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Photoacoustic Spectroscopy, Methods and Instrumentation☆
Photoacoustic Spectroscopy, Methods and Instrumentation☆ MW Sigrist, ETH Zu¨rich, Zurich, Switzerland ã 2017 Elsevier Ltd. All rights reserved.
Introduction Since the discovery of the photoacoustic (PA) effect by Bell in 1880 who used the Sun as radiation source, a foot-operated chopper for modulation and an earphone as acoustic detector, the PA effect has found numerous applications as a sensitive and rather simple technique for determining optical, thermal and mechanical properties of all kinds of samples. This article focuses on methods and instrumentation employed in spectroscopic applications. Since photothermal spectroscopy is discussed in another section, those schemes are only briefly mentioned here whereas emphasis is put on instrumentation used in photoacoustic spectroscopy. The spectroscopic applications of the PA effect are not discussed here but only the technique itself, its characteristics and implementation. Although the PA method has been around for more than a century it is particularly the advent of lasers as radiation sources with high spectral brightness that has initiated a renaissance of the PA effect. In the meantime a great variety of experimental schemes have been developed and render the PA method a very versatile spectroscopic tool.
The properties of piezoelectric transducers and microphones as pressure sensors are discussed below. If the use of pressure sensors is not appropriate, because, e.g., a piezoelectric transducer cannot be attached to the sample or measurements need to be done at high temperatures which hinders the application of microphones, noncontact techniques are to be applied. As shown in Figure 1(c) the induced spatial and temporal gradient of the refractive index can be sensed in this case by monitoring the deflection of a
Photoacoustic and Photothermal Schemes Experimental Arrangements As the photoacoustic (and related photothermal (PT)) phenomena comprise a large diversity of facets there exist various detection techniques which rely on the acoustic or thermal disturbances caused by the absorbed radiation. The selection of the most appropriate scheme for a given application depends on the sample, the sensitivity and specificity to be achieved, ease of operation, ruggedness, and any requirement for non-contact detection, e.g. in aggressive media or at high temperatures and/or pressures. Figures 1–3 present the most typical arrangements applied for solid, liquid and gaseous samples, respectively. Experimental schemes for PA studies on solid samples include the measurement of the generated pressure wave either directly in the sample with a piezoelectric sensor for the pulsed regime, or indirectly in the gas which is in contact with the sample by a microphone. These most widely used setups are depicted in Figure 1(a) and 1(b). The indirect detection of the generated acoustic wave in the gas phase with the microphone is inevitable if a direct contact with the sample is not readily possible, e.g. for samples such as powders, gels or grease. ☆
Change History: August 2014. MW Sigrist updated the text further readings to this entire article. Markus W. Sigrist, Photoacoustic Spectroscopy, Methods and Instrumentation, In Encyclopedia of Spectroscopy and Spectrometry (Second Edition), edited by John C. Lindon, Academic Press, Oxford, 1999, Pages 2146-2150.
598
Figure 1 Typical experimental arrangements used for photoacoustic (PA) and photothermal (PT) studies on solids. As indicated one differentiates between modulated or pulsed incident radiation. (a) Indirect PA detection by microphone in the gas phase. (b) Direct PA detection with PZT transducer or PVDF foil attached to solid. (c) PT sensing of the gradient of the refractive index with probe beam deflection in the (transparent) sample (probe beam 1), or above the sample surface (Mirage effect, probe beam 2). Monitoring of the generated surface displacement (probe beam 3) or of the change of surface reflectivity (probe beam 4). (d) PT radiometry senses the induced change of the IR radiation that is radiated off the sample surface.
Encyclopedia of Spectroscopy and Spectrometry, Third Edition
http://dx.doi.org/10.1016/B978-0-12-409547-2.11308-3
Photoacoustic Spectroscopy, Methods and Instrumentation
probe beam either within the (transparent) sample or directly above the (plane) sample surface (PT beam deflection or so-called mirage effect). Changes of the surface reflectivity or slight deformations of the surface (PT beam displacement) can also be detected in a non-contact manner by a probe beam. Finally, as depicted in Figure 1(d), variations of the thermal radiation from the surface can be monitored with an infrared detector (PT radiometry). This method is of particular interest for measurements at elevated temperatures owing to the increased radiation intensity according to Stefan–Boltzman’s law. Still other techniques include pyroelectric detection in thin films, thermal lensing and interferometric methods. The typical experimental arrangement for absorption spectroscopy in weakly absorbing liquids is shown schematically in Figure 2(a). The beam of a pulsed tunable laser is directed through the PA cell that contains the sample under study. The
Figure 2 Typical experimental arrangements used for PA detection in liquids. (a) PZT detection of aoustic wave generated by pulsed radiation in weakly absorbing liquid. (b) Optothermal window setup applied for studies on strongly or opaque liquids with modulated or pulsed radiation.
599
generated acoustic waves are detected by a piezoelectric transducer with fast response time. Usually just the first peak of the ringing acoustic signal is taken and further processed. Pulse-topulse variations of the laser power are accounted for by normalizing the piezoelectric signal with the laser power measured with the power meter after the cell. Another area of interest is the measurement of opaque or strongly absorbing liquids. A simple open PA cell called an optothermal window was developed for this purpose as displayed in Figure 2(b). It essentially consists of an uncoated ZnSe window to which an annular lead zirconate titanate (PZT) piezotransducer is glued from the bottom. The excitation beam from a laser passes unobstructedly through this window and is absorbed by a droplet of the sample deposited on the other side of the ZnSe disc. The generated heat diffuses into the disk which expands. The induced stress is then recorded by the PZT transducer. Unlike in conventional transmission spectroscopy where the cell thickness is the restricting factor in dealing with strongly absorbing samples, the magnitude of the optothermal window signal depends solely on the product between the absorption coefficient and the thermal diffusion length whereby the latter can be adapted via the modulation frequency. The typical setup for gas phase measurements is shown in Figure 3(a). A tunable laser with narrow linewidth or a conventional (broadband) radiation source followed by optical filters is used. In general, amplitude-modulated (or sometimes pulsed) or wavelength-modulated (WM) radiation is directed through the PA cell. The acoustic sensor is usually a commercial electret microphone or a condenser microphone. These devices are easy to use and sensitive enough for trace gas studies with very low absorptions. Often, the detection threshold is neither determined by the microphone responsivity Rmic itself nor by the electrical noise but rather by other sources (absorption by desorbing molecules from the cell walls, window heating, ambient noise, etc.). However, if this latter background is known from reference measurements, the ultimate detection sensitivity is determined solely by fluctuations of the radiation intensity, and by microphone and amplifier noise.
Figure 3 Typical experimental PA and PT arrangements used for gas monitoring with tunable laser sources. (a) PA detection with conventional microphone in resonant gas cell for modulated cw radiation or in non-resonant cell for pulsed radiation. (b) Noncontact refractive index sensing schemes with displaced collinear or transverse probe beam (PA deflection, 1), thermal lensing (2), or collinear probe beam (PT deflection, 3).
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Photoacoustic Spectroscopy, Methods and Instrumentation
The frequency dependence of Rmic is usually rather small and the temperature dependence may have to be taken into account in special cases only. If modulated radiation is employed the microphone signal is fed to a lock-in amplifier locked to the modulation frequency. Since according to theoretical considerations the microphone signal amplitude is proportional to the absorbed power for weakly absorbing media, the average radiation power is recorded simultaneously by a power meter for normalization. In more recent times, new schemes have been introduced, in particular quartz-enhanced photoacoustic spectroscopy (QEPAS) and cantilever-enhanced photoacoustic spectroscopy (CEPAS). If pulsed radiation is employed the microphone bandwidth is often not sufficient to resolve the temporal shape of the generated acoustic pulses. However, common microphones can still be used even for nanosecond laser pulses because the length of a single acoustic pulse is essentially determined by the transit time of the acoustic wave across the beam radius. Normalized PA amplitudes are obtained by dividing the microphone signal peaks by the corresponding laser pulse energy that is recorded by a sensor such as a pyroelectric detector. Averaging over several pulses improves the signal-to-noise ratio. Another approach for pulsed radiation consists of using an acoustic resonator with high Q-factor as gas cell, recording the microphone signals in the time domain but analyzing the PA signal amplitudes after Fourier transformation in the frequency domain. The excited cell resonances then appear in the PA frequency spectra. An important issue for many applications concerns the calibration of the entire PA or PT detection system. Since the PA signal depends on many factors that are not known with sufficient accuracy, a straigthforward calibration is often achieved by employing a reference sample with known absorption. As example, certified gas mixtures (trace gas diluted in a nonabsorbing buffer gas) or well characterized dye solutions in the case of liquids are used. The situation is more difficult with solid and biological samples, particularly layered media, powders, gels or tissue. In such cases, quantitative data are difficult, if not impossible, to obtain. But even qualitative instead of quantitative spectra are often valuable especially when other spectroscopic techniques fail owing to opaqueness or strong scattering of the sample. It should be emphasized that numerous different versions and modifications of these general schemes have been presented in the literature such as QEPAS and CEPAS mentioned above. Furthermore, combinations of conventional methods, such as Fourier-transform IR (FTIR) or gas-chromatography (GC), with PA detection have been reported. Some types of PA detection schemes are also implemented in commercial spectrometers. In the following the different components of PA spectrometers are briefly discussed.
Radiation Sources In commercial PA spectrometers, incoherent sources such as incandescent lamps are employed in combination with filters or with an interferometer, or also light-emitting diodes (LEDs). Devices equipped with a small light bulb with either a chopper or direct current modulation as modulated radiation source
and appropriate filters to avoid absorption interferences with other species are used as compact gas sensors, e.g. for indoor CO2 monitoring. However, since the generated PA signal is proportional to the absorbed (and thus to the incident) radiation power, powerful radiation sources, particularly lasers offering high spectral brightness, are advantageous for achieving high detection sensitivity and selectivity in spectroscopic applications. In the UV and visible spectral range, excimer and dye lasers have been employed whereas in the mid-infrared (fundamental or midIR) wavelength range line-tunable CO2 and CO lasers have dominated the applications for long times. Near infrared diode lasers with sufficient power for PAS are often used for monitoring overtones and combination bands of molecular fundamental absorptions. Widely tunable narrowband allsolid-state laser devices in the mid-IR region for accessing the (much stronger) fundamental absorptions include optical parametric oscillators (OPO’s) and difference frequency generation (DFG) in nonlinear crystals. Furthermore, near- to midinfrared interband cascade lasers (ICLs) and mid-infrared tunable quantum cascade lasers (QCLs, often with external cavity) represent very interesting sources for compact sensing devices. Most recent and ongoing developments of tunable midinfrared lasers such as diode-pumped lead salt vertical extended cavity surface emitting lasers (VECSELs), continue to change the situation.
Modulation Schemes Modulation schemes can be separated into the modulation of the incident radiation and the modulation of the sample absorption itself. The first technique includes the most widely used amplitude modulation (AM) of continuous radiation by mechanical choppers, electro-optic or acousto-optic modulators as well as the modulation of the source emission itself by current modulation or pulsed excitation. In comparison to AM, frequency- (FM) or wavelength- (WM) modulation of the radiation may improve the detection sensitivity by eliminating the continuum background caused by a wavelength-independent absorption, e.g., of the cell windows, known as window heating. This type of modulation is obviously most effective for absorbers with narrow linewidth and most easily performed with radiation sources whose wavelength can rapidly be tuned within a few wavenumbers. Pulsed excitation is often applied for liquids but is also of interest for gaseous samples because it permits time gating and the excitation of acoustic resonances. In certain cases the modulation of the absorption characteristics of the sample itself is advantageous. In gas studies the Stark or Zeeman effect has been employed, i.e., by applying a modulated electric or magnetic field to the sample. The result is a suppression of the continuum background and an enhancement of detection selectivity in multicomponent samples because, e.g., Stark modulation only affects molecules with a permanent electric dipole moment like ammonia (NH3) or nitric oxide (NO) while other, possibly interfering molecules are not affected. Finally, combinations of both amplitude and sample absorption modulation have been successfully applied, e.g., for the sensitive detection of ammonia in the presence of absorbing water vapor and carbon monoxide.
Photoacoustic Spectroscopy, Methods and Instrumentation
The scheme of QEPAS is based on the modulation of the source whose beam is guided through the gap between the prongs of a quartz tuning fork (QTF), filled with the sampling gas. The prongs are bent in opposite directions by the generated acoustic wave resulting in a piezoelectric signal which is normally recorded at the resonance frequency of typically 32 kHz.
Photoacoustic Cell Designs The PA cell serves as a container for the sample under study and for the microphone or some other device for the detection of the generated acoustic wave. An optimum design of the PA cell represents a crucial point when background noise ultimately limits the detection sensitivity. In particular for trace gas applications, many cell configurations have been presented including acoustically resonant and nonresonant cells, single- and multipass cells, as well as cells placed intracavity. Nonresonant cells of small volume are mostly employed for solid samples with modulated excitation or for liquids and gaseous samples with pulsed laser excitation. As unique example, we have developed at our laboratory a small-volume cell equipped with a “tubular” acoustic sensor consisting of up to 80 single miniature microphones. These microphones are arranged in eight linear rows with ten microphones in each row. The rows are mounted in a cylindrical geometry parallel to the exciting laser beam axis and located on a circumference around the axis. This configuration is thus ideally adapted to the geometry of the generated acoustic waves. Resonant cells in combination with modulated excitation are normally applied for gas monitoring. These cells are operated on longitudinal, azimuthal, radial, or Helmholtz resonances. The signal enhancement by the Q-factor (usually >100) is often advantageous. Resonance frequencies lie in the kHz range resulting in resonance widths of a few Hz. Furthermore, the gas handling for the cell can be designed in such a way that the gas in- and outlets are located at pressure nodes of the acoustic resonance which allows measurements in flowing gas with flow rates on the order of 1 l min 1 without increasing the noise level. Finally, cells developed for special purposes have been suggested like windowless cells equipped with acoustic baffles to reduce the influence of the ambient noise or heatable cells for studying liquid samples with low volatility. Quartz-enhanced photoacoustic spectroscopy (QEPAS) can be operated in the open air without cell although resonant microtubes are often added for improved performance. Cantilever-enhanced photoacoustic spectroscopy (CEPAS) generally uses a longitudinal cell.
601
For studies in the gas phase, commercial microphones are employed. These include miniature electret microphones such as Knowles or Sennheiser models with typical responsivities Rmic of 10–20 mV Pa 1 as well as condenser microphones, e.g. Bru¨el and Kjær models with typical Rmic of 100 mV Pa 1. Usually Rmic depends only weakly on frequency. The electret microphones produced for hearing aids exhibit a rather flat frequency response between, say, 20 Hz and 20 kHz whereas the bandwith Dn of condenser microphones extends to frequencies of 100 kHz. All these microphones are thus well suited for typical modulation frequencies in the 100 Hz to kHz range. For pulsed applications, the general relation between Rmic and Dn, as well as the occurrence of external noise implies a reduction of the signal-to-noise ratio for very large bandwidths so that miniature electret microphones are often appropriate detectors also in this case. The detection sensitivity can be enhanced by adding the signals of several microphones. In such a configuration the signal increases with the number of microphones used whereas the microphone random noise only increases with the square root of their number. Since electret microphones are small and cheap, a number of them can be arranged in a still compact geometry. A further improvement of sensitivity is expected from the insertion of an electrical filter that cuts the low-frequency components below, say, 1 kHz of the signal because these components contribute less to the increase of the signal-tonoise ratio than the higher-frequency components. Finally, an adaption of the frequency response of the microphone pre- and amplifier stages to that of the microphone is advantageous to fully exploit all the sensed frequency contributions except noise components at frequencies not contributing to the acquired signal. While the QEPAS scheme uses the tuning fork as sensing device, the CEPAS method relies on a cantilever whose bending by the acoustic wave generated in the cell is recorded with a diode laser-based interferometer. If there is a need for noncontact detection, refractive index sensing, notably thermal lensing and both PA and PT deflection, are employed. These methods use a pump beam and a probe beam (HeNe or diode laser) in either collinear or transverse arrangement as shown in Figure 3(b). In comparison to the conventional PA method with pressure sensors, these schemes offer similar sensitivity but require a somewhat more sophisticated setup and imply a more difficult calibration.
See also: Laser Induced Optoacoustic Spectroscopy; Photoacoustic Spectroscopy, Applications; Photoacoustic Spectroscopy, Theory.
Detection Sensors As mentioned above the acoustic disturbances generated in the sample are detected by some kind of pressure sensors. In contact with liquid or solid samples these are piezoelectric devices such as lead zirconate titanate (PZT), LiNbO3 or quartz crystals with a typical responsivity R in the range of up to Volts/ bar or thin polyvinilydene-difluoride (PVF2 or PVDF)-foils with lower responsivity R. These sensors offer fast response times and are thus ideally adapted for pulsed photoacoustics.
Further Reading Almond DP and Patel PM (1996) Photothermal Science and Techniques. London: Chapman & Hall. Mandelis A (ed.) (1992) Principles and Perspectives of Photothermal and Photoacoustic Phenomena. In: Progress in Photothermal and Photoacoustic Science and Technology, vol. 1. New York: Elsevier. Meyer PL and Sigrist MW (1990) Rev. Sci. Instrum. 61: 1779. Pao Y-H (ed.) (1977) Optoacoustic Spectroscopy and Detection. New York: Academic Press.
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Rosencwaig A (1980) Photoacoustics and Photoacoustic Spectroscopy. Chemical Analysis Series, vol. 57. New York: Wiley. Sigrist MW (1986) J. Appl. Phys. 60: R83. Sigrist MW (1994) In: Sigrist MW (ed.) Air Monitoring by Spectroscopic Techniques. Chemical Analysis Series, vol. 127. New York: Wiley Chapter 4. Tam AC (1983) In: Kliger DS (ed.) Ultrasensitive Laser Spectroscopy. New York: Academic Press.
Tam AC (1986) Rev. Mod. Phys. 58: 381. Zharov VP and Letokhov VS (1986) Laser Optoacoustic Spectroscopy. Springer Series in Optical Sciences, vol. 37. Berlin: Springer. Kosterev AA, Tittel FK, Serebryakov DV, Malinovski AL, and Morozov IV (2005) Rev. Sci. Instrum. 76: 043105. Kuusela T and Kauppinen J (2007) Appl. Spectr. Rev. 42: 443.
Introduction Since the discovery of the photoacoustic (PA) effect by Bell in 1880 who used the Sun as radiation source, a foot-operated chopper for modulation and an earphone as acoustic detector, the PA effect has found numerous applications as a sensitive and rather simple technique for determining optical, thermal and mechanical properties of all kinds of samples. This article focuses on methods and instrumentation employed in spectroscopic applications. Since photothermal spectroscopy is discussed in another section, those schemes are only briefly mentioned here whereas emphasis is put on instrumentation used in photoacoustic spectroscopy. The spectroscopic applications of the PA effect are not discussed here but only the technique itself, its characteristics and implementation. Although the PA method has been around for more than a century it is particularly the advent of lasers as radiation sources with high spectral brightness that has initiated a renaissance of the PA effect. In the meantime a great variety of experimental schemes have been developed and render the PA method a very versatile spectroscopic tool.
The properties of piezoelectric transducers and microphones as pressure sensors are discussed below. If the use of pressure sensors is not appropriate, because, e.g., a piezoelectric transducer cannot be attached to the sample or measurements need to be done at high temperatures which hinders the application of microphones, noncontact techniques are to be applied. As shown in Figure 1(c) the induced spatial and temporal gradient of the refractive index can be sensed in this case by monitoring the deflection of a
Photoacoustic and Photothermal Schemes Experimental Arrangements As the photoacoustic (and related photothermal (PT)) phenomena comprise a large diversity of facets there exist various detection techniques which rely on the acoustic or thermal disturbances caused by the absorbed radiation. The selection of the most appropriate scheme for a given application depends on the sample, the sensitivity and specificity to be achieved, ease of operation, ruggedness, and any requirement for non-contact detection, e.g. in aggressive media or at high temperatures and/or pressures. Figures 1–3 present the most typical arrangements applied for solid, liquid and gaseous samples, respectively. Experimental schemes for PA studies on solid samples include the measurement of the generated pressure wave either directly in the sample with a piezoelectric sensor for the pulsed regime, or indirectly in the gas which is in contact with the sample by a microphone. These most widely used setups are depicted in Figure 1(a) and 1(b). The indirect detection of the generated acoustic wave in the gas phase with the microphone is inevitable if a direct contact with the sample is not readily possible, e.g. for samples such as powders, gels or grease. ☆
Change History: August 2014. MW Sigrist updated the text further readings to this entire article. Markus W. Sigrist, Photoacoustic Spectroscopy, Methods and Instrumentation, In Encyclopedia of Spectroscopy and Spectrometry (Second Edition), edited by John C. Lindon, Academic Press, Oxford, 1999, Pages 2146-2150.
598
Figure 1 Typical experimental arrangements used for photoacoustic (PA) and photothermal (PT) studies on solids. As indicated one differentiates between modulated or pulsed incident radiation. (a) Indirect PA detection by microphone in the gas phase. (b) Direct PA detection with PZT transducer or PVDF foil attached to solid. (c) PT sensing of the gradient of the refractive index with probe beam deflection in the (transparent) sample (probe beam 1), or above the sample surface (Mirage effect, probe beam 2). Monitoring of the generated surface displacement (probe beam 3) or of the change of surface reflectivity (probe beam 4). (d) PT radiometry senses the induced change of the IR radiation that is radiated off the sample surface.
Encyclopedia of Spectroscopy and Spectrometry, Third Edition
http://dx.doi.org/10.1016/B978-0-12-409547-2.11308-3
Photoacoustic Spectroscopy, Methods and Instrumentation
probe beam either within the (transparent) sample or directly above the (plane) sample surface (PT beam deflection or so-called mirage effect). Changes of the surface reflectivity or slight deformations of the surface (PT beam displacement) can also be detected in a non-contact manner by a probe beam. Finally, as depicted in Figure 1(d), variations of the thermal radiation from the surface can be monitored with an infrared detector (PT radiometry). This method is of particular interest for measurements at elevated temperatures owing to the increased radiation intensity according to Stefan–Boltzman’s law. Still other techniques include pyroelectric detection in thin films, thermal lensing and interferometric methods. The typical experimental arrangement for absorption spectroscopy in weakly absorbing liquids is shown schematically in Figure 2(a). The beam of a pulsed tunable laser is directed through the PA cell that contains the sample under study. The
Figure 2 Typical experimental arrangements used for PA detection in liquids. (a) PZT detection of aoustic wave generated by pulsed radiation in weakly absorbing liquid. (b) Optothermal window setup applied for studies on strongly or opaque liquids with modulated or pulsed radiation.
599
generated acoustic waves are detected by a piezoelectric transducer with fast response time. Usually just the first peak of the ringing acoustic signal is taken and further processed. Pulse-topulse variations of the laser power are accounted for by normalizing the piezoelectric signal with the laser power measured with the power meter after the cell. Another area of interest is the measurement of opaque or strongly absorbing liquids. A simple open PA cell called an optothermal window was developed for this purpose as displayed in Figure 2(b). It essentially consists of an uncoated ZnSe window to which an annular lead zirconate titanate (PZT) piezotransducer is glued from the bottom. The excitation beam from a laser passes unobstructedly through this window and is absorbed by a droplet of the sample deposited on the other side of the ZnSe disc. The generated heat diffuses into the disk which expands. The induced stress is then recorded by the PZT transducer. Unlike in conventional transmission spectroscopy where the cell thickness is the restricting factor in dealing with strongly absorbing samples, the magnitude of the optothermal window signal depends solely on the product between the absorption coefficient and the thermal diffusion length whereby the latter can be adapted via the modulation frequency. The typical setup for gas phase measurements is shown in Figure 3(a). A tunable laser with narrow linewidth or a conventional (broadband) radiation source followed by optical filters is used. In general, amplitude-modulated (or sometimes pulsed) or wavelength-modulated (WM) radiation is directed through the PA cell. The acoustic sensor is usually a commercial electret microphone or a condenser microphone. These devices are easy to use and sensitive enough for trace gas studies with very low absorptions. Often, the detection threshold is neither determined by the microphone responsivity Rmic itself nor by the electrical noise but rather by other sources (absorption by desorbing molecules from the cell walls, window heating, ambient noise, etc.). However, if this latter background is known from reference measurements, the ultimate detection sensitivity is determined solely by fluctuations of the radiation intensity, and by microphone and amplifier noise.
Figure 3 Typical experimental PA and PT arrangements used for gas monitoring with tunable laser sources. (a) PA detection with conventional microphone in resonant gas cell for modulated cw radiation or in non-resonant cell for pulsed radiation. (b) Noncontact refractive index sensing schemes with displaced collinear or transverse probe beam (PA deflection, 1), thermal lensing (2), or collinear probe beam (PT deflection, 3).
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Photoacoustic Spectroscopy, Methods and Instrumentation
The frequency dependence of Rmic is usually rather small and the temperature dependence may have to be taken into account in special cases only. If modulated radiation is employed the microphone signal is fed to a lock-in amplifier locked to the modulation frequency. Since according to theoretical considerations the microphone signal amplitude is proportional to the absorbed power for weakly absorbing media, the average radiation power is recorded simultaneously by a power meter for normalization. In more recent times, new schemes have been introduced, in particular quartz-enhanced photoacoustic spectroscopy (QEPAS) and cantilever-enhanced photoacoustic spectroscopy (CEPAS). If pulsed radiation is employed the microphone bandwidth is often not sufficient to resolve the temporal shape of the generated acoustic pulses. However, common microphones can still be used even for nanosecond laser pulses because the length of a single acoustic pulse is essentially determined by the transit time of the acoustic wave across the beam radius. Normalized PA amplitudes are obtained by dividing the microphone signal peaks by the corresponding laser pulse energy that is recorded by a sensor such as a pyroelectric detector. Averaging over several pulses improves the signal-to-noise ratio. Another approach for pulsed radiation consists of using an acoustic resonator with high Q-factor as gas cell, recording the microphone signals in the time domain but analyzing the PA signal amplitudes after Fourier transformation in the frequency domain. The excited cell resonances then appear in the PA frequency spectra. An important issue for many applications concerns the calibration of the entire PA or PT detection system. Since the PA signal depends on many factors that are not known with sufficient accuracy, a straigthforward calibration is often achieved by employing a reference sample with known absorption. As example, certified gas mixtures (trace gas diluted in a nonabsorbing buffer gas) or well characterized dye solutions in the case of liquids are used. The situation is more difficult with solid and biological samples, particularly layered media, powders, gels or tissue. In such cases, quantitative data are difficult, if not impossible, to obtain. But even qualitative instead of quantitative spectra are often valuable especially when other spectroscopic techniques fail owing to opaqueness or strong scattering of the sample. It should be emphasized that numerous different versions and modifications of these general schemes have been presented in the literature such as QEPAS and CEPAS mentioned above. Furthermore, combinations of conventional methods, such as Fourier-transform IR (FTIR) or gas-chromatography (GC), with PA detection have been reported. Some types of PA detection schemes are also implemented in commercial spectrometers. In the following the different components of PA spectrometers are briefly discussed.
Radiation Sources In commercial PA spectrometers, incoherent sources such as incandescent lamps are employed in combination with filters or with an interferometer, or also light-emitting diodes (LEDs). Devices equipped with a small light bulb with either a chopper or direct current modulation as modulated radiation source
and appropriate filters to avoid absorption interferences with other species are used as compact gas sensors, e.g. for indoor CO2 monitoring. However, since the generated PA signal is proportional to the absorbed (and thus to the incident) radiation power, powerful radiation sources, particularly lasers offering high spectral brightness, are advantageous for achieving high detection sensitivity and selectivity in spectroscopic applications. In the UV and visible spectral range, excimer and dye lasers have been employed whereas in the mid-infrared (fundamental or midIR) wavelength range line-tunable CO2 and CO lasers have dominated the applications for long times. Near infrared diode lasers with sufficient power for PAS are often used for monitoring overtones and combination bands of molecular fundamental absorptions. Widely tunable narrowband allsolid-state laser devices in the mid-IR region for accessing the (much stronger) fundamental absorptions include optical parametric oscillators (OPO’s) and difference frequency generation (DFG) in nonlinear crystals. Furthermore, near- to midinfrared interband cascade lasers (ICLs) and mid-infrared tunable quantum cascade lasers (QCLs, often with external cavity) represent very interesting sources for compact sensing devices. Most recent and ongoing developments of tunable midinfrared lasers such as diode-pumped lead salt vertical extended cavity surface emitting lasers (VECSELs), continue to change the situation.
Modulation Schemes Modulation schemes can be separated into the modulation of the incident radiation and the modulation of the sample absorption itself. The first technique includes the most widely used amplitude modulation (AM) of continuous radiation by mechanical choppers, electro-optic or acousto-optic modulators as well as the modulation of the source emission itself by current modulation or pulsed excitation. In comparison to AM, frequency- (FM) or wavelength- (WM) modulation of the radiation may improve the detection sensitivity by eliminating the continuum background caused by a wavelength-independent absorption, e.g., of the cell windows, known as window heating. This type of modulation is obviously most effective for absorbers with narrow linewidth and most easily performed with radiation sources whose wavelength can rapidly be tuned within a few wavenumbers. Pulsed excitation is often applied for liquids but is also of interest for gaseous samples because it permits time gating and the excitation of acoustic resonances. In certain cases the modulation of the absorption characteristics of the sample itself is advantageous. In gas studies the Stark or Zeeman effect has been employed, i.e., by applying a modulated electric or magnetic field to the sample. The result is a suppression of the continuum background and an enhancement of detection selectivity in multicomponent samples because, e.g., Stark modulation only affects molecules with a permanent electric dipole moment like ammonia (NH3) or nitric oxide (NO) while other, possibly interfering molecules are not affected. Finally, combinations of both amplitude and sample absorption modulation have been successfully applied, e.g., for the sensitive detection of ammonia in the presence of absorbing water vapor and carbon monoxide.
Photoacoustic Spectroscopy, Methods and Instrumentation
The scheme of QEPAS is based on the modulation of the source whose beam is guided through the gap between the prongs of a quartz tuning fork (QTF), filled with the sampling gas. The prongs are bent in opposite directions by the generated acoustic wave resulting in a piezoelectric signal which is normally recorded at the resonance frequency of typically 32 kHz.
Photoacoustic Cell Designs The PA cell serves as a container for the sample under study and for the microphone or some other device for the detection of the generated acoustic wave. An optimum design of the PA cell represents a crucial point when background noise ultimately limits the detection sensitivity. In particular for trace gas applications, many cell configurations have been presented including acoustically resonant and nonresonant cells, single- and multipass cells, as well as cells placed intracavity. Nonresonant cells of small volume are mostly employed for solid samples with modulated excitation or for liquids and gaseous samples with pulsed laser excitation. As unique example, we have developed at our laboratory a small-volume cell equipped with a “tubular” acoustic sensor consisting of up to 80 single miniature microphones. These microphones are arranged in eight linear rows with ten microphones in each row. The rows are mounted in a cylindrical geometry parallel to the exciting laser beam axis and located on a circumference around the axis. This configuration is thus ideally adapted to the geometry of the generated acoustic waves. Resonant cells in combination with modulated excitation are normally applied for gas monitoring. These cells are operated on longitudinal, azimuthal, radial, or Helmholtz resonances. The signal enhancement by the Q-factor (usually >100) is often advantageous. Resonance frequencies lie in the kHz range resulting in resonance widths of a few Hz. Furthermore, the gas handling for the cell can be designed in such a way that the gas in- and outlets are located at pressure nodes of the acoustic resonance which allows measurements in flowing gas with flow rates on the order of 1 l min 1 without increasing the noise level. Finally, cells developed for special purposes have been suggested like windowless cells equipped with acoustic baffles to reduce the influence of the ambient noise or heatable cells for studying liquid samples with low volatility. Quartz-enhanced photoacoustic spectroscopy (QEPAS) can be operated in the open air without cell although resonant microtubes are often added for improved performance. Cantilever-enhanced photoacoustic spectroscopy (CEPAS) generally uses a longitudinal cell.
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For studies in the gas phase, commercial microphones are employed. These include miniature electret microphones such as Knowles or Sennheiser models with typical responsivities Rmic of 10–20 mV Pa 1 as well as condenser microphones, e.g. Bru¨el and Kjær models with typical Rmic of 100 mV Pa 1. Usually Rmic depends only weakly on frequency. The electret microphones produced for hearing aids exhibit a rather flat frequency response between, say, 20 Hz and 20 kHz whereas the bandwith Dn of condenser microphones extends to frequencies of 100 kHz. All these microphones are thus well suited for typical modulation frequencies in the 100 Hz to kHz range. For pulsed applications, the general relation between Rmic and Dn, as well as the occurrence of external noise implies a reduction of the signal-to-noise ratio for very large bandwidths so that miniature electret microphones are often appropriate detectors also in this case. The detection sensitivity can be enhanced by adding the signals of several microphones. In such a configuration the signal increases with the number of microphones used whereas the microphone random noise only increases with the square root of their number. Since electret microphones are small and cheap, a number of them can be arranged in a still compact geometry. A further improvement of sensitivity is expected from the insertion of an electrical filter that cuts the low-frequency components below, say, 1 kHz of the signal because these components contribute less to the increase of the signal-tonoise ratio than the higher-frequency components. Finally, an adaption of the frequency response of the microphone pre- and amplifier stages to that of the microphone is advantageous to fully exploit all the sensed frequency contributions except noise components at frequencies not contributing to the acquired signal. While the QEPAS scheme uses the tuning fork as sensing device, the CEPAS method relies on a cantilever whose bending by the acoustic wave generated in the cell is recorded with a diode laser-based interferometer. If there is a need for noncontact detection, refractive index sensing, notably thermal lensing and both PA and PT deflection, are employed. These methods use a pump beam and a probe beam (HeNe or diode laser) in either collinear or transverse arrangement as shown in Figure 3(b). In comparison to the conventional PA method with pressure sensors, these schemes offer similar sensitivity but require a somewhat more sophisticated setup and imply a more difficult calibration.
See also: Laser Induced Optoacoustic Spectroscopy; Photoacoustic Spectroscopy, Applications; Photoacoustic Spectroscopy, Theory.
Detection Sensors As mentioned above the acoustic disturbances generated in the sample are detected by some kind of pressure sensors. In contact with liquid or solid samples these are piezoelectric devices such as lead zirconate titanate (PZT), LiNbO3 or quartz crystals with a typical responsivity R in the range of up to Volts/ bar or thin polyvinilydene-difluoride (PVF2 or PVDF)-foils with lower responsivity R. These sensors offer fast response times and are thus ideally adapted for pulsed photoacoustics.
Further Reading Almond DP and Patel PM (1996) Photothermal Science and Techniques. London: Chapman & Hall. Mandelis A (ed.) (1992) Principles and Perspectives of Photothermal and Photoacoustic Phenomena. In: Progress in Photothermal and Photoacoustic Science and Technology, vol. 1. New York: Elsevier. Meyer PL and Sigrist MW (1990) Rev. Sci. Instrum. 61: 1779. Pao Y-H (ed.) (1977) Optoacoustic Spectroscopy and Detection. New York: Academic Press.
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Rosencwaig A (1980) Photoacoustics and Photoacoustic Spectroscopy. Chemical Analysis Series, vol. 57. New York: Wiley. Sigrist MW (1986) J. Appl. Phys. 60: R83. Sigrist MW (1994) In: Sigrist MW (ed.) Air Monitoring by Spectroscopic Techniques. Chemical Analysis Series, vol. 127. New York: Wiley Chapter 4. Tam AC (1983) In: Kliger DS (ed.) Ultrasensitive Laser Spectroscopy. New York: Academic Press.
Tam AC (1986) Rev. Mod. Phys. 58: 381. Zharov VP and Letokhov VS (1986) Laser Optoacoustic Spectroscopy. Springer Series in Optical Sciences, vol. 37. Berlin: Springer. Kosterev AA, Tittel FK, Serebryakov DV, Malinovski AL, and Morozov IV (2005) Rev. Sci. Instrum. 76: 043105. Kuusela T and Kauppinen J (2007) Appl. Spectr. Rev. 42: 443.