Photoacoustic spectroscopy is the measurement of the effect of absorbed electromagnetic energy (particularly of light) on matter by means of acoustic detection. 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 light 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.

A photoacoustic spectrum of a sample can be recorded by measuring the sound at different wavelengths. This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study solids, liquids and gases. ^[1]

Uses and Techniques

Exemplary assembly of a photoacoustic spectroscope for gas analysis

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 The ear has been replaced by sensitive microphones.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 ^[2] 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 applications such as 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.

External links

• General introduction to photoacoustic spectroscopy: [1]
• M.W. Sigrist, Air Monitoring by Spectroscopic Techniques, J. Wiley & Sons, New York, 1994.
• Photoacoustic spectroscopy in trace gas monitoring [2]
• MIniaturized-Cell-Enhanced PhotoAcoustic Spectroscopy MICEPAS [3]

References

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