Photoacoustic spectroscopy

Photoacoustic spectroscopy is based on the photoacoustic effect. The discovery of the photoacoustic effect dates to 1880 when Alexander Graham Bell showed that thin discs emitted sound when exposed to a beam of sunlight that was rapidly interrupted with a rotating slotted disk. The absorbed energy from the sunlight is transformed into kinetic energy of the sample by energy exchange processes. This results in local heating and thus a pressure wave or sound. Later Bell showed that materials exposed to the non-visible portions of the solar spectrum (i.e., the infrared and the ultraviolet) can also produce sounds. By measuring the sound at different wavelengths, a photoacoustic spectrum of a sample can be recorded that can be used to identify the absorbing components of the sample. Photoacoustic spectroscopy can be used to study solids, liquids and gases. An distinct advantage over many other spectroscopic techniques lies in the simple experimental set-up and alignment while similar sensitivies can be obtained.

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Uses and Techniques

Photoacoustic spectroscopy has become a powerful technique to study concentrations 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:

1. Use of intense lasers instead of the sun to illuminate the sample since the intensity of the generated sound is proportional to the light intensity.
2. The ear has been replaced by sensitive microphones. The microphone signals are further amplified and detected using lock-in amplifiers.
3. By enclosing the gaseous sample in a cylindrical chamber, the sound signal is amplified by tuning the modulation frequency to an acoustic resonance of the sample cell.

Example

The following example illustrates the potential of the photoacoustic technique: In the early 1970s, Patel and co-workers ^[1] measured the temporal variation of the concentration of nitric oxide in the stratosphere at an altitude of 28 km with a balloon-borne photoacoustic detector. These measurements provided crucial data bearing on the problem of ozone depletion by man-made nitric oxide emission.

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 with digital signal processors 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 detect microwave that generate by gigahertz 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.

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