Near-infrared spectroscopy types

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. 

Various accessories were designed for recording diffuse reflectance spectra. Apart from special devices developed by different groups and described in the literature, several commercially available types must be noted. A few accessories are shown schematically in Fig. 1, which are representative of the diversity of optics. For a long time, integrating spheres have been in use, in particular for UV/VIS and near-infrared spectroscopy, although a few applications with sensitive MCT detectors also can be found within the mid-infrared. Usually, a baffle is placed within the sphere... 

Table 6.7. Soybean Seed Composition in Percentage at 13.0% Moisture Content as Measured by Near-Infrared Spectroscopy for Oil from Commodity and Six Types of Genetically Modified Soybeans...

The first step in recycling plastics is to sort the plastics by their resin type, or resin identification code. The resin identification code is a number assigned to a plastic product (or container) according to the type of polymers it is made of. While it was once common to directly use this code to identify the types of polymer(s) present, there are now other methods, such as near-infrared spectroscopy or density sorting approaches, that are used to sort mass quantities of plastic samples for recycling. (See Polymer Chemistry for more information on resin identification codes.)... 

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. 

The technique commonly used to separate household packaging wastes is sensor-based near-infrared spectroscopy that is based on the fact that NIR spectra of different types of polymers are quite distinct. NIR is able to provide rapid and reliable identification of various polymers especially the polyolefin group, polyethylene (PE), and polypropylene (PP). However, the objects which are too small or too big are not suitable for this technology. Another drawback is its inability to detect dark polymers, like automotive parts and some of the electronics waste, since radiation is absorbed completely. In practice, for packaging polymers, around half of the total input materials end up in residuals. 

Antioxidant and antiozonant types most commonly used are aromatic amines or phenolics, though others are also employed, and can be determined using a variety of techniques such as UV-visible spectrophotometry, FTIR, near-infrared spectroscopy, TEC, GC (if the material can be volatilized), supercritical fluid chromatography, and HPLC. Identification of unknown antioxidants requires a separation technique like chromatography followed by mass spectrometry, NMR, ETIR, X-ray crystallography, etc. Standardized TEC methods are given in ASTM D3156 and... 

As previously noted, many commercial plastic bottle sorting lines use near-infrared spectroscopy to accurately ID bottles by resin type at very high speed. 

TiTech manufactures the TiTech Autosort beverage-carton sorter, originally developed to separate plastic-laminated paperboard. It uses near-infrared spectroscopy, particle detection, and selective impulses of compressed air. The position, size, and shape of the object are determined, in addition to determining the resin type. As with the systems described above, at each stage, only one sort is performed. Throughputs can be as much as 6000 kg/hr. Reportedly, two North American facihties are now using these systems. ... 

In near-infrared spectroscopy (NIRS), as we have seen in earlier chapters, one teaches the instru-ment/computer system what to look for in a given type of sample, then expects the hardware/software combination to produce valid answers when it is presented with unknown samples of the same type. But what kind of answers are we seeking Usually, they are quantitative answers, in other words, how much of substance XYZ is present in the sample. Occasionally we use what is called discriminant analysis to obtain a qualitative answer, in other words, is this sample A, B, C, or none of these ... 

While near-infrared spectroscopy was for many years a sleeping giant, the diversity of instrumentation available today attests to its new-found and widespread acceptance as an analytical method. The aim of this article is to outline the measurement principles for each of the various types of instruments that are commercially available as of this writing, and to outline the strengths of each. Because analysis is by far the most common application, we also include accounts of specialized measurement techniques that have emerged for on-line, in-situ, and remote spectral measurements, as well as briefly outlining dedicated analysers founded upon near-infrared technology. 

Infrared spectroscopy of the 3 1 compound showed two types of perylene molecules, one pair of molecules associated with TCNQ and one free molecule [17]. The charge distribution was clarified by UV and near infrared spectroscopy [20]. For both perylene-TCNQ compounds a charge-transfer band was identified and also the monomer TCNQ spectrum. The typical bands of (TCNQ)J are not observed. For Per3TCNQ the UV spectrum clearly shows the existence of neutral perylene. Thus the suggestion from the crystalline structure that the flanking perylene molecules are neutral seems confirmed. Therefore, given the existence of the anion TCNQ, each of the two other perylene species share one positive charge. EPR lines are narrow for both compounds 0.61 G and 0.97 G for the 3 1 and 1 1 compounds respectively at room temperature, and 0.67 G and 0.84 G at 77 K [17]. 

Ultraviolet and infrared spectroscopy indicate that quinoxaline-2,3-dione type structures are preferred to tlie tautomeric 3-hydroxy-quinoxalin-2 One or 2,3-dihydroxyquinoxaline forms. The light absorption properties (UV) of quinoxaline-2,3-dione have been compared with those of its NN -, ON-, and OO -dimethyl derivatives (79, 80, and 81), and also its N- and 0-monomethyl derivatives (43 and 82). The parent dicarbonyl compound and its mono- and di-A -methyl derivatives show very strong carbonyl absorption near to 1690 cm split into two peaks. 

Of the analytical techniques available for process analytical measmements, IR is one of the most versatile, where all physical forms of a sample may be considered - gases, liquids, solids and even mixed phase materials. A wide range of sample interfaces (sampling accessories) have been developed for infrared spectroscopy over the past 20 to 30 years and many of these can be adapted for either near-lme/at-lme production control or on-line process monitoring applications. For continuous on-line measurements applications may be limited to liquids and gases. However, for applications that have human interaction, such as near-line measurements, then all material types can be considered. For continuous measurements sample condition, as it exists within the process, may be an issue and factors such as temperature, pressure, chemical interfer-ants (such as solvents), and particulate matter may need to be addressed. In off-line applications this may be addressed by the way that the sample is handled, but for continuous on-line process applications this has to be accommodated by a sampling system. 

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