The isolation of anylyte molecules in a matrix or on a substrate makes PDMS-spectra more reproducible and often enhances the detection sensitivity, especially if the formation of stabile analyte ions is supported by chemical or other process-induced interactions between analyte and matrix/substrate. These interactions have a strong influence on the PDMS-yield. The PDMS-yield is also strongly dependent on analyte-analyte interactions if the analyte concentration is high. For example, high local concentrations which are produced by self-aggregation or cluster formation have a considerable effect on the PDMS-yield. Precise informations about the amount and the formation of molecules on the sample are required for studying these correlations. We applied optical spectroscopy methods for characterizing our samples. The absorption of light from the near IR to UV ( via diffuse reflectance ) and the fluorescence ( steady state and time-resolved ) were determined. Many informations about sample composition and structure can be obtained by evaluating the optical spectra. The determination of low analyte concentrations is one of the very useful advantages of this application. For example, a coverage of 6*10^12 adsorbates per cm^2 can be determined precisely enough by the diffuse reflectance method.