## Results and Discussion
In special cases, however, even processes in complex mixtures can be observed by the means of one single sensor element, e.g. when monitoring engine oil degradation [7], which are similar for different brands and types of oil. Therefore, it should be possible to design a sensor system that is capable of addressing all these oxidative processes and thus yielding a total cumulative signal. Organic polymers have been imprinted with the entire oil matrix of a fresh and a used material, the resulting sensors showed powerful selectivities. In this paper we want to concentrate on inorganic polymers, such as solgel materials. They have proven excellent properties concerning ruggedness, chemical tolerance and thermostability. When using Ti-alkoxides as precursors, thick layers with very low electronic loss are generated. This allows for the construction of a sensor system where the QCM is operated as the  
Figure 4: Trends for three key analytes obtained by an artificial neural network. Figure 3: Composter and sensor array used  
frequency-determining device in an oscillator circuit. Consequently, active measuring by applying a frequency counter can be used instead of (expensive) network analyzer equipment. An example of a degradation sensor mounted on a commercial oil level probe can be found in figure 5. The system is equipped with an electronic mixer device so that the chemical effects are directly read out, i.e. they are already corrected by the viscosity effects. Additionally, figure 5 contains a sensor response curve at the change from fresh oil to waste oil. The resulting mass effect of 1.2 kHz can be measured with high statistical significance, especially when regarding the noise level in the range of 10 Hz. Thus, by the means of suitable imprinted polymers, the complex procedures occurring during the oxidative degradation of engine oils can be translated into a single frequency shift that contains the entirety of chemical information. This gives a very straightforward measurand for the remaining lubricant usability based on chemical information (rather than a physical one). Usually, in molecular imprinting one single template is used to achieve ideal selectivity. However, there are analytical tasks where it has turned out to be advantageous to apply more than one template. Polycyclic aromatic hydrocarbons e.g. are members of a hazardous group of compounds that combine high toxicity and cancerogenous potential, so and thorough efforts are made for their analysis [8]. Earlier measurements with single-imprinted chemical sensors showed good selectivities and excellent sensitivities [9]. The sensitivity patterns obtained for different templates lead to the conclusion that the best re-inclusion can be observed for molecules being slightly larger than the template. To further investigate this phenomenon, mixtures of templates were tested [10]. Figure 6 shows a resulting set of data, where both the ratio between the two template molecules as well as the polymerization temperature is varied. Figure 5: Commercial oil level probe with mounted sensor and frequency response of the system on the change from fresh oil to waste oil. As with the single molecular templates, the temperature again has a high influence on the geometrical fit. At 20\({}^{\circ}\)C anthracene is obviously incorporated into the cavities generated by naphthalene, whereas the ones caused by pyrene are already too large to allow optimized interactions. This is the same observation as for single imprinted layers, where we could show that at low temperatures the cavities produced are larger than the templates. A second evidence for this phenomenon is the fact that the enrichment of the analyte monotonously depends on the amount of naphthalene in the template mixture. At higher temperatures, where the geometrical arrangement between the forming polymer and the template is much better, the larger pyrene molecule determines the re-inclusion properties. The smaller template molecule here acts as a porogen and opens otherwise inaccessible sites in the material.