Near infrared absorption spectroscopy sampling

Near-infrared absorption spectroscopy is increasingly used in agriculture, food science, medicine, fife sciences, pharmaceuticals, textiles, general chemicals, polymers, process monitoring, food quality control and in clinical in vivo measurements [215, 216]. The increase in popularity is largely due to the availability of miniaturised NIR-spectrometers by a variety of vendors (e.g. Ocean Optics Inc.). The most promising applications of NIR-absorbance spectroscopy clearly lie in process control, because of the relatively low complexity of the sample in chemical and biochemical processes, e.g. compared to biological tissues. Also in food quality control... 

Individual near-infrared absorption bands have absorptivities on the order of 10 1 Al /mm/mM for peak absorption bands in aqueous matrices.30 Such low absorptivities limit detection to the major components within skin tissue. As a general rule of thumb, substances must be present at concentrations above 1 mM to be quantified by near-infrared spectroscopy. Although such low absorptivities greatly restrict the number of possible analytes one can measure in clinical samples, the inability to measure chemicals present below millimolar concentrations enhances selectivity by rendering measurements insensitive to many different types of endogenous molecules. Only the major chemical components of these biological samples must be considered for selectivity purposes. 

Near-infrared absorption is therefore essentially due to combination and overtone modes of higher energy fundamentals, such as C-H, N-H, and O-H stretches, which appear as lower overtones and lower order combination modes. Since the NIR absorption of polyatomic molecules thus mainly reflects vibrational contributions from very few functional groups, NIR spectroscopy is less suitable for detailed qualitative analysis than IR, which shows all (active) fundamentals and the overtones and combination modes of low-energy vibrations. On the other hand, since the vibrational intensities of near-infrared bands are considerably lower than those of corresponding infrared bands, optical layers of reasonable size (millimeters, centimeters) may be transmitted in the NIR, even in the case of liquid samples, compared to the layers of pm size which are detected in the infrared. This has important consequences for the direct quantitative study of chemical reactions, chemical equilibria, and phase equilibria via NIR spectroscopy. 

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). 

Near-infrared spectroscopy is quickly becoming a preferred technique for the quantitative identification of an active component within a formulated tablet. In addition, the same spectroscopic measurement can be used to determine water content since the combination band of water displays a fairly large absorption band in the near-IR. In one such study [41] the concentration of ceftazidime pentahydrate and water content in physical mixtures has been determined. Due to the ease of sample preparation, near-IR spectra were collected on 20 samples, and subsequent calibration curves were constructed for active ingredient and water content. An interesting aspect of this study was the determination that the calibration samples must be representative of the production process. When calibration curves were constructed from laboratory samples only, significant prediction errors were noted. When, however, calibration curves were constructed from laboratory and production samples, realistic prediction values were determined ( 5%). 

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... 

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. 

Another consequence of the strong absorption properties of water is the spectral impact of the displacement of water by dissolved solutes. Generally, in absorption spectroscopy, the solvent is selected not to absorb over the wavelength range of interest. When the absorption properties of the solvent are negligible, any displacement of solvent molecules from the optical path by the dissolution of solute molecules has a negligible effect on the measured spectrum. For near-infrared spectra of aqueous solutions, however, the absorption spectrum depends heavily on the degree of water displacement by solutes in the sample. 

As several researchers have shown empirically, the use of —log(reflectance) can provide, analogous to a transmittance measurement, a linear relationship between the transformed reflectance and concentration, if the matrix is not strongly absorbing as can be found for many samples studied by near-infrared spectroscopy. This issue is presented in detail below. A different approach based on a physical model was considered for UV/VIS measurements and later also applied within the mid-infrared. A theory was derived by Kubelka and Munk for a simple, onedimensional, two-flux model, although it must be noted that Arthur Schuster (1905) had already come up with a reflectance function for isotropic scattering. A detailed description of theoretical and practical aspects was given by Korttim. The optical absorption... 

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