Near-infrared spectroscopy:

Near infrared spectroscopy (NIRS), a technique based on absorption and reflectance of monochromatographic radiation by samples over a wavelength range of 400-2500 run, has been successfully applied for food composition analysis, for food quality assessment, and in pharmaceutical production control. NIRS can be used to differentiate various samples via pattern recognitions. The technique is fast and nondestructive method that does not require sample preparation and is very simple to use compared too many other analytical methods such as HPLC. The drawback of NIRS, however, is that the instrument has to be calibrated using a set of samples typically 20-50 with known analyte concentrations obtained by suitable reference methods such as FIPLC in order to be used for quantitative analyses. Simultaneous quantification of the 

Principles and Characteristics The near-IR region, which extends from about 780 nm to 2500 nm (or 12,820 to 4000 cm ) and is located between mid-IR (from 2500 to 50,000 nm or 4000 to 200 cm ) and visible light, was essentially 

For the ideal harmonic oscillator only the fundamental vibrations are allowed and there would be no NIR spectrum. An important consequence of the an-harmonic nature of molecular vibrations is that transitions between more than one energy levels are allowed. These transitions give rise to overtone absorption bands. The near-IR bands result from transitions between the ground state and second or third excited vibrational states. The near-IR region of the spectrum thus contains mainly overtones and combination bands of fundamental mid-IR absorption bands cfr. Fig. 1.5). The intensity of the overtones depends on the anharmonicity of the vibration. Near-IR intensities are some 10 to 100 times lower than the corresponding fundamentals in mid-IR to compensate this, samples are 0.1 to 1 mm thick, which is a large virtue in comparison to mid-IR. There is no special theory of near-IR spectroscopy. 

In comparison with process liquids, solid samples are much more difficult to be handled by continuous analytical methods. In addition, solid materials, crystalline powders or pelletised plastics are 

Near-infrared instruments of the UV-VIS-NIR type have become commercially available about 1955 with applications for agricultural commodities. Instruments designed specifically for measuring NIR energy reflected from solids have been commercially available as from 1971 [214] the development of these devices was pioneered by Norris [215]. The first successful uses of modem MRS were in the 1100-2500 nm region. NIR instrumentation is now extremely varied from UV-VIS-NIR to FTIR instm-ments, NIR reflectance instruments, PAS technology, on-line and portable analysers. 

Various NIR technologies are available from over 50 manufacturers  

The analysis of polymers is one of the most important application fields for IR spectroscopy. This kind of spectroscopy can be successfully used for the determination of chemical structures like stereo-regularity, chain conformation, orientation and crystaUinity, for identification of complex polymeric systems, for monitoring reaction processes and for the study of dynamic properties like diffusion. All these apphcations are discussed in [15]. 

The near infrared (NIR) spans the range from 12500—4000 cm (800 2500 nm) and is dominated by overtones and combinations of O—H, N H, C—H and C=0 vibrations. Overtone and combination bands are rather weak. Band intensities 

Croup Type of Vibration (v, stretching, S, bending) Wavenumber, cm Wavelength, nm 

NIR is increasingly used in process and environmental analysis, the food industry, agriculture, the pharmaceutical industry and polymer analysis. In-line measurement with fiber optics and rapid multi-component quantification are the most important advantages of NIR spectroscopy. In comparison to mid-infrared, NIR analysis is much faster and more versatile. Most samples are analysed in one minute or less. Often chemometric methods must be applied to determine the parameter of interest 

The above principles will be exemplified through a brief discussion of near-infrared spectroscopy. 

The far-IR region, as its name implies, lies to the long wavelength (low wave-number) side of the mid-IR region it may be considered as from about 400 cm extending to about 10cm Recent developments in novel instrumentation for terahertz (THz) spectroscopy, particularly within the pharmaceutical industry and security applications, have created renewed interest in 

In contrast to the far-IR-THz region, Raman spectroscopy is currently considered a spectroscopic tool that does have a very high potential for medical diagnostic applications (see Chapter 4). 

Commonly used Raman scattering and collection geometries. Reproduced from Vibrational Spectroscopic Methods in Pharmaceutical Solid-state Characterization, J. M. Chalmers and G. Dent, pp. 95-138 in Polymorphism in the Pharmaceutical Industry, ed. R. Hilfiker, Wiley-VCH Verlag GmbH Co. KGaA, Weinheim (2006). 

Hollas, Modern Spectroscopy, 3 ed., John Wiley Sons Ltd., Chichester, 1996. 

Shurvell, Spectra-Structure Correlations in the Mid- and Far-IR, in Handbook of Vibrational Spectroscopy, eds. J. M. Chalmers and P. R. Griffiths, Vol. 3, John Wiley Sons Ltd., Chichester, 2002, pp. 1783-1816. 

The spectra observed in this region involve mainly hydrogen stretching vibrations in, for example, C—H, N—H, and O—H bonds. A fundamental absorption band at a given frequency may be accompanied by bands at all multiples of this frequency. These additional bands are called overtones. The first overtone is much weaker than the fundamental, and successive overtones are progressively weaker still. Absorption bands may also occur at a frequency which is the sum or difference of two fundamental frequencies, or the sum or difference of an overtone and a fundamental frequency. These are called combination frequencies. Overtone and combination bands are most readily observed on comparatively thick samples in the region between the visible and 2.5 pi, where there are no fundamental absorption bands, and all the bands arise from this cause. 

The vibration of the X—H group is large in amplitude because of the low atomic weight of hydrogen, and consequently, deviates appreciably from true harmonic motion. The overtone and combination bands are therefore relatively intense. The phenomena most studied with near-infrared spectroscopy have been intermolecular associations, the type most familiar to biochemists being hydrogen bonding. 

For two fundamental frequencies, a and b, first overtones will occur near 2a and 2b, second overtones near 3a and 3b, etc., and combination bands can appear at a + b and a — hem . Summation bands are commonly observed in the near-infrared spectra of many molecules, but combination bands arising from difference tones are improbable in the near-infrared region at room temperature (Kaye, 1954). 

We can calculate the expected absorption bands for n-octane as an example. For a CH2 group, if we use 1460 cm as the value of the symmetrical deformation frequency (5) and 2900 cm as the value of the stretching frequency (v), we should expect some of the bands listed in Table 2.1. The approximate values of observed frequencies are also recorded in the table, and are seen to support the calculated results. 

Many near-infrared absorption bands occur with sufficient regularity to allow the characterization of certain molecular groups, just as is done with fundamental bands in the 3-15 /r range. 

Practical instructions regarding this method of analysis are beyond the scope of this book, however, some details will be given to provide a background for those wishing to make use of commercial services. Some material was originally published in Faithfull (1996). 

Absorbance signals seen in NIR consist of combination and overtone bands of hydrogen bonds such as C-H, N-H, 0-H, and S-H, which are aroused by large force constants and small mass. NIR spectra thus cover precious information on chemical as well as physical properties of analyzed samples due to characteristic reflectance and absorbance patterns [121-123], which makes this analysis method applicable to the characterization of monolithic stationary phases. 

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]. 

In-line measurements are frequently used to perform kinetic studies to follow chemical reactions or to visualize emerging physical and chemical properties like quantities of analytes, particle, or pore size. 

The absorption fraction of a particle is related to the volume of the particle. Thus, the larger the volume of a particle, the more of the incident light is absorbed. In contrast, reflectance is related to the particles surface area, being in turn dependent on material porosity. The absorption/remission function relates to the fraction of absorbed light, the fraction of remitted (or back scattered) light, and the fraction of light transmitted by a representative layer 

The actual limits, particularly the lower wavelength one, seem to be subject to arbitrary 

The absorption of commonly used solvents in this spectral range requires the use of thin layer cells in order to avoid spectra with poor signal-to-noise ratios. 

The design described above for use with UV-Vis spectroelectrochemistry can be used here provided that both the glass used as cell window (cuvet) and the ITO-coated glass have sufficiently low NIR absorption. As shown in Fig. 5.37, the optical absorption of ITO-coated glass shows some major bands where the signal-to-noise ratio might be worse than in other parts of the spectrum. 

A setup suitable for work with highly reflective solid electrodes (e.g. platinum, gold or glassy carbon discs) has been described by Salbeck [145]. As shown in the cross section in Fig. 5.39, the polished electrode surface is mounted close to the NIR-transparent cell bottom, leaving only a thin layer of electrolyte solution in the narrow gap. Connection to the NIR spectrometer is accomplished with a fiber optic 

Investigated systems include a broad variety of dissolved organometallic species, electrochemically active organic molecules and redox active polymers like polyani-line (for a review, see [142]). Both dissolved species and species attached by adsorption, covalent bonding or film-forming deposition have been studied. Dissolved poly aniline dispersions as prepared by chemical oxidation [145] show various transitions in the NIR, as depicted in a set of NIR spectra in Fig. 5.40. The bands around X = 1490 nm and 1950 nm are overtones of the N-H stretch mode of an aromatic amine, whereas the band around X = 2300 nm is caused by the oligomer itself, which presumably indicates the presence of mobile charge carriers. 

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Fast Fourier Transformation is widely used in many fields of science, among them chemoractrics. The Fast Fourier Transformation (FFT) algorithm transforms the data from the "wavelength" domain into the "frequency" domain. The method is almost compulsorily used in spectral analysis, e, g., when near-infrared spectroscopy data arc employed as independent variables. Next, the spectral model is built between the responses and the Fourier coefficients of the transformation, which substitute the original Y-matrix. 

Infrared spectra of fats and oils are similar regardless of their composition. The principal absorption seen is the carbonyl stretching peak which is virtually identical for all triglyceride oils. The most common appHcation of infrared spectroscopy is the determination of trans fatty acids occurring in a partially hydrogenated fat (58,59). Absorption at 965 - 975 cm is unique to the trans functionaHty. Near infrared spectroscopy has been utilized for simultaneous quantitation of fat, protein, and moisture in grain samples (60). The technique has also been reported to be useful for instmmental determination of iodine value (61). 

Characterization. In many cases, ftir is a timely and cost-effective method to identify and quantify certain functionaHties in a resin molecule. Based on developed correlations, ftir is routinely used as an efficient method for the analysis of resin aromaticity, olefinic content, and other key functional properties. Near infrared spectroscopy is also quickly becoming a useful tool for on-line process and property control. 

The acetyl content of cellulose acetate may be calculated by difference from the hydroxyl content, which is usually determined by carbanilation of the ester hydroxy groups in pyridine solvent with phenyl isocyanate [103-71-9J, followed by measurement of uv absorption of the combined carbanilate. Methods for determining cellulose ester hydroxyl content by near-infrared spectroscopy (111) and acid content by nmr spectroscopy (112) and pyrolysis gas chromatography (113) have been reported. 

A critical study has been carried out in order to evaluate the capabilities of Near Infrared spectroscopy for the analysis of commercial pesticide formulations using transmittance measurements. In this sense, it has been evaluated the determination of active ingredients in agrochemical formulations after extraction with an appropriate solvent. 

Naes, T., Isaksson, T., "Selection of Samples for Calibration in Near-Infrared Spectroscopy. Part I General Principles Illustrated by Example", Appl. Spec. 1989 (43) 328-335. 

Chen LJ, Thosor SS, Forbess RA, Kemper MS, Rubinovitz RL, Shukla AJ. Prediction of drug content and hardness of intact tablets using artificial neural networks and near-infrared spectroscopy. Drug Dev Ind Pharm 2001 27 623-31. 

Advanced techniques like molecularly imprinted polymers (MIPs), infrared/near infrared spectroscopy (FT-IR/NIR), high resolution mass spectrometry, nuclear magnetic resonance (NMR), Raman spectroscopy, and biosensors will increasingly be applied for controlling food quality and safety. 

Downey, G. and Kelly, J.D., Detection and quantification of apple adulteration in diluted and sulfited strawberry and raspberry purees using visible and near-infrared spectroscopy, J. Agric. Food Chem., 52, 204, 2004. 

Urbano Cuadrado, M. et al.. Comparison and joint use of near infrared spectroscopy and Fourier transform mid-infrared spectroscopy for the determination of wine parameters, Talanta, 66, 218, 2005. 

Wetzel, D.L.B., Analytical near infrared spectroscopy, in Instrumental Methods In Food and Beverage Analysis, Wetzel, D.L.B. and Charalambous, G., Eds., Elsevier, Amsterdam, 1998. 

Osborne, B.G., Near-infrared spectroscopy in food analysis. Encyclopedia of Analytical Chemistry, Meyers, R.A., Ed., John Wiley Sons, Chichester, 2000. 

E. Vigneau, D. Bertrand and E.M. Qannari, Application of latent root regression for calibration in near-infrared spectroscopy. Comparison with principal component regression and partial least squares. Chemometr. Intell. Lab. Syst., 35 (1996) 231-238. 

Davies, A. M. C., Radovic, B., Fearn, T., and Anklam, E. A. (2002). Preliminary study on the characterisation of honey by near infrared spectroscopy. /. Near Infrared Spectrosc. 10, 121-135. 

Iwahashi, M. Hayashi, Y. Hachiya, N. Matsuzawa, H. Kobayashi, H., Self-association of octan-l-ol in the pure liquid state and in decane solutions as observed by viscosity, selfdiffusion, nuclear magnetic resonance and near-infrared spectroscopy measurements, J. Chem. Soc. Faraday Trans. 89, 707-712 (1993). 

Helminen J, Leppamaki M, Paatero E and Minkkinen (1998) Monitoring the Kinetics of the Ion-exchange Resin Catalysed Esterification of Acetic Acid with Ethanol Using Near Infrared Spectroscopy with Partial Least Squares (PLS) Model, Chemometr Intell Lab Syst, 44 341. 

The K-edge spectra of [Ni(287)2]2 and [Ni(cdt)2]2 are remarkably similar to each other and to those of natural hydrogenases.196 Some complexes with ligand (289) (R = NMe2, R = H, Me, NMe2) have been characterized using electronic and near infrared spectroscopy.823 Complex [Ni(289)2]2 served to study phase transformation behavior by microscopy and DSC.824... 

Jagemann K-U, Fischbacher C, Danzer K, Muller UA, Mertes B (1995) Application of near-infrared spectroscopy for non-invasive determination of blood/tissue glucose using neural networks. Z Physikal Chem 191 179... 

Mtlller UA, Mertes B, Fischbacher C, Jagemann K-U, Danzer K (1997) Non-invasive blood glucose monitoring by means of near infrared spectroscopy methods for Improving the reliability of the calibration models. Int J Artific Organs 20 285... 

The phase composition of glycine crystal forms during the drying step of a wet granulation process has been studied, and a model developed for the phase conversion reactions [88], X-ray powder diffraction was used for qualitative analysis, and near-infrared spectroscopy for quantitative analysis. It was shown that when glycine was wet granulated with microcrystalline cellulose, the more rapidly the granulation... 

Heise H.M., Non-invasive monitoring of metabolites using near infrared spectroscopy state of the art, Horm. Met. Research 1996 28 (10) 527-534. 

T. Khan, B. Soller, M. Naghavi, and W. Casscells, Tissue pH determination for the detection of metabol-ically active, inflamed vulnerable plaques using near-infrared spectroscopy an in-vitro feasibility study. Cardiol. 103, 10-16 (2005). 

J. Puyana, B. Soller, S. Zhang, and S. Heard, Continuous measurement of gut pH with near infrared spectroscopy during hemorrhagic shock. J. Trauma 46, 9-15 (1999). 

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