Differential Photoacoustic Spectroscopy
The differential photoacoustic method combines the sensitivity of photoacoustic spectroscopy with the long absorption path length used by conventional absorption spectroscopy and allows open-path and flow-through detection of gases.
The differential system consists of three separate gas cells: sample, reference, and differential photoacoustic cell. The sample cell is filled with the gas to be analysed. The reference cell is filled with a non absorbing zero-gas, e.g. nitrogen. The differential photoacoustic cell contains the gas to be analyzed, typically at high concentrations. The differential cell is divided into two equal halves, separated by the ultra-sensitive cantilever sensor.
The light emitted by the light source is also split into two equal parts, the top beam going through the sample cell into the top half of the differential cell and the bottom beam going through the reference cell into the bottom half of the differential cell. When there is no absorption in the sample cell, the two beams arrive at the differential cell with equal intensity and identical pressure waves are generated in the top and bottom halves which cancel each other out. If there is absorption in the sample cell, then there is a net pressure effect in the differential cell which is measured with the silicon cantilever sensor. The cantilever displacement is directly proportional to the concentration of the gas in the sample cell. Since the sample cell is not part of the photoacoustic detection cell, open-path measurements can be performed, even without relevant interference from the ambient noise.
The IR source can be a blackbody, LED or a laser. The modulation is performed by a mechanical chopper or electrically modulating the source. The most interesting advantage of this technique is that no optical filter or spectrograph is necessary, since the gas inside the differential cell acts like an optical filter (so-called gas correlation method). The selectivity is given by the real absorption spectrum and is not affected by any instrument function.
The differential photoacoustic spectroscopy is an ideal platform for single gas sensors. This measurement concept is the motivation for the MINIGAS EU-project that aims to a miniaturized high performance gas sensor based on combining cantilever sensor based differential PAS and infrared LED sources.
A fast and sensitive gas analyzer can be achieved by combining laser sources with the differential photoacoustic spectroscopy concept. This is ideal technology e.g. for gas flux measurements where low detection limits and short response time are required.
