Diffuse-reflection measurements examples

To continue the derivation, the next step is to determine the variation of the absorbance readings starting with the definition of absorbance. The extension we present here, of course, is based on Beer s law, which is valid for clear solutions. For other types of measurements, diffuse reflectance for example, the derivation should be based on a suitable function of T that applies to the situation, for example the Kubelka-Munk function for diffuse reflectance should be used for that case ... 

A number of recent analytical techniques have been used to improve the reliability of chemical analysis from a qualitative and quantitative point of view. The work of Tubino et al.13 should be mentioned as an example of using fiberoptic devices for spot tests of diffuse reflectance measurements. 

Various forms of spectroscopy have been applied to in situ studies of catalysis, and it is appropriate to cite a few examples. FT-IR is frequently employed for in situ investigations. The experimental configurations used can be either transmission studies of free-standing catalyst wafers [2] or diffuse reflectance measurements on samples in catalytic reaction chambers 13]. In situ Raman spectroscopy has also been applied [4]. X-rays have been used to study catalysts in situ, either by powder diffraction methods [5,6] or XAFS [7]. In situ imaging techniques are beginning to be applied to the measurement of spatial distributions and residence times in catalytic reactors. A recent example of this method employed positron-emission tomography [8]. 

As diffuse reflection spreads out over a large solid angle from the surface of a powder sample, an optical system is required to collect reflection radiation as efficiently as possible over a large solid angle. An example of an accessory for diffuse-reflection measurements is schematically illustrated in Figure 12.5. Two types of cups for diffuse-reflection measurements are commonly commercially available the diameter of the larger (ordinary) one... 

There are other sources of noise, whose behavior cannot be described analytically. They are often principally due to the sample. A premier example is the variability of the measured reflectance of powdered solids. Since we do not have a rigorous ab initio theory of diffuse reflectance, we cannot create analytic expressions that describe the variation of the reflectance. Situations where the sample is unavoidably inhomogeneous will also fall into this category. In all such cases the nature of the noise will be unique to each situation and would have to be dealt with on a case-by-case basis. 

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. 

The application example outlined in detail in this chapter refers to diffuse-reflection NIR chemical imaging measurements of a pharmaceutical dmg... 

Figure 18. Example of diffuse reflectance spectroscopy in vivo. The reflectance spectrum from normal skin/muscle was measured in an animal model following intravenous administration of a non-fluorescent photosensitizer. The spectra show monomeric and aggregated drug and the changes in these components with time after injection.

IR spectroscopy is one of the few analytical techniques that can be used for the characterization of solid, liquid, and gas samples. The choice of sampling technique depends upon the goal of the analysis, qualitative identification or quantitative measurement of specific analytes, upon the sample size available, and upon sample composition. Water content of the sample is a major concern, since the most common IR-transparent materials are soluble in water. Samples in different phases must be treated differently. Sampling techniques are available for transmission (absorption) measurements and, since the advent of FTIR, for several types of reflectance (reflection) measurements. The common reflectance measurements are attenuated total reflectance (ATR), diffuse reflectance or diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and specular reflectance. The term reflection may be used in place of reflectance and may be more accurate specular reflection is actually what occurs in that measurement, for example. However, the term reflectance is widely used in the literature and will be used here. 

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