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 gases. An distinct advantage over many other spectroscopic techniques lies in the simple experimental set-up and alignment while similar sensitivies can be obtained.

Uses and Techniques

Photoacoustic 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 The ear has been replaced by sensitive microphones.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 photoacoustic imaging has been developed.

References

  1. ^ C.K.N. Patel, E.G. Burkhardt, C.A. Lambert, ‘Spectroscopic Measurements of Stratospheric Nitric Oxide and Water Vapor’, Science, 184, 1173–1176 (1974)
 
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