5.3.3.   Thickness of the Sensitive Layer

In figure 25, the sorption and desorption of ethanol is shown for two Makrolon layers with a thickness of 100 nm and 300 nm. It is obvious that the thinner layer shows a significantly faster kinetics than the thicker layer. This effect can be explained by the different mechanisms of sorption proposed in section 5.3.2. A thicker layer means a longer diffusion path resulting in a higher proportion of the kinetic volume in the effective volume, which is consequently bigger. This effect allows tailoring the sensitive layer to a specific analytical problem. For example, the thicker layer allows an efficient discrimination of ethanol from the smaller and consequently "faster" methanol (see figure 15) whereas the thinner layer allows discrimi­nating ethanol from the bigger and consequently slower 1-propanol and 1-butanol (see figure 24). The ability of tailoring the layers is only limited for very thick layers by the consumption of time needed for a measurement and for thin layers by a poor signal to noise ratio as only few molecules can sorb into very thin layers. Further investigations of the influences of the thickness of the sensitive Makrolon layer can be found in [227].

figure 25:  Kinetics of sorption and desorption of ethanol for a thin and a thick sensitive layer.