The photoacoustic spectroscopy (PAS) is a spectroscopic method which utilizes the photoacoustic effect. The sample, for example gas, is irradiated with modulated light of a predefined wavelength. A certain part of light energy is absorbed by the sample and converted into acoustic waves. These signals can be detected by a microphone and then evaluated.
IR-laser-diodes are often used as light source, because for many applications the particular wavelength (color) of the examined material is in the infrared range. The light is electronically or mechanically modulated, for example by using a chopper.
Gas molecules absorb a part of the light, when the light frequency corresponds with an absorption band of the gas in the cell. The higher the concentration of the gas, the more light is absorbed. In this process heat is generated, which leads to a change of pressure in the measuring cell. Normally, such a pressure difference balances out again immediately. This is different for chopper modulated light. When impigning on the molecules a pressure wave and thus an acoustic signal is generated which can be detected with a microphone.
Advantages of PAS
Thus, the method of the PAS shows no interfering signals and provides extremely accurate results, because the absorption is determined directly and not in relation to the background.This measurement method is thus one of the most sensitive methods for the detection of gases and is therefore often used in trace gas analysis; as we operate, too.
The small size of PAS cells enables the measurement of very small gas volume. In contrast to conventional methods, the sample volume can be reduced drastically.
Another advantage is that the PAS is generally cheaper than other gas analysis methods, because microphones are less expensive than (infrared) detectors.
Therefore, the photoacoustic spectroscopy finds today application in various fields, e.g. the emissions testing of vehicles, in environmental technology (detection of air pollutants) and in the medical technology and in the biology.