Selectivity enhancement of metal oxide gas sensors using a micromachined gas chromatographic column

Indoor air quality monitoring applications require both high sensitivity and selectivity, which are difficult to reach with solid state gas sensors. While for some gas species like, e.g. CO and NO2, the use of optimized operating conditions allows to reach the necessary selectivity, the discrimination between single volatile organic compounds (VOC) is generally not possible with stand-alone arrays of gas sensors. This limitation represents a major drawback, since not all indoor VOC are equally harmful for the health of the human beings living in the polluted environment. For this reason, a highly selective hybrid microsystem based on a gas chromatographic-like (GC) approach is proposed. In this miniaturized gas chromatographic system a solid state gas sensor is used as detector, together with a Silicon micromachined packed GC column, a zero grade air unit, a commercial minipump and a minivalve. With this prototype, Benzene, Toluene and m-Xylene eluted in synthetic or real indoor air were reliably detected and measured at concentrations as low as 5 ppb. The use of chromatographic units together with solid state gas sensors both integrated in silicon allows to develop low-cost, handheld portable devices having exceptional selectivity and reproducibility. Possible applications which could benefit of these performances include security control, air quality monitoring as well as food quality control.