.

In this work, we report on the measurement of methane (CH4) effective non-radiative relaxation rate in a mixture containing 1% of CH4 and 0.15% of water vapor in nitrogen, by using a set of custom quartz tuning forks (QTFs). The dependence of quartz-enhanced photoacoustic spectroscopy (QEPAS) peak signal and QTF quality factor as a function of operating pressure allowed the estimation of the radiation-to-sound conversion efficiency and, consequently, the calculation of the effective relaxation rate of the investigated gas mixture. We measured an effective relaxation rate of 3.2 ms·Torr, in good agreement with values reported in literature.

In oil and gas exploration and environmental monitoring fields is extremely important to rely on versatile and rugged trace gas sensors that can be mounted on remote-controlled unmanned vehicles to monitor dangerous areas inaccessible to a human operator. Thereby, a compact fiber-coupled dual-gas QEPAS sensor employing a custom tuning fork was designed and tested. Two distributed feedback pigtailed diode lasers emitting at 1654 nm and 1684 nm, for methane and ethane detection respectively, were also employed. The gas sensor was calibrated for ethane and methane trace detection and a minimum detection level of 18 parts per million and 570 parts per billion at 1 s integration time were respectively achieved. Methane detection in atmosphere was also performed.

We report on a quartz-enhanced photoacoustic (QEPAS) sensor employing a monolithic distributed-feedback quantum cascade laser array operating in a pulsed mode as a light source. The array consists of 32 quantum cascade lasers emitting in a spectral range from 1190 cm-1 to 1340 cm-1, which covers two absorption branches of nitrous oxide (N2O) and several absorption features of (CH4). The versatility of the QEPAS technique combined with the rapid wavelength tuning provided by the ultra-compact, low-power consuming laser source allowed the detection of N2O and CH4 with detection sensitivities below a part-per-million at atmospheric pressure.

We report on the performance of new quartz tuning fork (QTF) designs optimized for quartz-enhanced photoacoustic spectroscopy (QEPAS). We investigated the impact on resonance properties of prong geometries differing from the standard rectangular one. We proposed a QTF with T-shaped prongs and a QTF with prongs having rectangular grooves carved on the surface. QTFs were implemented in a QEPAS sensor and performances were compared in terms of signalto-noise ratio (SNR). Then, QTFs were acoustically coupled with single- and dual-tube micro-resonator systems. A record x60 SNR enhancement factor with respect to the bare QTF was achieved with QTF having T-shaped prongs.

We report on a comparison between two quartz tuning forks (QTFs) employed for quartz-enhanced photoacoustic spectroscopy (QEPAS) having quadrupole and octupole electrode pattern configurations. With respect to the quadrupole, the implementation of the octupole pattern suppresses the fundamental mode and reduces by a factor of ~ 4.4 the electrical resistance for the first overtone mode with negligible variations of the related Q-factors. Both QTFs operating at the first overtone mode were implemented in a QEPAS sensor and the results showed that the octupole configuration provides a ~2.3 signal enhancement factor.

Simultaneous detection of different trace gases is an important topic in many applications, such as breath analysis, environmental monitoring and oil exploration. Quartz-enhanced photoacoustic spectroscopy offers a good solution of this problem, due to the possibility to operate the quartz resonators at the fundamental and overtone resonance frequencies by simultaneously exciting two different antinodes with two different laser sources. In this work we employed a custom spectrophone coupled with a diode laser and a quantum cascade laser for simultaneous detection of water vapor and methane/nitrous oxide with minimum detection limits in the ppb range.

Hydrocarbon detection in the gas phase can be a powerful tool to guide downstream operations for the oil and gas industry. This application requires highly sensitive, selective and robust spectroscopic techniques. In this work we present: i) a quartz-enhanced photoacoustic (QEPAS) sensor that can individually detect methane and ethane in the part per billion range and propane in the ppm range by employing a single interband cascade laser emitting at 3345 nm; ii) a QEPAS sensor detecting 12CH4 and 13CH4 isotopes at the part-per billion sensitivity level, by employing a quantum cascade laser emitting at 7730 nm.

Hydrocarbon detection in the gas phase can be a powerful tool to guide downstream operations for the oil & gas industry. This application requires highly sensitive, selective and robust spectroscopic techniques. In this work, a quartz-enhanced photoacoustic sensor system designed to detect methane and ethane concentration levels in the part-per billion concentration range by employing a single interband cascade laser emitting at 3345 nm is reported. Measurements at both low and high pressure were carried out and mixtures simulating typical downhole compositions were analyzed.