Applications of NIR Spectroscopy

NIR spectroscopy has been used for more than 30 years in the x)d industry and agriculture. The main applications are determination of moisture and characterization of other compounds, e. g. protein content in grain and milk products. Usually diffuse reflection is measured because then the samples need not be prepared extensively. Compared to wet chemical analysis or other instrumental methods of analysis, NIR in particular allows rapid detection even under field conditions. Some basic characteristic wavelengths are fisted in Tab. 6.4. 

Moisture and hydroxyl number are important parameters, which are determined by measuring either the first overtone at 6890 cm or the combination band at 5180 cm . A few details about chemical structure are accessible by interpretation of these bands. Changes in hydrogen bonding lead to changes in the band shape and band location. Difference spectra or second derivatives must be calculated in order to detect minor chemical interactions of OH with other molecular species in the sample. The number of double bonds is another important parameter to describe the properties of fats and oils, e. g. their degree of unsaturation. 

NIR spectroscopy can also be used to identify different makes or different charges of the same product. Chemometric evaluation even of very similar looking spectra provides access to the parameter of interest or enables distinction between 

In the food industry NIR spectroscopy is the most common in-line method to monitor moisture, oil, fat and protein to analyze grains, feeds, meat, dairy and other products. Metabolites in leaves of spice plants can be determined by using NIR reflection measurements [21]. Accuracy and precision achieved are better than 0.2 % [22]. On-line measurements are also made for diverse snack food products. 

In the polymer industry, packing material, laminates including multilayer films, pellets or molded products can be analyzed by NIR. Even polymer latex particles with up to 99 % water content may be analyzed. NIR provides information about reaction mechanisms, polymerization, crystallinity, orientation, water content and hydrogen bonding, even during the process of polymer manufacture. For example the disappearance of the double bonds in polyethylene and polypropylene can be monitored. In the NIR spectrum C=C bonds lead to a combination band at about 4740 cm and a first overtone at about 6170 cm NIR spectroscopy is applied to characterize ester-, nitrile-, or amide-based acrylic and methacrylic polymers. Other examples are the identification of polyvinylchloride, polyvinyl alcohol and polyvinyl acetates or the analysis of polymerization in epoxy and phenolic resins. 

The most important bands are overtones or combinations of the stretching modes of C—H, 

and N—H. These bands enable the quantitative characterization of polymers, chemicals, foods, and agricultural products for analytes such as water, fatty acids, proteins, and the like. In many cases, the use of NIR reflectance spectroscopy has been able to replace time consuming, classical wet chemical analyses, such as the Kjeldahl method for protein nitrogen and the Karl Fischer titration for water content. The NIR region has been used for qualitative studies of hydrogen bonding, complexation in organometallic compounds, and solute-solvent interactions because the NIR absorptions are sensitive to intermolecular forces. 

Polymer characterization is an important use of NIR spectrometry. Polymers can be made either from a single monomer, as is polyethylene, or from mixtures of monomers, as are styrene-butadiene rubber from styrene and butadiene and nylon 6-6, made from hexamethylenediamine and adipic acid. An important parameter of such copolymers is the relative amount of each present. This can be determined by NIR for polymers with the appropriate functional groups. Styrene content in a styrene-butadiene copolymer can be measured using the aromatic and aliphatic C—H bands. Nylon can be characterized by the NH band from the amine monomer and the C=0 band from the carboxylic acid monomer. Nitrogen-containing polymers such as nylons, polyurethanes, and urea formaldehyde resins can be measured by using the NH bands. Block copolymers, which are typically made of a soft block of polyester and a hard block containing aromatics, for example, polystyrene, have been analyzed by NIR. These analyses have utilized the 

aromatic and aliphatic C—H bands. NIR is used to measured hydroxyl content in polymers with alcohol functional groups (polyols), both in final product and online for process control. 

Fibers and textiles are well suited to NIR reflectance analysis. Analyses include identifying the type of fiber, the uptake of dyes, the presence of processing oil in polyester yarns, and the presence of fabric sizing agents. 

Proteins in foods can be measured by NIR reflectance spectrometry with no sample preparation. This has replaced the standard Kjeldahl protein nitrogen determination, which required extensive sample preparation to convert protein nitrogen to ammonia, distillation of the released ammonia, and subsequent titration of the ammonia. The replacement of the Kjeldahl method for routine analysis by NIR has permitted online measurement of protein in food and beverage products. The Kjeldahl method is required for assaying the materials used to calibrate the NIR and for method validation. 

The use of NIR for process analysis and real-time analysis of complex samples is impressive. Grains such as wheat and corn can be measured with no sample preparation for protein content. 

The cost savings provided by the use of NIR instead of titrations for water and protein are enormous, not just in labor cost savings but in the cost of buying and then properly disposing of expensive reagents. NIR permits increased efficiency and increased product quality by onUne and at-line rapid analysis in the agricultural, pharmaceutical, polymer, specialty chanicals, and textile industries, among others. 

In general, symmetric vibrations give rise to intense Raman lines nonsymmetric ones are usually weak and sometimes unobserved. 

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S.H. Tabasi, R. Fahmy, D. Bensley, C.O Brien and S.W. Hoag, Quality by design. Part II Application of NIR Spectroscopy to monitor the coating process from a pharmaceutical sustained release product, J. Pharm. Scl, 97 (9), 4052 066 (2008). 

NIR Spectroscopy in Pharmaceutical Analysis 479 Table 14.1 Applications of NIR spectroscopy to API and excipient Identification... 

Shenk, J.S., Workman, J.J. Jr and Westerhaus, M.O. (1992) Application of NIR spectroscopy to agricultural products. In Flandbook of Near-Infrared Analysis. Marcel Dekker, New York, pp. 383 29. 

The calibration methods most frequently used to relate the property to be measured to the analytical signals acquired in NIR spectroscopy are MLR,59 60 principal component regression (PCR)61 and partial least-squares regression (PLSR).61 Most of the earliest quantitative applications of NIR spectroscopy were based on MLR because spectra were then recorded on filter instruments, which afforded measurements at a relatively small number of discrete wavelengths only. However, applications involving PCR and PLSR... 

Fevotte, G. Calas, J. Puel, F. Hoff, C., Applications of NIR spectroscopy to monitoring and analyzing the solid state during industrial crystallization processes Int. J. Pharm. 2004, 273, 159-169. 

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). 

Quantification of phenolic compounds in wine and during key stages in wine production is therefore an important quality control goal for the industry and several reports describing the application of NIR spectroscopy to this problem have been published. 

Although near-infrared radiation was discovered by Herschel in 1800, NIR spectroscopy has only recently become an established technique for process analysis [6, 7]. The increasing success and widespread application of NIR spectroscopy in this area is a result of several advantageous features and technical developments. 

The applications of NIR spectroscopy to process analysis and control are far too numerous to comprehensively cover in a text of this type. A regular review of literature in this area is given in NIR News [10]. Examples in Tab. 17.1 illustrate the breadth of NIR applications to process analysis. Table 17.2 summarises the use of NIR in process analysis. 

Table 17.1 Examples of the application of NIR spectroscopy to on-line process analysis...

This section has been included to illustrate the factors that can coalesce to defy application of NIR spectroscopy to grains and seeds—but the more rebellious beUev-ers in the early days of NIRS application managed to overcome the more hazardous hazards, and in doing so were able to uncover the plethora of factors that can affect the technology. For example, the importance of grain texture has been mentioned above, and over 20 factors have been shown to affect wheat kernel texture (110). 

J. S. Shenk, J. K. Workman Jr., M. O. Westerhaus. Application of NIR Spectroscopy to Agricultural Materials. In Handbook of Near-Infrared Analysis. 2nd ed. A. Donald, Burns, Emil W. Ciurczak, co-eds. Marcel Dekker, Inc. New York, 2001, p. 431-433. 

The application of NIR spectroscopy to the analysis of cereal food products has been reviewed by Osborne (39) and Kays (22). These reviews cover the use of NIR spectroscopy for the analysis of the composition of diverse cereal foods. The current chapter describes the approaches used in the development of NIR models for cereal foods. These approaches include cereal product sample preparation, sample conditioning, the importance of selecting the correct reference method where, in most cases, several reference methods are available, and the use of specific instruments to optimize sampling and performance. In addition, a table is presented with a comprehensive list of published research on the application of NIR spectroscopy to cereal food products from 1979 to the present. The application of NIR to the analysis of cereals processed into flour and baking products is reviewed in a prior section of this book. 

A comprehensive list of research published on the application of NIR spectroscopy to the analysis of cereal food products is presented in Table 8.2.1. The table includes notes on the instrumentation and reference analyses used in the studies. 

TABLE 8.2.1. Research Published on Application of NIR Spectroscopy to Analysis of Cereal Food Products... 

J. Rodgers and S. Ghosh, At-Line Application of NIR Spectroscopy for Textiles, Eastern Analytical Symposium, Somerset, NJ, 1995. 

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