## Results and Discussion  
Even trace amounts of organic solvent contamination in water impose a huge threat to the proper functionality of wastewater treatment plants. Therefore, they have to be detected and removed before reaching the biological clarifier. Although the detection directly in aqueous phase is in principle possible, it should be considered to use an apparatus that allows the application of gas phase sensing. This is favorable due to the substantially lower noise level occurring in the oscillators driving the respective mass-sensitive device, which results from decreased electronic loss of the sensor caused by viscous interactions between the sensor and its environment [5]. Figure 1 shows both the experimental  
Figure 1: Liquid and gas cell for determining organic trace contaminants in wastewatersetups for liquid as well as for gaseous phase used for these measurements. Whereas the first consists of a flow-cell, where the sensitive area is directly exposed to the samples, in the second case the sensor chamber is separated from the water stream by the means of a thin Teflon membrane with pores sizing 200 nm. Preliminary experiments showed that this is impermeable for water molecules, whereas organic solvents pass through readily. The results of this procedure are given in Figure 2, where typical sensor responses for either type of measurement are given. In both cases toluene was used for templating. Comparing the two datasets with each other, one can clearly see that especially the response time is highly affected by the measuring environments. The sensor immersed into the liquid reacts much faster than the one with the Teflon membrane separating the measurement chamber and the flow chamber from each other. The resulting sensor effects are in the same order of magnitude, therefore the thin polymer material obviously is fully permeable for the toluene molecules. The system with the gas sensing chamber reacts sluggishly, however, it shows the much lower noise level (around 1 Hz compared with about 10 Hz for the fluid cell) thus leading to lower detection limits, namely less than one ppm, whereas in the liquid it is about two orders of magnitude higher. This is a result of the higher electronic damping that QCMs undergo in contact with a fluid, because here a part of the oscillation energy is dissipated into the adjacent liquid layers due to viscous coupling with the resonating device. Thus, by suitable alterations of the sensor housing, the resulting system can be tuned to either fast reaction times or lower detection limits. Of course, on first sight it is not logical why the gas measurements proceed more slowly, because usually sensors exposed to gas streams have substantially lower response times than in liquids samples. Therefore, the sensor response in this case is determined by the coupled kinetics of analyte evaporation from the water through the porous membrane and the sensor-analyte interaction. Of course, chemical sensors are not restricted to determine distinct analytes but also more complex phenomena can be evaulated. Although one could apply modern separation techniques in this case, sensors still offer the advantage of providing direct on-line measurements. However, a single sensor will usually fail in this case, as it is only capable of yielding a sum signal of all analyte changes. Therefore, applying an array of devices [6] with up to six channels or more showing different  
Figure 2: Sensor responses against toluene obtained in liquid and in gas phase. selectivities followed by multivariate data analysis leads to reliable sensor results. We demonstrated this principle by mounting a QCM containing six electrode pairs in the headspace of a small composter and monitoring the concentrations of four predefined key-analytes, namely small aliphatic alcohols, ethyl acetate, limonene and humidity. Each of these compounds can be used as a lead analyte characterizing a different stage of plant degradation.