NIR diffuse reflectance

W.F. McClure, A. Hamid, F.G. Giesbrecht and W.W, Weeks, Fourier analysis enhances NIR diffuse reflectance spectroscopy. Appl. Spectrosc., 38 (1988) 322-329. 

In spectroscopy applications, a hrst derivative effectively removes baseline offset variations in the spectral prohles. Second-derivative pretreatment resnlts in the removal of both baseline offset differences between spectra and differences in baseline slopes between spectra. Its historical effectiveness in NIR diffuse reflectance applications snggests that baseline slope changes are common in these applications, although there is no theoretical basis for snch variations. 

Figure 12.6 The effect of Savistky-Colay first and second derivative preprocessing on a set of NIR diffuse reflectance spectra (A) the raw (uncorrected) spectra, (B) spectra after Ist derivative preprocessing, (C) spectra after 2nd derivative preprocessing. In both cases the window width was 15 points (7.5 nm), and the polynomial order was 2.

Figure 12.7 shows the results obtained when MSC preprocessing (using the mean as the reference spectrum) is applied to the same NIR diffuse reflectance spectra that were shown previously. Note that both SNV and MSC accomplish some degree of correction of multiplicative variations, although these corrections are somewhat different. Although these two methods make different assumptions about the multiplicative variations in the spectral data, in most practical cases they perform rather similarly when these variations are present in the data. 

To illustrate some commonly encountered classification methods, a data set obtained from a series of polyurethane rigid foams will be used [76]. In this example, a series of 26 polyurethane foam samples were analyzed by NIR diffuse reflectance spectroscopy. The spectra of these foams are shown in Figure 12.16. Each of these foam samples belongs to one of four known classes, where each class is distinguished by... 

Figure 12.16 NIR. diffuse reflectance spectra of 26 poly(urethane) foams, used to demonstrate different classification methods.

In diffuse reflection spectroscopy, the spectrometer beam is reflected from, scattered by, or transmitted through the sample, whereas the diffusely scattered light is reflected back and directed to the detector. The other part of the electromagnetic radiation is absorbed or scattered by the sample [124,125]. Changes in band shapes or intensity as well as signal shifts can be affected by morphological and physicochemical properties of the sample or combinations thereof (e.g., chemical absorptions, particle size, refractive index, surface area, crystallinity, porosity, pore size, hardness, and packing density [126]). Therefore, NIR diffuse reflection spectra can be interpreted in dependence of various physical parameters [127]. 

UV-VIS-NIR diffuse reflectance (DR) spectra were measured using a Perkin-Elmer UV-VIS-NIR spectrometer Lambda 19 equipped with a diffuse reflectance attachment with an integrating sphere coated by BaS04. Spectra of sample in 5 mm thick silica cell were recorded in a differential mode with the parent zeolite treated at the same conditions as a reference. For details see Ref. [5], The absorption intensity was calculated from the Schuster-Kubelka-Munk equation F(R ,) = (l-R< )2/2Roo, where R is the diffuse reflectance from a semi-infinite layer and F(R00) is proportional to the absorption coefficient. 

Some of the pre-treatment methods discussed below will be illustrated using a set of NIR diffuse reflectance spectra. For reference, the uncorrected NIR spectra are shown in Figure 8.3. 

This pre-treatment method1,23 has been effectively used in many NIR diffuse reflectance applications, and in other applications where there are multiplicative variations between sample response profiles. In spectroscopy, such variations can be caused by differences in sample pathlength (or effective pathlength, in the case of reflectance spectroscopy). It is important to note that multiplicative variations cannot be removed by derivatives, mean-centering, or variable-wise scaling. 

McClure, W.F., Hamid, A., Giesbrecht, F.G. and Weeks, W.W., Fourier Analysis Enhances NIR Diffuse Reflectance Spectroscopy Appl. Spectrosc. 1984, 38, 322-329. 

The application of NIR spectroscopy has been further stimulated by the development of NIR diffuse reflectance techniques which are widely used in the analysis of agricultural, pharmaceutical, biochemical and synthetic polymer materials (Siesler, 1991). The rapidly increasing use of NIR spectroscopy is illustrated in the book Making Light Work Advances in Near infrared spectroscopy , edited by Murray and Cowe (1992) as well as in the Handbook of Near-Infrared Analysis by Bums and Ciurcak (1992). 

The advantages of NIR diffuse reflectance techniques which rapidly develop are due to the direct analysis of solids without any necessity for special sample preparation (Weyer, 1985 Stark et al., 1986 Murray and Cowe, 1992). In NIR absorption and reflectance techniques fibre optics may be used which allow analysis remote from the spectrometer. 

Gao, X. and Wachs, I.E. (2000) Investigation of surface structures of supported vanadium oxide catalysts by UV-vis-NIR diffuse reflectance spectroscopy. Journal of Physical Chemistry B, 104 (6), 1261-8. 

North, N. Young, K. Leng, M. A comparison between NIR diffuse reflectance and transmittance for the analysis of pharmaceutical tablets. 8th FOSS/NIR Systems European Pharmaceutical User Group Meeting, SmithKline Beecham, Harlow, UK, October 1-8, 1997. 

Whitfield [13] was one of the first NIR spectroscopists to discuss the use of NIR diffuse reflection analysis for veterinary products in 1986. Even then, he recognized the need for specificity and included an identification step in the analysis. This discriminant analysis program, named DISCRIM (Technicon, Tarrytown, NY) was an early version of the types of algorithms available commercially now. 

G. E. Ritchie, L. Gehrlein, and E. W. Ciurczak, Simultaneous Development, Validation and Implementation of a Near-Infrared (NIR) Diffuse Reflectance Spectroscopic Identification Method For Pharmaceutically Active And Inactive (Placebo) Clinical Dosage Forms, Proc. PittCon, New Orleans, March, 2000. 

Ritchie, G. E., Gehrlein, L., and Ciurczak, E. W. Simultaneous development, validation and implementation of a near-infrared (NIR) diffuse reflectance spectroscopic identification method for pharmaceutically active and inactive (placebo) clinical dosage forms. In Pittsburgh Conference of Analytical Chemistry and Applied Spectroscopy, March 2000, New Orleans, LA. 

NIR Diffuse-reflection (Spectral resolution 16 5300-4000 cm ) single-element detector spectroscopy cmspectral calibration range 9050-7450, 7100-5570, ... 

NIR Diffuse-reflection single-element detector spectroscopy ... 

There have been many studies of the most effective way to address a common feature in NIR and Raman spectra of reacting systems, viz. the change in spectral baseline. Figure 3.48(a) shows a typical output from a NIR diffuse-reflectance spectral measurement of 12 kinds of ethylene-vinyl acetate (EVA) copolymers differing in vinyl acetate content (Shimoyama et al, 1998). 

V wide-line NMR spectra were collected on a Bmker MSL 300 FT NMR spectrometer operating at 78.9 MHz and equipped with a special probe head for measurements in the absence of air. UV-Visible-NIR diffuse reflectance spectra (DRS) were measured in air using a Perkin Elmer Lamda 19 instrument after transformation of reflectivity (R) according to the Kubelka-Munk function F(R) = (l-R)V2R.o. BaS04 was used as a reference. 

VIS-NIR diffuse reflectance spectroscopy, by which the processes taking place (decarbonylation, changes in the oxidation state, clustering of metal atoms) are monitored. 

Marbach, R. Kosehinsky, T. Gries, F.A. Heise, H.M. Noninvasive blood glucose assay by NIR diffuse reflectance spectroscopy of the human inner Up. Appl. Spectrosc. 1983, 47 (7), 875. 

Balsam, W. L. B. C. Deaton, 1996. Determining the composition of late Quaternary marine sediments from NUV, VIS, and NIR diffuse reflectance spectra. Marine Geol. 134 31-55. 

UV/VIS and NIR diffuse reflection spectra are usually measured by an integrating sphere. The inner surface of the so-called Ulbricht sphere (Fig. 5.8) is coated by strongly scattering, non-absorbing powder. After repeated reflections at the inside of the sphere aU radiation will eventually reach the detector. 

A surface method of measurement of a sizing agent (partly hydrolysed polyvinyl alcohol) on the warp yam substrate (polyester/cotton) using NIR diffuse reflectance spectroscopy has been described (337). A partial least-squares (PLS) modelling procedure used a frequency segment of the NIR spectrum that is most sensitive to changes in size concentration relative to the warp yarn. 

A dominant feature of NIR diffuse reflectance spectra is the increase in absorbance values from 1100 to 2500 nm. This trend is curvilinear for densely packed samples. A second-degree polynomial function has proved to be an adequate model to linearize the spectral baseline. 

Figure 6 Canonical variates calibration and prediction sample plot of NIR diffuse reflectance spectra characterized as A, A, country A , Ocountry B (authentic) and , , country B (adulterated). Solid symbols, calibration samples hollow symbols, prediction samples.

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