Kubelka-Munk unit

Diffuse reflectance FTIR spectra of the ground Mo03/Al203 catalysts were recorded on an FTIR instrument (Nicolet, Model 740, MCT detector). The microreactor in the flow system was replaced by an FTIR cell. The cell used a Harrick diffuse reflectance accessory (DRA-2CO) fitted with a controlled environmental chamber (HVC-DRP). Spectra (500 scans, 4 cm 1 resolution) were presented in Kubelka-Munk units and recorded at RT. 

Figure 8A Diffuse reflectance spectra recorded following the steady-state photolysis (visible light) of Acid Orange 7 adsorbed on Ti02 nanoparticles (0.02 mmol A07/g of Ti02). The ordinate scale is expressed in Kubelka-Munk units, where R is the reflectivity measured at the corresponding wavelength. (From Ref. 258.)...

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 (in Kubelka-Munk units) of silylated Si02 and AI2O3 substrates.

Fig. 3. v(CH) range of different epoxysilanes SA-layers on silica (absorbance given in Kubelka-Munk units) (A) pure silane (B) silane in H2O (C) silane in toluene. 

The measurement of diffuse reflectance effectively involves focusing the infrared source beam onto the surface of a powder sample and using an integrating sphere to collect the scattered infrared radiation.59 The technique requires careful attention to sample preparation, and often one must dilute the analyte with KBr powder to reduce the occurrence of anomalous effects.60 In practice, one obtains the spectrum of the finely ground KBr dispersant, and then ratios this to the spectrum of KBr containing the analyte. The relative reflectance spectrum is converted into Kubelka-Munk units using standard equations,61 thus obtaining a diffuse reflectance spectrum that resembles a conventional IR absorption spectrum. 

FIGURE 2 Infrared absorption intensity (expressed in terms of Kubelka-Munk units) of sulfamethoxazole Form-1 (absorbance at 3468 cm-1) and Form-ll (absorbance at 3476 cm-1) as a function of concentration in a KCI matrix.The figure was adapted from data presented in reference 62. 

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. 

Instrumentation. Spectra were acquired with a Nicolet 60SX FTIR spectrometer, continuously purged with dry air and equipped with a liquid-nitrogen-cooled, wideband mercury-cadmium telluride detector. Coaddition of 100 interferometer scans at 8-cm 1 resolution was employed. The location of absorption maxima was confirmed by spectra taken at l-cm 1 resolution. All spectra were converted into Kubelka-Munk units prior to use. Integration of peak areas was accomplished by using software available on the Nicolet 60SX. All peak areas were normalized to the 1870-cm-1 Si-O-Si combination band (15). 

Fig. 2 - Part (a) ftom bold to dotted curves, evolution of the UV-Vis spectra, collected in transmission mode, of the NaaPdCh bare mother solution (3.2-10" M) as a function of the pH increase (only a selection of curves has been reported). Part (b) as part (a) for the Na2PdCU solution previously acidified with HCl down to pH = 1.4. Both insets report absorption intensity of the two main CT components as a function of pH (triangles and squares refer to the high and low frequency component, respectively). Part (c) UV-Vis DRS spectra of the precipitates obtained from the two solutions analyzed in parts (a) and (b), grey and black curve, respectively. Part (d) XRD pattern of the precipitate phase from the bare mother solution, a.u. = absorbance imits K-M u. = Kubelka-Munk units.

When diffuse reflectance measurements are taken of the adsorbed tetrahedral species and the transmittance data are transformed into Kubelka-Munk units (Eq. 1), a plot versus loading affords a linear correlation analogous to the... 

Figure 10. Linearity of Kubelka-Munk function with surface loading of [Co(neo)]. Transmittance values were obtained from diffuse reflectance UV-vis spectra taken at 654 nm. Data are shown for low loadings of [Co(neo)] on Merck silica grade 9385 and the values were converted to Kubelka-Munk units. Transmittance values < 5% were not used. The concentration of Co was determined by removal of the adsorbed complex by treatment with 1M HCl followed by colorimetric determination with ammonium thiocyanate. [Adapted from (57).]...

The Kubelka-Munk type is designed for spectra that are measured in diffuse reflectance. The spectral intensities in Kubelka-Munk units are more linear in concentration than those in absorbance units. The conversion formula is... 

In this method,the sample can be analyzed either directly in bulk form or as dispersions in IR transparent matrices such as KBr and KCl. Sometimes, a thin film of KBr powder placed on the sample surface to improve the quality of the spectrum. Dilution of analyte in a nonabsorbing matrix increases the proportion of diffuse reflectance in the reflected light. Typically, the solid sample is diluted homogeneously to 5 to 10% by weight in KBr. The spectra of diluted samples are similar to those obtained from pellets when plotted in units such as log 1/R (R is the reflectance) or Kubelka- Munk units. 

Figure 7. Diffuse reflectance infrared difference spectra from 100 to 910 cm of only the plumbosiloxane contribution near 965 cm f. Spectra were obtained using the digital subtraction method and are not scale expanded. The adsorbate concentrations and measured peak to baseline intensity differences in Kubelka-Munk units are respectively, (A) 0.40 mg/m and 0.005, (B) 0.80 mg/m and 0.009, (C) 1.50 mg/m2 and 0.018, (D) 2.00 mg/m2 and 0.015, (E) 4.00 mg/m2 and 0.016, (F) 6.00 mg/m2 and 0.014.

The spectra (Figures 3.16, 3.17 and 3.18) are presented in transmittance mode and not Kubelka-Munk units as discussed previously. However, the purpose of this investigation was to understand the chemical reactions taking place at the filler surface and not to make any quantitative measurements. 

One important fact to note this is only applicable to spectra that have responses that are fairly linear in concentration. Therefore, any spectra collected in reflectance units should be converted to log(l/R) or Kubelka-Munk units before applying MSC. In addition, MSC works very well with spectra of samples that are chemically similar. If the appearance of the spectra in the training set are substantially different from one another due... 

The Fourier transformed interferograms provide IR spectra that can be recorded at will in normalized reflectance spectra (reflectance units R) (Fig. 17A), in quasi-absorbance units that are not proportional to concentration (-log/ ) (Fig. 17B), or in Kubelka-Munk units that are proportional to concentration (Fig. 17C). The substances can be localized on the TLC plate by using either spectral windows chosen at will (Fig. 18A and also dashed line in B) or the Gram-Schmidt technique (Fig. 18 dotted line in B). The first method can be used to increase selectivity (e.g., the spectral window can be chosen so as to detect only compounds with carbonyl groups), while the latter is universally applicable and independent of wave number. 

Figure 17 HPTLC-FTIR spectra of nitrazepam in reflectance units (A), in quasi-absorbance units (B), and in Kubelka Munk units (C) on a silica gel plate.

Diffuse reflection spectra are often transformed to Kubelka-Munk units as defined below, just as spectra measured in transmission are converted to absorbance ... 

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. 

Kubelka-Munk Units Y-axis units used to plot diffuse reflectance spectra when they are used for quantitative analysis. 

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