Photoacoustic spectroscopy

The geometry of the measuring cell is essential for the sensitivity of the photoacoustic detector. The detector can either be operated resonantly or non-resonantly. In resonant operation, the modulation frequency of the light source matches the acoustic resonance frequency. This amplifies the detected signal. In non-resonant operation of a photoacoustic detector, the modulation frequency of the light source (typically a broadband IR emitter) is far below the acoustic resonance frequency of the measuring cell. The generated acoustic waves are therefore not amplified further.

Depending on the target gas and application, different geometries of measuring cells and different light sources are selected. Both factors essentially determine the price and size of the gas measurement system. Cost-effective measurement systems that use LEDs as light sources, for example, can determine the target gas concentrations very sensitively (in the 1 ppm range). If lasers are used as light source, the sensitivity of the measurement systems reaches values in the low ppb range.

Two examples: For a compact SO₂ measurement system used for emission measurement in the exhaust gas scrubber on ships, Fraunhofer IPM employs a resonant measurement cell with a UV LED as light source. Thus, the required high resolution of 1 ppm in the measuring range up to 50 ppm SO₂ is achieved. For a miniaturized photoacoustic CO₂ sensor used for the detection of indoor air quality, a non-resonant measuring cell was combined with a broadband IR emitter and a MEMS microphone. This allows a particularly small sensor, which in this case has a sufficient sensitivity of 50 ppm in the measuring range up to 5000 ppm CO₂.