Gas Phase Photoacoustic Spectroscopy (PAS)
The need for rapid and reliable monitoring of air pollutants and hazardous gases is growing and the photoacoustic spectroscopy provides an efficient technology for this demand.
The photoacoustic infrared spectroscopy is based on the fact that infrared light energy is absorbed by gas molecules. In the photoacoustic spectroscopy the light energy is converted into pressure variations i.e. sound energy. The sound energy in the gas sample cell is then converted into electric signal using a microphone.
Photoacoustic principle
The sample gas is sealed into a photoacoustic measurement chamber and irradiated with infrared light of a frequency that corresponds to a resonant frequency of a known sample gas molecule. If this sample gas is present in the measurement chamber a portion of the infrared energy is absorbed by that gas. This results in local increase of the heat energy in the gas molecules and the pressure and temperature of the sample gas will increase. When the infrared radiation is modulated with a certain frequency the pressure variation in the photoacoustic sample chamber creates acoustic wave of the same frequency. This acoustic wave is then converted into electric signal by the use of a microphone.
Advantages of photoacoustics
Very high sensitivity in gas detection can be achieved with the photoacoustic spectroscopy. Especially utilizing Gasera’s novel optical cantilever microphone, below ppb limits of detection can be reached.
In the photoacoustic spectroscopy the absorption is measured directly, not relative to the background as in other infrared absorption techniques. This means that it is a zero-background technology and the zero-point stability of the system is extremely good. Furthermore, the response of the optical cantilever microphone is extremely stabile. This means that very low amount of drift occurs and calibration interval is very long.
Dynamic measurement range of over 100 000 times the detection limit is possible with photoacoustic spectroscopy. This allows simultaneous analysis of very high and low concentrations without any range adjustments.
Low sample volume, only a few milliliters, is required to achieve similar sensitivity compared to multi-pass gas cells of several meters and several liters in volume in other infrared techniques. This is particularly useful when only a small amount of sample gas is available for analysis.
Due to the short optical path length required the response is highly linear over a wide dynamic range. This is advantageous in compensating for the effect of other gases in the sample gas mixture. This is particularly useful when analyzing wet gases.
