Mirrors transmission considerations

In solvent-elimination LC-FTIR, basically three types of substrates and corresponding IR modes can be discerned, namely, powder substrates for diffuse reflectance (DRIFT) detection, metallic mirrors for reflection-absorption (R-A) spectrometry, and IR-transparent windows for transmission measurements [500]. The most favourable solvent-elimination LC-FTIR results have been obtained with IR-transparent deposition substrates that allow straightforward transmission measurements. Analyte morphology and/or transformation should always be taken into consideration during the interpretation of spectra obtained by solvent-elimination LC-FTIR. Dependent on the type of substrate and/or size of the deposited spots, often special optics such as a (diffuse) reflectance unit, a beam condenser or an FITR microscope are used to scan the deposited substances (typical diameter of the FITR beam, 20 pm). 

To validate the whole technique, a transmission spectrum of the neat liquid silane was compared with one obtained by specular reflectance from an aluminium mirror surface on which a film of silane had been cast from 1% solution in anhydrous methanol. These two spectra were virtually indistinguishable, as regards both position and intensity of all peaks. They contained a considerable number of peaks, most of which could be assigned with complete satisfaction in terms of the known structure of y-glycidoxypropyltrimethoxy-silane. 

If the spatial resolving power has to be high, then the Raman radiation must be observed through microscope objectives (Fig. 3.5-10 b). Unfortunately, these objectives have a somewhat lower optical conductance than the regular sample arrangement (Schrader, 1990). As a result, the observed Raman spectrum is also considerably weaker. A microscope may be connected to the spectrometer by a mirror system or by optical fibers, as shown in Fig. 3.5-10 b. Optical fibers are e.specially useful for NIR FT Raman spectroscopy, because the transmission of the fibers may be at its maximum exactly in the range of a Raman spectrum excited by a Nd YAG laser (Fig. 3.3-5). 

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