Diffuse-reflection measurements

Willey R R 1976 Fourier transform infrared spectrophotometer for transmittance and diffuse reflectance measurements Appl. Spectrosc. 30 593-601... 

Hexafluoroiridate(IV) salts are again obtained by hydrolysis of the corresponding M(V) species, MFe (48). For caesium hexafluoroiridate (IV) Cs2IrFe, a magnetic moment of 1.42 B.M. at 298 °K has been reported (48), confirming that the ground state is the low-spin 2T2g (t g). For the hexafluoroiridate(IV) anion Hepworth et al. (48) reported two bands in the solution spectrum — an intense absorption at 46.9 kK. and a much weaker band at 31.6 kK. — but Brown et al. and Allen et al. have both carried out diffuse reflectance measurements. 

III. THEORY ASSOCIATED WITH DIFFUSE REFLECTANCE MEASUREMENTS... 

S. J. Madsen, B. C. Wilson, M. S. Patterson, Y. D. Park, S. L. Jacques, and Y. Hefetz. Experimental tests of a simple diffusion model for the estimation of scattering and absorption coefficients of turbid media from time-resolved diffuse reflectance measurements. Applied Optics, 31 3509-3517, 1992. 

Fuller, M.P. and Griffiths, P.R. (1978) Diffuse reflectance measurements by Fourier-transform infrared spectroscopy. Anal. Chem., 50 (11), 1905-1910. 

A further benefit of the low absorbtivity of most samples in the NIR is that measurements involving scattering effects (both diffuse transmission and diffuse reflectance) are possible. The penetration depth of the sampling beam in diffuse reflectance measurements of powders in the NIR can be on the scale of millimeters, despite the numerous refractive index interfaces and extended optical path involved. Thus relatively large volumes of material can be interrogated, avoiding problems of surface contamination and sample nonhomogeneity. 

Modern infrared (IR) spectroscopy is a versatile tool applied to the qualitative and quantitative determination of molecular species of all types. Its applications fall into three categories based on the spectral regions considered. Mid-IR (MIR) is by far the most widely used, with absorption, reflection, and emission spectra being employed for both qualitative and quantitative analysis. The NIR region is particularly used for routine quantitative determinations in complex samples, which is of interest in agriculture, food and feed, and, more recently, pharmaceutical industries. Determinations are usually based on diffuse reflectance measurements of untreated solid or liquid samples or, in some cases, on transmittance studies. Far-IR (FIR) is used primarily for absorption measurements of inorganic and metal-organic samples. 

When light is directed onto a sample it may either be transmitted or reflected. Hence, one can obtain the spectra by either transmission or reflection. Since some of the light is absorbed and the remainder is reflected, study of the diffuse reflected light can be used to measure the amount absorbed. However, the low efficiency of this diffuse reflectance process makes it extremely difficult to measure 120) and it was speculated that infrared diffuse reflection measurements would be futile 120). Initially, an integrating sphere was used to capture all of the reflected light121) but more recently improved diffuse reflectance cells have been designed which allow the measurement of diffuse reflectance spectra using FT-IR instrumentation 122). 

Abstract—This study extends previous work on silanized kaolin clays to other substrates, such as aluminum hydroxide. It will also show that high precision quantitative Fourier Transform Infrared Spectroscopy (FT-IR) diffuse reflectance measurements can be performed on this vinyl silanized substrate and predict that other silanized finely divided powders can be analyzed using these techniques. 

FT-IR diffuse reflectance measurements have been used to quantitatively determine the amount of silane deposited on a substrate. The advantages of FT-IR are speed and sensitivity, the latter relating to the ultimate analytical precision or the smallest amount of the analyte one can determine. Other classical uses of infrared spectroscopy are as a qualitative identification or structure elucidation tool. 

Hanning [4], Miller and Ishida [5], and McKenzie and Koenig [6] favored transmission measurements using FT-IR for monitoring silanized mica quantitatively. Berger and Desmond [3] demonstrated the ability of FT-IR diffuse reflectance measurements to quantitate various silanized substrates including mercapto silane on hydrous clay and epoxy silane on alumina trihydrate. Later Vagberg el at. 

More recently, the need to analyze vinyl silane on an aluminum hydroxide (alumina trihydrate) substrate has arisen. The silanized aluminum hydroxide substrate system was investigated to determine whether quantitative diffuse reflectance measurements were practical for routine control of the vinyl silaniz-ation process. In this paper, we discuss a method to quantify the amount of silane adsorbed on the aluminum hydroxide substrate. 

Unlike classical analytical spectroscopy performed on liquids or dilute solutions of analytes, diffuse reflectance measurement in the near-infrared must deal with a composite effect of spectroscopic absorption and scattering from the analyte and the matrix in which it is found. Differences in refractive indices of the sample material, specular reflection and observance of relatively small differences are all dealt with in this technique. 

Granulation Sorting from Diffuse Reflectance Measurement in the Near-Infrared". Proc. Third Ann. Users Conference for NIR Researchers. Pacific Scientific, Silver Springs, MD. Feb. 1984. 

Hamada et al. 1992 Saito et al. 1992), this increase of absorption is caused by high-frequency conductivity of the free carriers in metallic nanotubes. Relative intensities of the spectra of Fig. 11.7 we have found as a result of the diffuse reflection measurements of powders at low wavenumbers. The discussed above Drude approximations of the low-energy part of the absorption spectra are shown by dashed curves in Fig. 11.7. Comparison of the spectra 1 and 2 shows that hydrogenation decreases high-frequency conductivity of the SWNTs by one order of magnitude. 

These are GC-IR (5), LC-IR (6), and diffuse reflectance (7). On-the-fly GC-IR systems are commercially available, and lower detection limits are being continually reported. While GC-IR may not replace GC/MS in residue and metabolism work, it can provide valuable data in these areas. On-the-fly LC-IR systems have been developed and are also commercially available. The major problem in these systems is the strong infrared absorbence of many common LC solvents. However, with proper selection of solvents and the development of LC conditions specifically designed for the LC-IR experiment, these problems may be overcome. Recent reports on diffuse reflectance measurements by FTIR indicate the technique may provide a method of examining formulated material or TLC spots with no sample preparation. While this technique is still in the development stage, it may become quite significant in the future. 

The reactivity of oxide supported metals has received considerable attention because of the importance of such systems in heterogeneous catalysis. The morphology (structure and size) of the supported particle and its stability, the interaction of the particle with the support, and the crossover of adsorbed reactants, products and intermediates between the metal and oxide phases are all important in determining the overall activity and selectivity of the system. Because of the relative insensitivity of an optical technique such as IR to pressure above the catalysts, and the flexibility of transmission and diffuse reflection measurement techniques, vibrational spectroscopy has provides a considerable amount of information on high area (powder) oxide supported metal surfaces. Particularly remarkable was the pioneering work of Eichens and Pliskin [84] in which adsorbed CO was characterised by IR spectroscopy on... 

In the past, extensive investigations were made to obtain better insight into the limitations of the diffuse reflectance measurement technique. Studies demonstrated that sample properties such as particle size and packing affect, in addition to the optical constants, the diffuse reflectance spectrum. The characteristics of the diffuse and specular components were studied for different particle sizes and dilution within a non-absorbing inert matrix. It was found that specularly and diffusely reflected radiation coexist in the measured diffuse reflectance spectrum, even in KCl-diluted samples. In addition, the specular component, which is certainly sample-dependent, is not necessarily the same as from the front-surface reflection.To prove this, a top layer of pure KCl powder was... 

In diffuse reflection measurements of particulate materials, scattering effects also lead to maximum pathlengths of several hundred micrometers of the photons within the sample (see below and Chapter 1). 

Figure 9.1 (a) The NIR imaging system (b) The optical scheme of the instrument for diffuse-reflection measurements of a powder sample. 

Some possible schematic NIR radiahon pathways in diffuse reflection measurements of a powder sample are outlined in Figure 9.2. The important issue with reference to the spatial resoluhon of imaging measurements is, on the one hand, the lateral extension of photons in the sample (Figure 9.2b, point 1) and, on the other hand, the maximum penetration depth of NIR radiation into the sample (Figure 9.2b, point 2). 

Figure 9.2 (a) Schematic pathways of NIR radiation in diffuse-reflection measurements of a powder sample (b) Lateral extension (b, point 1) and maximum penetration depth (b, point 2) of NIR radiation in the powder sample (for details, see the text). 

In the application discussed below, the derivation of quantitative results from NIR spectroscopic imaging data of solid drug formulations is reported. In order to assess the vaUdity of these procedures, however, the results will be compared to the compositional analysis of the same sample set by conservative NIR spectroscopic diffuse-reflection measurements with a single-element detector [68]. 

Figure 9.39 Schematic representation of the diffuse-reflection measurements with a single-element detector FT-NIR spectrometer. Reproduced with permission from Ref [68] 2008, Society for Applied Spectroscopy.

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