- Photoacoustic spectroscopy
Photoacoustic spectroscopy is based on the
photoacoustic effect . The discovery of the photoacoustic effect dates to 1880 whenAlexander Graham Bell showed that thin discs emittedsound when exposed to a beam ofsunlight that was rapidly interrupted with a rotating slotted disk. The absorbedenergy from the sunlight is transformed intokinetic energy of the sample by energy exchange processes. This results in localheat ing and thus apressure wave or sound. Later Bell showed that materials exposed to the non-visible portions of the solarspectrum (i.e., theinfrared and theultraviolet ) can also produce sounds. By measuring the sound at differentwavelength s, a photoacoustic spectrum of a sample can be recorded that can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to studysolid s,liquid s andgas es.Uses and Techniques
Photoacoustic
spectroscopy has become a powerful technique to studyconcentration s of gases at the part per billion or even part per trillion levels. Modern photoacoustic detectors still rely on the same principles as Bell’s apparatus, however to increase the sensitivity the following modifications have been made:
# Use of intenselaser s instead of the sun to illuminate the sample since the intensity of the generated sound is proportional to the light intensity.
# The ear has been replaced by sensitivemicrophone s. The microphone signals are further amplified and detected usinglock-in amplifier s.
# By enclosing the gaseous sample in a cylindrical chamber, the sound signal is amplified by tuning themodulation frequency to anacoustic resonance of the sample cell.Example
The following example illustrates the potential of the photoacoustic technique: In the early 1970s, Patel and co-workers [C.K.N. Patel, E.G. Burkhardt, C.A. Lambert, ‘Spectroscopic Measurements of Stratospheric Nitric Oxide and Water Vapor’, Science, 184, 1173–1176 (1974)] measured the temporal variation of the concentration of
nitric oxide in thestratosphere at an altitude of 28 km with a balloon-borne photoacoustic detector. These measurements provided crucial data bearing on the problem ofozone depletion by man-made nitric oxide emission. Nibu George et al [N.A.George and R.Vinayakrishnan, ‘Photoacoustic evaluation of the thermal diffusivity of coconut shell', J.Phys.: Cond.Matter 14, 4509-4513 (2002); ] . employed the photoacoustic technique to investigate the thermal properties of coconut shell.Academic Research
While most academic research has concentrated on high resolution instruments, some work has gone in the opposite direction. In the last twenty years, very low cost instruments for leakage detection and for the control of
carbon dioxide concentration have been developed and commercialized. Typically, low cost thermal sources are used which are modulated electronically.Diffusion through semi-permeable disks instead of valves for gas exchange, low cost microphones and proprietary signal processing withdigital signal processor s has brought down the costs of these systems. The future of low cost applications of photoacoustic spectroscopy may be the realization of fully integrated micromachined photoacoustic instruments. Other academic research use SAW (Surface acoustic wave ) in order to detectmicrowave that generate bygigahertz modulation . The use of Laser Ultrasonic Sensor enable to work with high sonic frequency that allowed with Piezoelectric Ultrasonic Sensor. Recently, a photoacoustic based noninvasive biomedical imaging technology known as the photoacoustic imaging has been developed.References
External links
* General introduction to photoacoustic spectroscopy: [http://www.spectroscopyeurope.com/PAS14_5.pdf#search='photoacoustic%20spectroscopy']
* M.W. Sigrist, Air Monitoring by Spectroscopic Techniques, J. Wiley & Sons, New York, 1994.
* Photoacoustic spectroscopy in trace gas monitoring [http://www.mlf.science.ru.nl/publ/2000/0718a.pdf]
* Some recent publications in photoacoustic spectroscopy. [http://nibuageorge.tripod.com/id1.html]
* [http://www.lbl.gov/Science-Articles/Archive/EETD-papersensor-Ridgway.html EETD Ultrasound microphone @ Transducer]
* [http://eetdnews.lbl.gov/nl24/2lus.html Laser Ultrasonic sensor]
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