Photoacoustic Spectroscopy (PAS)

The photoacoustic spectroscopy (PAS) is a spectroscopic method which utilizes the photo-acoustic effect. For example, gas, irradiated with modulated light of a predefined wavelength. A certain part of light energy is absorbed by the sample and is converted into acoustic waves. These signals can be detected by a microphone and then evaluated. As a light source, infrared laser diodes are frequently used because the specific wavelength (color) of the investigated material is in the infrared range in many applications. The light is modulated in an electronic or mechanical way, for example with a chopper. Gas molecules absorb a portion of 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. This produces heat, which leads to a change of pressure in the measuring cell. Normally, such a pressure difference balances out immediately. In case of light which is modulated by chopper it is different. The light is generating a pressure wave and therefore an acoustic signal while impinging on the molecules what can be detected with a microphone. The photoacoustic spectroscopy anebles the very accurate determination of e.g. nitrogen dioxide (NO₂). It is one of the measurement principles used in the ecom-J2KNpro TECH.