9.1.1.   Single Analytes

The changes of the resonance wavelength when the device was exposed to alternating sequences of different concentrations of analyte and to synthetic air are shown in figure 15 in section 4.5.2.1 with the shift of the SPR resonance wavelength plotted versus the time and versus the relative saturation pressure of analyte (0 to 0.045). It is visible that 10 seconds of exposure to methanol vapor are enough to reach equilibrium between the methanol vapor and the methanol, which is sorbed into the polycarbonate layer, and thus the resonance wavelength does not change any more. In contrast, even after 120 s of exposure of the polycarbonate layer to ethanol vapor equilibrium has not been reached. However, after 120 s the signals of ethanol are higher than the signals of methanol for the same relative saturation pressure as ethanol has a higher refractive index (n_D20 =1.362) than methanol (n_D20 =1.329). Since the diffusion of ethanol is slower, the signals at short exposure times are lower compared to methanol.