Photoacoustic Spectroscopy for C1-C2 Hydrocarbon Monitoring in Ambient Air

Die vorliegende Dissertation erörtert das Potenzial der photoakustischen Spektroskopie (PAS) für die Detektion von Methan und Ethan in der Umgebungsluft. Der theoretische Teil widmet sich zunächst ausführlich den Grundlagen der Absorptionsspektroskopie und insbesondere der PAS. Zusätzlich werden für diese Arbeit relevante Messmethoden wie die Absorptionsspektroskopie nach Lambert-Beer, Cavity ring-down Spektroskopie (CRDS) und Wellenlängen-modulierte Spektroskopie (WMS) vorgestellt und ihre jeweiligen Vor- und Nachteile erörtert. Im Theoriekapitel wird zusätzlich die photoakustische Signalgenerierung, einschließlich der Signalverstärkung durch akustische Resonanzverstärkung, detailliert hergeleitet. Besonderes Augenmerk liegt auf der Diskussion des Einflusses der nicht-strahlenden Relaxation auf das photoakustische Signal, wobei die zwei dominanten Formen der stoßbasierten Energieübergänge, Schwingungs-Translation (VT) Relaxation und Schwingungs-Schwingungs (VV) Relaxation, intensiv betrachtet und diskutiert werden. Eine Literaturzusammenfassung rundet den theoretischen Teil ab, wobei der Fokus auf dem Einfluss der Relaxation auf das photoakustische Signal für verschiedene Analyten liegt. Neben der Charakterisierung der Laserquellen hinsichtlich der Emissionswellenzahl wird ein in die photoakustische Messzelle integriertes System (Acoustic Resonance Monitoring System - ARMS) präsentiert, welches eine schnelle Quantifizierung akustischer Parameter, wie Resonanzfrequenz und Q-Faktor ermöglicht. Im Ergebnisteil wird der photoakustische Methansensor auf Quereinflüsse gegenüber Sauerstoff, Luftfeuchte, Kohlenstoffdioxid sowie Messzellentemperatur und -druck hin evaluiert. Dabei wurden akustische und relaxationsbedingte Effekte als dominierende Einflussgrößen identifiziert. Die Abhängigkeit der photoakustischen Amplitude und Phase von der Effizienz der nicht-strahlenden Relaxation lässt sich mithilfe des entwickelten Algorithmus CoNRad berechnen und kompensieren, wodurch die Zuverlässigkeit der Sensoren gesteigert wird. Das Konzept des digitalen Zwillings kombiniert theoretische Berechnungen, ARMS-Messungen und die emittierte optische Laserleistung, um ein vollst¨andig theoretisch zu erwartendes photoakustisches Signal zu berechnen und gemessene Rohwerte für sämtliche Quereinflüsse zu kompensieren. Dieser Ansatz wurde in einer mehrtägigen Messreihe mit einem Referenzgerät zum atmosphärischen Methanmonitoring evaluiert. Die Ergebnisse verdeutlichen das Potenzial der PAS zur Spurengasdetektion in der Umgebungsluft, unterstreichen aber auch die Notwendigkeit des digitalen Zwillings zur Quereinflusskompensation. Messungen mit einem quartz-enhanced PAS Sensor zeigten ebenfalls relaxationsbedingte Signalveränderungen, die mithilfe der etablierten statistischen Methode der Partial Least Squares Regression (PLSR) und dem digitalen Zwilling kompensiert wurden. Mit beiden Ansätzen konnten vergleichbar gute Ergebnisse erzielt werden und den mittleren relativen Fehler der vorhergesagten Analytkonzentration auf den einstelligen Prozentbereich verringert werden. Darüber hinaus wurde eine Methode entwickelt, um spektrale Quereinflüsse in der Wellenlängen-modulierten PAS zu berechnen. Der Einfluss von spektralen Überlappungen auf das Messsignal wurde zusätzlich für Amplitudenmodulation untersucht. Zusammenfassend befasst sich diese Dissertation vorrangig mit der detaillierte Analyse von relaxationsbedingten, akustischen und spektralen Einflüssen auf das photoakustische Signal. Hierbei werden verschiedene innovative Ansätze zur Quantifizierung und Kompensation dieser Einflüsse präsentiert, mit dem übergeordneten Ziel, die Genauigkeit und Zuverlässigkeit photoakustischer Sensoren in langfristigen Feldanwendungen signifikant zu erhöhen. Die Erkenntnisse dieser Arbeit unterstreichen nicht nur das Potenzial der photoakustischen Spektroskopie zur präzisen Detektion von Spurengasen in der Umgebungsluft, sondern betonen auch die Notwendigkeit von Kompensationsmechanismen. Insgesamt trägt diese Arbeit entscheidend dazu bei, dass photoakustische Sensoren in kontinuierlichen Feldanwendungen eingesetzt werden können

Breath Analysis Using Laser Spectroscopic Techniques: Breath Biomarkers, Spectral Fingerprints, and Detection Limits

Breath analysis, a promising new field of medicine and medical instrumentation, potentially offers noninvasive, real-time, and point-of-care (POC) disease diagnostics and metabolic status monitoring. Numerous breath biomarkers have been detected and quantified so far by using the GC-MS technique. Recent advances in laser spectroscopic techniques and laser sources have driven breath analysis to new heights, moving from laboratory research to commercial reality. Laser spectroscopic detection techniques not only have high-sensitivity and high-selectivity, as equivalently offered by the MS-based techniques, but also have the advantageous features of near real-time response, low instrument costs, and POC function. Of the approximately 35 established breath biomarkers, such as acetone, ammonia, carbon dioxide, ethane, methane, and nitric oxide, 14 species in exhaled human breath have been analyzed by high-sensitivity laser spectroscopic techniques, namely, tunable diode laser absorption spectroscopy (TDLAS), cavity ringdown spectroscopy (CRDS), integrated cavity output spectroscopy (ICOS), cavity enhanced absorption spectroscopy (CEAS), cavity leak-out spectroscopy (CALOS), photoacoustic spectroscopy (PAS), quartz-enhanced photoacoustic spectroscopy (QEPAS), and optical frequency comb cavity-enhanced absorption spectroscopy (OFC-CEAS). Spectral fingerprints of the measured biomarkers span from the UV to the mid-IR spectral regions and the detection limits achieved by the laser techniques range from parts per million to parts per billion levels. Sensors using the laser spectroscopic techniques for a few breath biomarkers, e.g., carbon dioxide, nitric oxide, etc. are commercially available. This review presents an update on the latest developments in laser-based breath analysis

Development, characterization and miniaturization of a trace gas detection system for NO₂ in air based on photoacoustic spectroscopy

This thesis provides a detailed theoretical discussion about common absorption spectroscopy (AS) and, in particular, about photoacoustic spectroscopy (PAS). The physical concepts of signal generation are illustrated in view of amplitude modulation (AM) and wavelength modulation (WM). Furthermore the advantages and disadvantages of the techniques are presented. As a result, PAS was identified to outclass AS, thus it turned out to be the method of choice in view of developing a miniaturized trace gas sensing application. The theoretical part of this work further outlines various approaches of signal enhancement, e.g. by acoustic and/or mechanical resonance amplification. Besides, several phenomena of signal attenuation are addressed, e.g. acoustic detuning, vibrational-translational (VT) relaxation and vibrational-vibrational (VV) energy transfer processes, which have to be considered with regard to the individual measuring conditions. Simulation and experimental chapters illustrate the pre-development and the practical implementation of a laboratory photoacoustic setup, a portable trace gas monitoring device and various photoacoustic cell (PAC) designs. These include a conventional bulky design, an optimized low-cost 3D printed PAC, a miniaturized quartz enhanced photoacoustic spectroscopic (QEPAS) scheme and a further integrated microelectromechanical system (MEMS) based sensor chip, respectively. Although several parts of this thesis also provide preparatory work for multi-component analysis, nitrogen dioxide (NO2) was used as primary analyte in order to characterize the above mentioned photoacoustic cell designs. This involves acoustic resonance and noise analysis, determination of optimal operating parameters (e.g. gas flow rate and lock-in time constant), performance evaluation (e.g. response behavior, optical performance, calibration characteristics and long-term signal stability) as well as interference studies towards oxygen (O2), carbon dioxide (CO2), humidity (H2O) and acoustic noise. In conclusion, NO2 detection by means of the low-cost 3D printed PAC and the QEPAS configuration even revealed two world record detection limits (1sigma) of 33 pptV and 600 pptV, respectively

Optical Gas Sensing: Media, Mechanisms and Applications

Optical gas sensing is one of the fastest developing research areas in laser spectroscopy. Continuous development of new coherent light sources operating especially in the Mid-IR spectral band (QCL—Quantum Cascade Lasers, ICL—Interband Cascade Lasers, OPO—Optical Parametric Oscillator, DFG—Difference Frequency Generation, optical frequency combs, etc.) stimulates new, sophisticated methods and technological solutions in this area. The development of clever techniques in gas detection based on new mechanisms of sensing (photoacoustic, photothermal, dispersion, etc.) supported by advanced applied electronics and huge progress in signal processing allows us to introduce more sensitive, broader-band and miniaturized optical sensors. Additionally, the substantial development of fast and sensitive photodetectors in MIR and FIR is of great support to progress in gas sensing. Recent material and technological progress in the development of hollow-core optical fibers allowing low-loss transmission of light in both Near- and Mid-IR has opened a new route for obtaining the low-volume, long optical paths that are so strongly required in laser-based gas sensors, leading to the development of a novel branch of laser-based gas detectors. This Special Issue summarizes the most recent progress in the development of optical sensors utilizing novel materials and laser-based gas sensing techniques

Laser Spectroscopy for Atmospheric and Environmental Sensing

Lasers and laser spectroscopic techniques have been extensively used in several applications since their advent, and the subject has been reviewed extensively in the last several decades. This review is focused on three areas of laser spectroscopic applications in atmospheric and environmental sensing; namely laser-induced fluorescence (LIF), cavity ring-down spectroscopy (CRDS), and photoluminescence (PL) techniques used in the detection of solids, liquids, aerosols, trace gases, and volatile organic compounds (VOCs)

Review—Non-Invasive Monitoring of Human Health by Exhaled Breath Analysis: A Comprehensive Review

Exhaled human breath analysis is a very promisingfield of research work having great potential for diagnosis of diseases in non-invasive way. Breath analysis has attracted huge attention in thefield of medical diagnosis and disease monitoring in the last twodecades. VOCs/gases (Volatile Organic Compounds) in exhaled breath bear thefinger-prints of metabolic and biophysicalprocesses going on in human body. It’s a non-invasive, fast, non-hazardous, cost effective, and point of care process for diseasestate monitoring and environmental exposure assessment in human beings. Some VOCs/gases in exhaled breath are bio-markers ofdifferent diseases and their presence in excess amount is indicative of un-healthiness. Breath analysis has the potential for earlydetection of diseases. However, it is still underused and commercial device is yet not available owing to multiferrious challenges.This review is intended to provide an overview of major biomarkers (VOCs/gases) present in exhaled breath, importance of theiranalysis towards disease monitoring, analytical techniques involved, promising materials for breath analysis etc. Finally, relatedchallenges and limitations along with future scope will be touched upon.will be touched upon

Photoacoustic spectroscopy for multi-gas sensing using near infrared lasers

Photoacoustic spectroscopy is a widely recognised technique to measure trace gases at parts-per-million (ppm) or parts-per-billion (ppb) level using semiconductor laser in the near infrared range. This technique is based on the generation of an acoustic wave in a gas excited by a modulated laser beam at a wavelength corresponding to a absorption line of the gas species, and on the detection of this sound using a sensitive microphone. Various sensors have been developed in the past decades in the field of atmospheric pollution monitoring, in the semiconductor industries, in medical applications and in life science applications. This work aims at presenting the development of a new sensor for multi-gas detection at sub-ppm level using distributed feedback (DFB) semiconductor lasers developed for the optical telecommunication market in the near infrared range. A novel resonant photoacoustic cell that consists in three resonators was designed and characterised. The sensor was developed to monitor up to three different gases for the monitoring of microclimatic parameters of living organisms and for the manufacturing of the next generation of optical fibres used in the optical telecommunication network application. The buffer gases used in these two applications are extremely different and have a very important impact on the calibration of the photoacoustic sensor. In particular, effects of the physical gas parameters on the photoacoustic signal are theoretically and experimentally compared. Relaxation effects related to these different buffer gases were observed in particular situations and gave rise to drastic changes in the photoacoustic response. A model is developed and quantitatively compared with experimental data. Finally, the sensitivity of the sensor is an important parameter, since many applications require detection limits down to ppb levels. The use of an Erbium-doped fibre amplifier made ammonia detection at concentration of 2.4 ppb possible. Ammonia monitoring with typical ambient concentration of water vapour and carbon dioxide at atmospheric pressure could be successfully achieved using an innovative approach

Development of a direct monitoring approach of CO/H₂ in bioreactors for syngas to biofuels conversion

Syngas fermentation is a novel technology to produce biofuels from syngas using acetogens. Precise monitoring of dissolved CO/H₂ concentrations helps to maintain the proper activities of the acetogens and is of great importance for a high product yield and operation stability. A new approach was developed to measure the dissolved CO/H₂ concentration with optical sensing and electrochemical methods, respectively.An indirect mid-infrared measurement system was developed to measure dissolved CO concentration. The measurement system included a gas extraction system for dissolved CO degasification and a non-dispersive mid-infrared CO sensor for gas phase CO concentration measurement. The gas extraction system used a hollow fiber membrane contactor to extract dissolved CO in a fixed volume of liquid sample. A vacuum was applied to the contactor to accelerate the degasification. The non-dispersive mid-infrared CO sensor used a room-temperature, mid-infrared light emitting diode as a mid-infrared source. The sensor was designed to detect CO absorption in the mid-infrared range between 4.7 µm and 4.9 µm as this range has minimal spectral interference from other chemicals in the syngas fermentation. The infrared signal was detected with a thermoelectric cooled photodetector. A digital lock-in amplifier was designed to improve the quality of the collected signals from the preamplifier.A commercial electrochemical dissolved H₂ sensor was introduced to measure the dissolved H₂ concentration. An integrated dissolved CO/H₂ measurement system was built by integrating the dissolved H₂ sensor to the developed dissolved CO measurement system. Performance evaluation of the integrated measurement system showed a maximum mean absolute percentage error of 10.83% and a root-mean-square error of 1.40 mg/L for the dissolved CO concentration measurement. The measurement on the dissolved H₂ concentration measurement was affected by the supersaturated dissolved H₂.The developed integrated measurement system provides a new method that can measure the dissolved CO/H₂ concentrations in the syngas fermentation medium. The method would be helpful to future optimization and commercialization of the syngas fermentation technology

Optoelectronics – Devices and Applications

Optoelectronics - Devices and Applications is the second part of an edited anthology on the multifaced areas of optoelectronics by a selected group of authors including promising novices to experts in the field. Photonics and optoelectronics are making an impact multiple times as the semiconductor revolution made on the quality of our life. In telecommunication, entertainment devices, computational techniques, clean energy harvesting, medical instrumentation, materials and device characterization and scores of other areas of R&D the science of optics and electronics get coupled by fine technology advances to make incredibly large strides. The technology of light has advanced to a stage where disciplines sans boundaries are finding it indispensable. New design concepts are fast emerging and being tested and applications developed in an unimaginable pace and speed. The wide spectrum of topics related to optoelectronics and photonics presented here is sure to make this collection of essays extremely useful to students and other stake holders in the field such as researchers and device designers

Ppbv-Level Ethane Detection Using Quartz-Enhanced Photoacoustic Spectroscopy with a Continuous-Wave, Room Temperature Interband Cascade Laser

A ppbv-level quartz-enhanced photoacoustic spectroscopy (QEPAS)-based ethane (C2H6) sensor was demonstrated by using a 3.3 μm continuous-wave (CW), distributed feedback (DFB) interband cascade laser (ICL). The ICL was employed for targeting a strong C2H6 absorption line located at 2996.88 cm−1 in its fundamental absorption band. Wavelength modulation spectroscopy (WMS) combined with the second harmonic (2f) detection technique was utilized to increase the signal-to-noise ratio (SNR) and simplify data acquisition and processing. Gas pressure and laser frequency modulation depth were optimized to be 100 Torr and 0.106 cm−1, respectively, for maximizing the 2f signal amplitude. Performance of the QEPAS sensor was evaluated using specially prepared C2H6 samples. A detection limit of 11 parts per billion in volume (ppbv) was obtained with a 1-s integration time based on an Allan-Werle variance analysis, and the detection precision can be further improved to ~1.5 ppbv by increasing the integration time up to 230 s