Diffuse-reflection measurements sample-layer thickness

The quantitative description of diffuse reflectance measurements is known from thin-layer chromatography. Taking a powdery sample with such a large thickness (d = oo) that no reflection at the support material can be observed, the reflectivity of the sample is connected to the stray coefficients as well as with the absorption coefficient k. Kubelka and Munk [97]... 

Measurement oftransmittance Td and reflectance Ro of one sample with known layer thickness d. Diffuse irradiation and detection. Evaluation with Eqs. (8.16) and (8.17). 

UV-VIS-NIR diffuse reflectance (DR) spectra were measured using a Perkin-Elmer UV-VIS-NIR spectrometer Lambda 19 equipped with a diffuse reflectance attachment with an integrating sphere coated by BaS04. Spectra of sample in 5 mm thick silica cell were recorded in a differential mode with the parent zeolite treated at the same conditions as a reference. For details see Ref. [5], The absorption intensity was calculated from the Schuster-Kubelka-Munk equation F(R ,) = (l-R< )2/2Roo, where R is the diffuse reflectance from a semi-infinite layer and F(R00) is proportional to the absorption coefficient. 

This method is compared to a totally independent procedure, the VLT method, where diffuse reflection spectra are collected at several controlled powder thicknesses. Assuming that the backing is either completely transparent or completely opaque, the diffuse reflectance of the powder layers increases with increasing sample thickness until the reflectance of an optically thick sample (Roo) is reached. For each measured wavelength, an exponential function is fitted to the experimental data (plots of log(l /R) vs. sample thickness, where R is the measured reflectance). Using the 98% limit, the effective sample size can be obtained from the exponential fit (see [75] for a detailed discussion of the VLT method). 

Diffuse reflectance is a technique where source radiation strikes a powdered sample and is diffusely reflected in different directions." This weak diffuse radiation is collected in a manner which minimizes the specular reflectance component and is measured by the spectrometer which is usually an FT-IR instrument. Usually the finely powdered sample is diluted to a concentration of 5 to 10% in powdered KBr or KCl for mid-IR diffuse reflectance spectra. No dilution is required in the near-IR region, where the bands are weaker. Pure powdered KBr or KCl is used as a reference against which the sample spectrum is ratioed as shown below, where is the reflectance of a thick scattering layer. 

For measurements of samples with 1-5% by weight, an ordinary cup with a sample layer of about 1 mm will suffice, but for measuring more dilute samples, the use of a 2-3 mm-thick sample layer will be more appropriate. In Figure 12.7, the diffuse-reflection spectra of a powder sample of j6-naphthylmethylether (0.047% by weight diluted in KBr) measured... 

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