Preconcentrator module for the implementation in optical gas measurement systems

Fabrication process monitoring in industrial applications, safety and environmental monitoring e.g. for the determination of toxic gases are typical fields of applications in modern gas measurement tasks. In the lower price segment solid state devices like metal oxide based gas sensors are typically used. These sensors are very sensitive but they show high cross-sensitivities and a significant signal drift. Optical techniques distinguish by the compensation of these two disadvantages and it is still possible to get inexpensive absorption spectroscopic systems for e.g. C2H4. In general these systems are based on thermal IR-sources, a simple absorption cell without optical elements and a detector. The decreasing of the detection limit is mainly reached by increasing of the optical path. We p resent a new approach to improve the sensitivity of these systems based on a preconcentrating unit mounted to absorption cell Preconcentrating is a well known method in analytical chemistry and is based on the collection of an analyte during a period. For the first experiments the collection of gas molecules is done using the commercial available adsorbent resin Tenax® TA. Preliminary test were performed to show the proof of principle. The measurement set-up consists of a filter-photometer with a small gas measurement cell (White-cell type) and a preconcentrating module, which is mounted close to the gas cell. The temperature of the preconcentrating module can be varied from -10°C to 150°C. As test gas ethylene was chosen. Ethylene measurements were performed with a constant analyte f low and different operation temperatures of the preconcentrator in order to check the interaction between the gas molecules and the adsorbent resin. An increasing of the adsorbent temperature resulted in lower adsorption of ethylene. At a temperature of -10°C a signal enhancement of almost factor two could be reached. This positive results yield in a development of an improved and integrated version of the preconcentrator. Due to the shorter distance between adsorbent resin and measurement cell, the enlarged amount of adsorbent resin and the improved thermal decoupling should be result in improvement factor of 4 to 5 in comparison to the system without preconcentrator