Photoacoustic Spectroscopy
The photoacoustic spectroscopy (PAS) also optoacoustic spectroscopy called (OAS) is a physical examination method based on the photoacoustic effect based. This was first described by Alexander Graham Bell in 1880 . Energy is supplied to a medium in rapid succession by flashes of light . The resulting constant alternation between heating and cooling leads to alternating thermal expansion and contraction. This vibration can be perceived as sound under appropriate circumstances .
The effect has been used in various areas since the 1970s to examine gases , solids and tissues , among other things .
Solid surfaces
For material investigations, an absorbing surface is heated by modulated light. The time-dependent temperature generates sound (or ultrasound ) in the solid through thermal expansion . This can e.g. B. measured with a piezoelectric detector .
The temperature fluctuations also propagate into the gas on the surface via thermal conduction and generate pressure fluctuations or sound in the gas through thermal expansion. A normal microphone is used for detection in this variant .
If the excitation is carried out periodically at different modulation frequencies , a distinction can be made within limits according to the depth below the surface. PAS analyzes can also be carried out in solutions .
Investigation of gas components
For the investigation of gases one makes use of the fact that gas molecules only absorb light of very specific wavelengths . Which these are depends on the chemical structure.
To detect a specific gas component in a mixture , light pulses from a laser are used, the frequency of which can only be absorbed by one type of molecule. The sound waves measured by a microphone in a container with the gas are then louder, the higher its proportion in the gas mixture. The volume can be increased by a factor of 100 (40 dB) through acoustic resonance .
The method can be used, for example, for exhaust gas tests or the detection of air pollutants. For example, concentrations of methane in nitrogen of only ten parts in a billion can be detected.
Simpler measuring devices use an infrared emitter in conjunction with a narrow-band filter as the light source . They compare the acoustic excitation between the sample to be examined and a known gas. Both are located in adjacent chambers that are separated by a thin membrane. If the concentrations of the constituent to be examined differ in the two gases, the membrane begins to vibrate.
Tissue examinations
In medicine and biology, the photoacoustic effect is used as an imaging process . The photoacoustic tomography (PAT) works with very short laser pulses (a few nanoseconds pulse duration) that is to be examined tissue ultrasound generate. The difference to conventional ultrasound examinations is that not only are "echoes" examined on the surface of organs, but these generate the sound themselves. The resulting images correspond in their richness of detail to computed tomography without generating dangerous radiation.
The method was also used in laboratory experiments to measure the blood sugar content of diabetics.
literature
- DJ O'Conner, BA Sexton, RSC Smart: Surface Analysis Methods in Materials Science . Springer-Verlag, Heidelberg 1992, ISBN 0-387-53611-6 .
- Frans JM Harren, Gina Cotti, Jos Oomens, Sacco te Lintel Hekkert: Photoacoustic Spectroscopy in Trace Gas Monitoring. In: R .A. Meyers (Ed.): Encyclopedia of Analytical Chemistry. Chichester 2000, ISBN 0471976709 , pp. 2203-2226 ( PDF , English).
- Manfred Euler: Can you hear light? Photoacoustic experiments in the kitchen . In: Physics in Our Time . tape 32 , 2001, p. 180-182 , doi : 10.1002 / 1521-3943 (200107) 32: 4 <180 :: AID-PIUZ180> 3.0.CO; 2- # .
Individual evidence
- ↑ Bert M. Weckhuysen, Pascal Voort, Gabriela Catana: Spectroscopy of transition metal ions on surfaces. Leuven University Press, 2000, p. 170.
- ^ Measuring blood sugar with light. eurekalert.org, October 25, 2013.