Chemistry

What is Photoacoustic Spectroscopy



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Photoacoustic Spectroscopy (PAS) is a type of Spectroscopy that uses light or other energy for "photo" and sound for “acoustic.”  Spectroscopy, by itself, deals with emitting and absorbing light, matter and energy.  PAS translates optical energy to mechanical energy through wavelengths during excitation.  When the “de-excitation process occurs it produces kinetic energy or heat” Middlebury college (N.d.).  Photoacoustic light spectrum is the wavelength of light used in Photoacoustic Spectroscopy.

History of Photoacoustic Spectroscopy

Between “1880–1881, Alexander Graham Bell found that when a thin disk was exposed to mechanically chopped sunlight, sound was emitted “(Ball, 2006).  As a result the finding determined that both light and sound can occur together.  Further studies and use of Photoacoustic Spectroscopy discontinued until microphones were developed and its use began once again.

Photothermal Techniques

Photoacoustic Spectroscopy can be used as a photothermal sampling technique.  A light beam is used and alters the thermal temperature as a result.  When using solids in photothermal techniques; the ‘measurement of the thermal diffusivity of a solid sample’ (Marin Et. Al., 2009) is checked.

Sample gases

Photoacoustic Spectroscopy is used when sampling gases and the process is pretty simple. “An increase in the temperature of the gas leads to an increase in the pressure of an isochoric (constant-volume) sample. If the incoming light is modulated — modulation frequencies can vary from single to several thousand hertz — the gas pressure increases and decreases accordingly, creating sound.” (Ball, 2006).  The different wavelengths effect sound through light received.

Condensed Plates

Photoacoustic Spectroscopy can be used with an accepted mechanism to have a photoacoustic effect.  “The commonly accepted mechanism for the photoacoustic effect is called RG theory, after its developers Rosencwaig and Gersho. The main source of the acoustic wave is the repetitive heat flow from the absorbing condensed-phase sample to the surrounding gas, followed by propagation of the acoustic wave through the gas column to microphone-based detector” (Ball, 2006).  Radiant heat gets transferred through a solid vibrational mode referred to as a phonon mode which is dependent on the size it is.

Applications of  Photoacoustic Spectroscopy

There many uses of photoacoustic spectroscopy people may not be aware of that exist, with some more common than others.  It can be used in laboratory processes when studying biological samples to identify bacteria through looking at blood and other tissues such as skin and eyes.  It ‘is used for depth profiling and can be used to measure pollutants such as NOx or soot in the atmosphere or in automobile exhaust’ (Ball, 2006).  It can be useful for people who do work in these areas.


References

Ball, D. (Sept. 1st, 2006). Photoacoustic Spectroscopy, Retrieved on March 18th, 2011 from http://spectroscopyonline.findanalytichem.com/spectroscopy/article/articleDetail.jsp?id=373774

Marín, E., Juárez-Gracia, G., Vera-Medina G., and Calderón, A. (2009). A simple approach to introduce photothermal techniques basic principles for thermal diffusivity measurement, ISSN: 1870-909, Retrieved on March 18th, 2011 from http://www.journal.lapen.org.mx/May09/LAJPE%20264%20preprint%20f.pdf

Middlebury college (No date). Photoacoustic Spectroscopy, Power point, Retrieved on March 18th, 2011 from community.middlebury.edu/.../Photoacoustic_Spectroscopy.ppt


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