Kubelka DRIFT

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

If the scattering coefficient does not depend on the infrared frequency, the Kubelka-Munk function transforms the measured spectrum RJ V) into the absorption spectrum K v). In situ cells for DRIFT studies of catalysts have been described [10] and are commercially available. 

Diffuse reflectance IR spectroscopy has become an attractive alternative to mulls with the introduction of DRIFT cell by Griffiths,29 later modified by Yang.30 Since materials are dispersed in a nonabsorbing medium and not subjected to thermal or mechanical energy during sample preparation, DRIFT spectroscopy is especially suitable for the qualitative/quantitative analysis for polymorphs, which are prone to solid-state transformations. The Kubelka-Munk (K-M) equation,31 which is analogous to Beer s law for transmission measurements, is used to quantitatively describe diffusely-reflected radiation ... 

The DRIFT spectra were recorded by a FTS 165 spectrometer of BIO-RAD Laboratories (Philadelphia, USA) using the praying-mantis-diffuse-reflection attachment. Unmodified metal or metal oxide particles were used as reference. The spectra were measured in Kubelka-Munk units collecting 32 scans. For data post processing the spectra were transformed in ASCII-files and processed in Origin 5.0. Smoothing was not done. 

Fig. 2 DRIFT spectra of PVFA-co-PVAm modified ZnO particles where the degree of the PVFA-co-PVAm hydrolysis was varied a degree of hydrolysis = 0% b degree of hydrolysis = 10% c degree of hydrolysis = 30% d degree of hydrolysis = 50% and e degree of hydrolysis > 90%, desalted. The molar mass of the non-hydrolysed PVFA polymer was 340 000 g mol-1, during the adsorption the pH was pH = 8. The spectra show the overtone amide-II band at v = 3053 cm-1 (A), the amide-I band at v = 1675 cm-1 (71), the NH deformation vibration at v = 1590 cm-1 (C), the amide-H band at v = 1540 cm-1 (/.)). (v = wave number, a.u. = arbitrary units according to Kubelka and Munk)...

Fig. 2. DRIFT spectra (in Kubelka-Munk units) of silylated Si02 and AI2O3 substrates.

Qualitative and Quantitative Analysis of the DRIFT Spectra. DRIFT spectra are usually presented in Kubelka-Munk units. DRIFT spectra with small baseline errors can be obtained when measurements are made at ambient temperature. However, if measurements are performed at higher temperatures, IR radiation emitted from the heated sample can affect the collected spectra, especially if MCT detectors are employed. This is even more pronounced when the refractivity of the sample changes with time. The baseline artifacts are added to the collected spectra. 

Figure 6. DRIFT spectra of the surface of a-alumina particles before and after coating with l-wt% silica. F(RoS) is the Kubelka-Munk function of reflectance.

Specifically, data was presented describing moisture desorption and intermediate temperature air oxidation of a powdered sub-bituminous coal. In comparison to its companion method, PA-IR, DRIFT spectroscopy would appear to be the technique of choice for the study of such reaction processes involving powdered samples since the temperature and environment of the sample are more conveniently controlled. Also PA-IR in general requires longer data acquisition times than DRIFT to produce a similar quality S/N ratio (34), No effort has been made in this report to treat in any way the quantitative aspects which most surely at some point must be considered. Most quantitative work involving DR spectra has utilized the Kubelka-Munk Equation to mathematically treat the data. This Equation seems to apply mainly to species in highly reflecting matrices at low dilution. Therefore, it remains to be determined what treatment may be required for DR spectral data obtained from neat materials such as coal. 

DRIFT spectra are displayed according to the Schuster-Kubelka-Munk remission function, Eq. (28) ... 

In diffuse reflectance spectroscopy, there is no linear relation between the reflected light intensity (band intensity) and concentration, in contrast to traditional transmission spectroscopy in which the band intensity is directly proportional to concentration. Therefore, quantitative analyzes by DRIFTS are rather complicated. The empirical Kubelka -Munk equation relates the intensity of the reflected radiation to the concentration that can be used for quantitative evaluation. The Kubelka-Munk equation is defined as ... 

The DRIFT spectra were taken in a Nicolet 510P instrument in which a diffuse reflectance cell (Spectra-Tech) was fitted. For obtaining a reasonable signal-to-noise ratio. 200 interferograms were collected with a resolution of 4 cm. All the spectra are presented without manipulation and only Kubelka-Munk transformations are employed for ensuring quantitivity. 

Diffusive reflectance infrared spectroscopy (DRIFT, presented as the Kubelka-Munk transformation) was used to identify the nature of the carbon and hydrogen species of the polymer-deriv amorphous products. The spectra of the pyrolyz PNMS and PCMS are... 

The results of the DRIFTS analyses reported in literature are also expressed in % A (absorbance) and Kubelka-Munk (K-M), which relates /(/ oo) functions with concentration and the scattering factor of the samples, and it is used for quantitative analyses ... 

As you can see, the peaks in the Kubelka-Munk spectrum are smaller and have different relative intensities than those in the absorbance spectrum. To be clear, the only difference between these two spectra is how the y-axis units are plotted peak positions are the same. The equation derived by Kubelka and Munk relates the intensity of diffusely reflected light to concentration [8]. If you are going to perform quantitative DRIFTS you must measure the spectra in Kubelka-Munk units to obtain a calibration line, similar to how absorbance spectra must be used for quantitation in transmission experiments (see Chapter 5). However, if one is doing qualitative analysis, DRIFTS spectra plotted in absorbance or Kubelka-Munk units may be used. Since the DRIFTS experiment does not produce a true absorbance spectrum, it is best to call the y-axis units of a DRIFTS spectrum plotted in absorbance diffuse absorbance. All the DRIFTS spectra in this chapter are plotted in diffuse absorbance. 

FIGURE 4.44 Top (solid) DRIFTS spectrum of an aspirin tablet plotted in Diffuse Absorbance. Bottom (dashed) DRIFTS spectrum of an aspirin tablet plotted in Kubelka-Munk units. 

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