Photoacoustic Detection (PAS)

PAS Principle (Click to enlarge)

In Photoacoustic Spectroscopy (PAS) the gas to be measured is irradiated by modulated light of a pre-selected wavelength. The gas molecules absorb some of the light energy and convert it into an acoustic signal which is detected by a microphone.The IR-source is a spherical, heated black body. A mirror focuses the light onto the window of the PAS cell after it has passed the light chopper and the optical filter. The chopper is a slotted disk which rotates and effectively "switches" the light on and off. The optical filter is a narrow-band IR interference filter.After passing through the window, the light beam enters the PAS-cell. If the frequency of the light coincides with an absorbtion band of the gas in the cell, the gas molecule will absorb part of the light. The higher the concentration of gas in the cell, the more light will be absorbed.As the gas absorbs energy, it is heated and therefore expands and causes a pressure rise. As the light is chopped, the pressure will alternately increase and decrease - an acoustic signal is thus generated. The acoustic signal is detected by two microphones. The electrical output signals from the two microphone signals are added in an amplifier, before they are processed.

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Advantages of Photoacoustic Technology (PAS)

Photoacoustic gas measurement is based on the same basic principles as conventional IR-based gas analysers - the ability of gases to absorb infrared light. However there are some important differences between PAS and these conventional techniques.

The absorbtion (proportional to the concentration) is measured directly – not relative to a background. This means that PAS is highly accurate, with very little instability.Furthermore, with PAS all gases and vapours can be monitored simultaneously in a single measurement chamber as it is possible to detect each substance to be monitored individually.

Finally, the sample volume required is very small due to the small volume of the cell. Volumes down to 10 ml. can be measured.

The original experiment carried out by Alexander Bell
The original experiment carried out by Alexander Bell

The Light Source

The type of light source most suited to gas detection and analysis is one, which emits radiation in the infrared region of the electromagnetic spectrum, particularly between 650 and 4000 cm-1

Sunlight is the most common infrared light source and is what Alexander Bell used in his pioneering experiments in photoacoustics. Although undoubtedly the cheapest source available it is not, as he himself commented, the most reliable!

An excellent and more dependable alternative to sunlight is an incandescent source. The simplest type is a wire filament heated to a high temperature. It has the major advantage of being stable, inexpensive and long lasting. The spectral output is continuous, with 70 – 80 % of it in the infrared region.

Narrow bandwidth radiation is required for spectroscopy and for this reason an incandescent lamp is used in conjunction with an optical system which selectively admits the desired wavelength band. Filters are used for fixed wavelength irradiation. For continuous tunability, diffraction gratings, prisms or interferometry can be used.

You can get more information about Photo Acoustic if you  download the PAS Booklet. (1.14 Mb)

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