Angela Elia

Photoacoustic Techniques for Trace Gas Sensing Based on Semiconductor Laser Sources

The paper provides an overview on the use of photoacoustic sensors based on semiconductor laser sources for the detection of trace gases. We review the results obtained using standard, differential and quartz enhanced photoacoustic techniques.

Photoacoustic detection of nitric oxide by use of a quantum-cascade laser

A photoacoustic trace-gas sensor for the measurement of nitric oxide with a detection limit of 500 parts in 10(9) has been demonstrated. The radiation source was a thermoelectrically cooled distributed-feedback quantum-cascade laser operating in pulsed mode near 5.3 microm with an average laser power of 8 mW. A resonant photoacoustic cell was excited in its first longitudinal mode by the modulated laser light. Preliminary measurements have been performed to test the performance of our photoacoustic sensor; possible improvements to reach lower detection limits are discussed.

Quantum Cascade Laser-Based Photoacoustic Sensor for Trace Detection of Formaldehyde Gas

We report on the development of a photoacoustic sensor for the detection of formaldehyde (CH2O) using a thermoelectrically cooled distributed-feedback quantum cascade laser operating in pulsed mode at 5.6 μm. A resonant photoacoustic cell, equipped with four electret microphones, is excited in its first longitudinal mode at 1,380 Hz. The absorption line at 1,778.9 cm−1 is selected for CH2O detection. A detection limit of 150 parts per billion in volume in nitrogen is achieved using a 10 seconds time constant and 4 mW laser power. Measurements in ambient air will require water vapour filters.

Optical and Electronic NOx Sensors for Applications in Mechatronics

Current production and emerging NOx sensors based on optical and nanomaterials technologies are reviewed. In view of their potential applications in mechatronics, we compared the performance of: i) Quantum cascade lasers (QCL) based photoacoustic (PA) systems; ii) gold nanoparticles as catalytically active materials in field-effect transistor (FET) sensors, and iii) functionalized III-V semiconductor based devices. QCL-based PA sensors for NOx show a detection limit in the sub part-per-million range and are characterized by high selectivity and compact set-up. Electrochemically synthesized gold-nanoparticle FET sensors are able to monitor NOx in a concentration range from 50 to 200 parts per million and are suitable for miniaturization. Porphyrin-functionalized III-V semiconductor materials can be used for the fabrication of a reliable NOx sensor platform characterized by high conductivity, corrosion resistance, and strong surface state coupling.

Advanced NOx Sensors for Mechatronic Applications

Vehicle emissions represent an increasing contributor to air pollution in urban and country areas which gives rise to a range of environmental problems related to air quality. This has led to increasingly stringent regulatory laws on exhaust emissions level and composition. The US regulations, signed in Dec. 2000, set new standards for measurement of automotive exhaust emissions. The emission limits for regulated species, such as carbon oxides (CO and CO2), hydrocarbons (THC), and nitrogen oxides, i.e. NO, NO2, N2O (referred as NOx), are being reduced, and new components such as methanol and formaldehyde are being added to the list of monitored species. In Japan, diesel emission standards require that in-use onroad light commercial vehicles should meet NOx emission of 0.25 g/km starting from the end of 2005 and achieve full implementation by 2011 (Nakamura et al., 2006). The European Commission has also introduced a series of regulations, the so called EURO Emission directives, from Euro I in 1991 to tighter limitations in 2009 (Euro V 80 g/km) and 2014 (Euro VI 0.08 g/Km), to meet the air quality standards stated by the international agencies (van Asselt & Biermann, 2007). The standard and validated on-line technologies for regulated emissions are effective at monitoring few components, but are limited in their use for measuring other gases. Single-wavelength non-dispersive infrared filters for CO and CO2 monitoring cannot be used for other species due to interferences from water and other molecules. Chemiluminescence analyzers, which traditionally are used to measure NOx compounds, cannot differentiate NO from NO2 in the same test, nor identify the other NOx gases. Flame ionization detectors cannot differentiate individual hydrocarbons. To measure raw exhaust, each technique requires a cold trap for water vapour, which can affect the concentrations of other gases. In addition, calibrations are necessary for each analysis, avoiding on-line and real-time emission monitoring. To sample other gases in the exhaust, bag samples must be collected and taken to a laboratory for further analysis. Expensive dilution equipment must be used to prevent water condensation in the bag. Each bag sample gathers exhaust for several minutes, therefore the final result of the test is an integrated average of the gas concentrations and all time resolution is lost. Methanol and formaldehyde samples are collected with impingers, which dissolve the gases by passing them through a water-based solution. The extract is then analyzed using high-performance liquid chromatography (HPLC). Gas chromatography

Quantum cascade laser technology for the ultrasensitive detection of low-level nitric oxide.

Several spectroscopic methods based on mid-infrared quantum cascade lasers for the ultrasensitive detection of nitric oxide have been developed with detection limit in ppbv and sub-ppbv range. We will describe here a selection of the most effective techniques, i.e., laser absorption spectroscopy, cavity-enhanced spectroscopy, photoacoustic spectroscopy, and Faraday modulation spectroscopy. For each technique, advantages and drawbacks will be underlined.

Photoacoustic trace gas sensing with mid-IR quantum cascade lasers

We report on the realization of an optoacoustic sensor based on mid-infrared quantum-cascade lasers (QCLs) for the detection of nitric oxide (NO) and formaldehyde (CH2O). A resonant photoacoustic cell equipped with 4 electret microphones was excited in its first longitudinal mode by the modulated laser light. A detection limit of 150 parts in 109 (ppbv) for NO and CH2O was found, using distributed feedback QCLs operating in pulsed mode at 5.34 µm and 5.62 µm, respectively.

Quantum-cascade-laser-based optoacoustic detection: application to nitric oxide and formaldehyde

The detection and quantification of trace gases is of great interest in a wide range of applications such as environmental monitoring, industrial process control and medical diagnostics. In combination with quantum cascade lasers, photoacoustic spectroscopy offers the advantage of high sensitivity (parts per billion detection limits), compact set-up, fast time-response and simple optical alignment. We will report here on the design and fabrication of optoacoustic sensors based on two different cell configurations to detect nitric oxide and formaldehyde.

Trace gas sensing using quantum cascade lasers and optoacoustic detection

We report on the realization of a optoacoustic sensor based on mid-infrared quantum-cascade lasers (QCLs) for the detection of nitric oxide (NO) and formaldehyde (CH2O). A resonant photoacoustic cell equipped with 4 electret microphones was excited in its first longitudinal mode by the modulated laser light. A detection limit for of 300 parts in 109 (ppbv) for NO and 150 ppbv for CH2O is found, using distributed feedback QCLs operating in pulsed mode at 5.34 μm and 5.6 μm, respectively.