Diffuse-reflectance spectroscopy limitations

Scheinost, A.C. Chavernas, A. Barron,V. Tor-rent, J. (1998) Use and limitations of second-derivative diffuse reflectance spectroscopy in the visible to near-infrared range to identify and quantify Fe oxide minerals in soils. Clay Min. Soc. 46 528-536... 

Along this line, the limitations of the technique used must be recognized. Some measure predominantly bulk properties, e.g., X-ray diffraction and magnetic susceptibility whereas, others are sensitive to surface composition, e.g., adsorption and ESCA. For example, in one reported study only cobalt in tetrahedral coordination was found on a catalyst by diffuse reflection spectroscopy, but magnetic measurements revealed that octahedral cobalt must also be present (10). Thus, it is dangerous to rely on any one method to characterize these catalysts. 

Diffuse reflectance spectroscopy has been widely used to characterize the surface of solids as well as films and coatings present on a solid substrate (6). The application of this technique to the study of adsorbed species has been much more limited (26) and, thus far, has not involved the use of FT spectroscopy. 

As illustrated by the examples discussed here, the use of FT spectrometers for the observation of surface structures is favored by situations in which the flux of radiation coming from the sample is very low or the data acquisition time is limited. Such cases arise in transmission spectroscopy using strongly absorbing or scattering samples, specular and diffuse reflectance spectroscopy from opaque samples, and emission spectroscopy from low temperature sources. FT spectroscopy is also well suited for observing the dynamics of surface species during adsorption, desorption, and reaction. 

Fig. 1 Accessories for diffuse reflectance spectroscopy (A) Integrating sphere with hemispherical radiation collection (B) Accessory based on ellipsoidal mirrors, used within the sample compartment of the spectrometer (C) Rotational ellipsoidal mirror device with dedicated detector and (D) Bifurcated fiber optic-based accessory (also shown is the random mixture of fibers for illumination and detection compared with devices based on reflection optics the acceptance cone for radiation delivery and collection is limited and depends on the refractive indices of the core and cladding material).

Alternative 4. This is in fact a combination of alternative 2 and 3 (MSFIA-GD-optosensor coupling) that overcomes the loss of sensitivity of membrane-based separation techniques through sorptive flow-through diffuse reflectance spectroscopy, yielding an limits of detection (LOD) of 1 pg/1 (Figure 7.7). The resulting method is an attractive choice for an online sample processing whenever trace-level concentrations of sulfide are to be determined in environmental complex matrices [16]. 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

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 ... 

Measurements of supported catalysts in diffuse reflection and transmission mode are in practice limited to frequencies above those where the support absorbs (below about 1250 cm-1). Infrared Emission Spectroscopy (IRES) offers an alternative in this case. When a material is heated to about 100 °C or higher, it emits a spectrum of infrared radiation in which all the characteristic vibrations appear as clearly recognizable peaks. Although measuring in this mode offers the attractive advantage that low frequencies such as those of metal-oxygen or sulfur-sulfur bonds are easily accessible, the technique has hardly been explored for the purpose of catalyst characterization. An in situ cell for IRES measurements and some experiments on Mo-O-S clusters of interest for hydrodesulfurization catalysts have been described by Weber etal. [11],... 

Direct NIR or MIR measurements of whole soil can be made using attenuated total reflectance (ATR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) samplers. This type of measurement detects only components on the surface and so has severe limitations when information about the bulk soil is needed [4],... 

Because chemical and structural properties of natural and artificial gems are very similar in this case, the possibilities of Raman and LIBS methods are rather limited. It was found that another laser-based techniques could be very effective for rapid spectroscopic discrimination between natural and synthetic emeralds, rubies, and alexandrite (Armstrong et al. 2000a,b). The first one is DRIFTS (Diffuse Reflectance Fourier Transformed Infra-Red Spectroscopy)... 

Newton MA, Dent AJ, Fiddy SG, Jyoti B, Evans J. Combining diffuse reflectance infrared spectroscopy (DRIFTS), dispersive EXAFS, and mass spectrometry with high time resolution potential, limitations, and application to the study of NO interaction with supported Rh catalysts. Catal Today. 2007 126 64. 

An investigation was carried out into the fire retardant behaviour of zinc hydroxystannate-coated fillers (alumina trihydrate and magnesium hydroxide) in PVC and EVA cable formulations. Measurements were made of the limiting oxygen index, peak rate of heat release and smoke parameter and the data for unfilled and filled formulations compared. X-ray photoelectron spectroscopy and diffuse reflectance infrared Fourier-transform spectroscopy were used to study the filler-coating interaction. 16 refs. 

Fig. 5 Comparison of mid-infrared spectra of caffeine obtained by diffuse reflectance and transmission spectroscopy. (A) Diffuse reflectance spectrum of the pure powdered substance with transformed intensity data in K-M units. (B) Same diffuse reflectance spectrum, but using —log(i ) transformation (top trace), the lower spectral range was limited by the cut-off of the MCT detector used the bottom trace shows a transmission spectrum using the conventional KBr pellet technique transformed into absorbance, i.e., —log(transmittance).

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