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overview, attention is focused onto the description of Photoacoustic Spectroscopy in devices and equipment; they determine the detection limits Trace Gas Monitoring and selectivity. Applications are discussed with emphasis on environmental monitoring, medical applications and biological applications (such as post-harvest physiology, Frans J.M. Harren, Julien Mandon, plant physiology, microbiology, and entomology). Simona M. Cristescu Molecular and Laser Physics, Radboud University Nijmegen, Nijmegen, The Netherlands 1 INTRODUCTION A gaseous molecule that absorbs electromagnetic radi- 1 Introduction 1 ation is excited to a higher electronic, vibrational, or 2 Historical Overview 1 rotational quantum state. Generally, depopulation of this 3 Devices and Equipment 2 quantum state to lower lying states occurs either via 3.1 Nonlaser Light Sources 2 fluorescence or via collisions; the latter gives rise to a temperature increase of the gas due to energy transfer to 3.2 Laser Light Sources 4 translation. This nonradiative relaxation process occurs 3.3 Photoacoustic Cells 5 when the relaxation time can compete with the radiative 3.4 Photoacoustic Detectors 6 lifetime of the excited energy levels. Radiative decay has 3.5 Special Designs 9 a characteristic lifetime of 10−7 s at visible wavelengths − 3.6 Sensitivity, Selectivity, Limitations, as compared to 10 2 seconds at 10 µm. For nonradiative Interference, Detection Limits 9 decay these values depend on the pressure (decay time τ 4 Applications 12 inversely proportional to the pressure) and can vary strongly at atmospheric pressures (10−3 –10−8 s). 4.1 Atmospheric Applications 12 By modulating the radiation source at an acoustic 4.2 Chemical Detection of Warfare Agents frequency, the temperature changes periodically, giving and Explosives 14 rise to a periodical pressure change. The modulated 4.3 Biological and Agricultural Applications 14 pressure will result in an acoustic wave, which can be 4.4 Human Health, Noninvasive Breath detected with a sensitive microphone. The amplitude of Analysis 18 the detected sound is proportional to the concentration 5 Comparison with other Spectroscopic of the probed molecules. Laser-based photoacoustic Methods in Trace Gas Monitoring 19 detectors are able to monitor trace gas concentrations Acknowledgments 20 at atmospheric conditions with orders of magnitude Abbreviations and Acronyms 20 better sensitivity as compared to conventional scientific instrumentation; in addition, they are able to monitor Related Articles 20 noninvasively and on-line under dynamic conditions. References 20 2 HISTORICAL OVERVIEW Gas phase spectroscopy is nowadays very common in a wide variety of fields next to chemistry and physics. The photoacoustic effect was first reported by Alexander From research involving living organisms to air pollution Graham Bell in 1880(1); he discovered that thin discs monitoring, spectroscopic gas sensors have proven to be emitted sound when exposed to a rapidly interrupted indispensable tools. There are various ways of utilizing beam of sunlight. In a later experiment,(2) he removed gas sensors, and each application has different demands. the eye piece of a commercial spectroscope and placed Some applications require a very high sensitivity for one absorbing substances at the focal point of the instrument. specific gas compound, while others benefit more from The substances were put in contact with the ear by sensors able to measure a wide range of gases. A high time means of a hearing tube (Figure 1) and he found resolution is also desirable, as well as selectivity, robustness, ‘good’ sounds in all parts of the visible and invisible and little or no need for sample preparation. This paper electromagnetic spectrum of the sun. Other publications discusses photoacoustic spectroscopy as a sensitive, on- on this phenomenon followed this first work; we mention line and non-invasive tool to monitor the concentration here the works of Rontgen,¨ (3) Tyndall,(4) and Preece.(5) of trace gases. After a short introduction and a historic However, owing to the lack of a quantitative description Encyclopedia of Analytical Chemistry, Online 2006–2012 John Wiley & Sons, Ltd. This article is 2012 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Analytical Chemistry in 2012 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a0718.pub2 2 ENVIRONMENT: TRACE GAS MONITORING Figure 1 (a) The eye piece of a spectroscope is removed and the substances are placed in the focal point of the instrument behind a slit. These substances are put in contact with the ear by means of a hearing tube.(2) (b) Sunlight is modulated with chopper B and focused onto a glass bulb A. and the lack of sensitive microphones, the interest in the of broadband infrared lamps is orders of magnitude photoacoustic effect soon declined. lower as compared to lasers, they have substantial In 1938, Viegerov refined the photoacoustic technique advantages such as wide wavelength coverage, reliability, for the first spectroscopic gas analysis(6); thereafter and cost effectiveness. Such infrared lamp sources Pfund(7) and Luft(8) measured trace gas absorption in combination with various photoacoustic detection spectra with an infrared broadband light source down schemes are commercially available for trace gas to the part per million level. By the end of the detection at ppmv levels for a wide range of molecular 1960s, after the invention of laser, the scientific interest gasses such as CO, CO2,NO,N2O, SO2,CH4,C2H4, and (9) expanded again. In 1968, Kerr and Atwood utilized C3H8. Such design is a direct result of the developments laser photoacoustic detection to obtain the absorption made by Luft and coworkers in the 1950s and 1960s.(8,12) spectrum of small gaseous molecules. Owing to the The infrared-lamp-based gas analyzer uses a photoa- high spectral brightness of lasers and improved phase- coustic detection scheme that is able to detect a specific sensitive lock-in techniques to amplify the acoustic signal, gas out of a multicomponent gas mixture avoiding cross they were able to determine low concentrations of air interferences. In this instrument, selectivity is achieved by pollutants. Kreuzer(10) demonstrated that it was possible comparing the direct absorption in a sample cell to that to detect concentrations of 10 ppbv (1ppbv = 1:109) in a reference cell (Figure 2). After passing the sampling of methane in nitrogen, using an intensity-modulated cells, each attenuated light beam enters a second detec- µ infrared (3 m) He–Ne laser. Patel demonstrated the tion cell, filled only with the gas of interest (e.g. CO2); potential of the technique by measuring the NO and the detection cells are interconnected via a membrane H2O concentrations at an altitude of 28 km with a connected to a capacitance. Since the dual beam is modu- balloon-borne spin-flip Raman laser.(11) From hereon, lated by a chopper, the difference in acoustic energy the photoacoustic effect was introduced into the field of reflects the difference in absorption and thus the concen- trace gas detection with all its environmental, biological, tration difference between sample cell and reference cell. and medical applications. Selection of wavelength occurs by the species under inves- tigation itself in such a way that all wavelengths at which absorption occurs are simultaneously active. When there 3 DEVICES AND EQUIPMENT is no spectral overlap from other gases, additional absorp- tions in the sample cell will not contribute to the acoustic signal; the light passes the detection cell unattenuated. 3.1 Nonlaser Light Sources When a specific compound, e.g. H2O, causes spectral Lasers are not essential to operate photoacoustic gas overlap, an extra cell can be placed in the light path filled detection systems. Although the spectral power density with the interfering gas. This cell completely attenuates Encyclopedia of Analytical Chemistry, Online 2006–2012 John Wiley & Sons, Ltd. This article is 2012 John Wiley & Sons, Ltd. This article was published in the Encyclopedia of Analytical Chemistry in 2012 by John Wiley & Sons, Ltd. DOI: 10.1002/9780470027318.a0718.pub2 PHOTOACOUSTIC SPECTROSCOPY IN TRACE GAS MONITORING 3 Equalizer optical filters in a rotation cartwheel before the light passes through the photoacoustic gas sampling cell, the Membrane M1 specific infrared wavelength is selected at which the gas of interest has its strongest absorption bands. Such a system can measure concentrations of up to five component Filter gases and water vapor in any air sample. Detection limit is gas dependent, but is typically in the sub- IR source ppmv region. Such instruments require no consumables Splitter Chopper M2 Reference and very little maintenance, and are therefore ideally Figure 2 Infrared gas analyzer with photoacoustic detection suited for permanent monitoring tasks (environment and scheme to detect a specific gas out of a multicomponent gas industrial). mixture thereby avoiding cross interference (ABB GmbH, Instead of using optical filters, if photoacoustic Frankfurt). Light from the infrared source is split into two paths. detection is also combined with Fourier transformed The chopper modulates the intensity for both paths. The filter infrared (FTIR) spectroscopy higher spectral resolution volume in each path serves to filter out light of wavelengths not needed for the detection process; they can be filled with gases, can be achieved. FTIR is a widely used method for the absorption spectra of which do not overlap with those of obtaining broad infrared spectra of a sample. Infrared the species under scrutiny. M1 and M2 serve as measuring cell radiation is split in a Michelson interferometer where and reference cell, respectively. With the help of the equalizer, half of the light passes through to a fixed mirror and both light intensities become equal before entering the last cell. the other half is reflected toward a moving mirror. The The last cell consists of two compartments with a membrane in between. Both compartments are filled with the gas under two beams recombine and pass through a gas cell where investigation so that all wavelengths characteristic for this gas the sample absorbs light at molecule-specific frequencies.
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