High Sensitivity, High Reliability Trace Gas Sensors using optical detection - C. Kumar N. Patel

Pranalytica, Inc. located in Santa Monica, CA specializes in technology and products for ultra low level, high reliability detection of trace gases. Pranalytica uses tunable laser based photoacoustic spectroscopy (O-Nose™ Technology) for the detection of many molecular gases at ppb and sub-ppb levels without interference from other gases that may be present in the sampled gas. Pranalytica’s instruments are field proven, rugged and reliable; they are able to operate autonomously without operator intervention or assistance; they provide data on a continuous basis with a fresh measurement every 12 seconds and they can be interfaced with central data collection systems in using a broad variety of communication protocols.

Principle of photoacoustic spectroscopy can be understood through figure 1. The sample gas is illuminated by a radiation at frequency ν. When the frequency ν coincides with the absorption frequency of the target molecule, the photon can be absorbed by the molecule and the molecule is now excited to a higher state from which the molecule can relax back to the initial state by radiative or nonradiative mechanisms. Nonradiative relaxation converts the internal excitation energy of the molecule into translational energy and the molecular gas is now heated. If the incoming radiation is periodically interrupted, as shown in the figure, the gas is periodically heated. Such periodic heating is equivalent to periodic variation in the gas pressure. If the amplitude modulation occurs at an audio frequency, the resultant pressure fluctuations, sound, can be detected using a sensitive microphone as shown and the electrical signal corresponds to the rate at which the excited molecules decay back to their initial state. It is instructive to point out that