Near-infrared analysis principles

E.W. Ciurczak, Principles of near-infrared spectroscopy, in Handbook of Near-Infrared Analysis, 2nd edn, D.A. Burns and E.W. Ciurczak (eds), Marcel Dekker, New York, 2001. 

Siesler HW, Ozaki Y, Kawata S, Heise HM. Near-Infrared Spectroscopy Principles, Instruments and Applications. Weinheim, Germany Wiley, 2002. Wetzel DL. Near Infrared reflectance analysis Sleeper among the spectroscopic techniques. Anal Chem 1983 55(12) 1165A-1175A. 

Near-Infrared Spectroscopy. Near-infrared (NIR) spectroscopy is a technique that has been around for some time but, like NMR spectroscopy, has only recently been improved and developed for on-line applications. Near-infrared analysis (NIRA) is a nondestructive technique that is versatile in the sense that it allows many constituents to be analyzed simultaneously 112, 113). The NIR spectrum of a sample depends upon the anharmonic bond vibrations of the constituent molecules. This condition means that the temperature, moisture content, bonding changes, and concentrations of various components in the sample can be determined simultaneously. In addition, scattering by particles such as sand and clay in the sample also allows (in principle) the determination of particle size distributions by NIRA. Such analyses can be used to determine the size of droplets in oil-water emulsions. 

Samola and Urleb [15] reported qualitative and quantitative analysis of OTC using near-infrared (NIR) spectroscopy. Multivariate calibration was performed on NIR spectral data using principle component analysis (PCA), PLS-1, and PCR. 

Another difficulty comes from the spectroscopic technique used to probe the degree of electronic interaction, which relies basically on the observation of a metal-to-metal charge transfer band (intervalence band) in the near-infrared (NIR). A simple analysis based on the Franck-Condon principle leads to the conclusion that its shape and energy may depend on the geometry change between the reduced and... 

See also Activation Analysis Neutron Activation. Atomic Absorption Spectrometry Principles and Instrumentation. Atomic Emission Spectrometry Principles and Instrumentation. Chromatography Overview Principles. Gas Chromatography Pyrolysis Mass Spectrometry. Headspace Analysis Static Purge and Trap. Infrared Spectroscopy Near-Infrared Industrial Applications. Liquid Chromatography Normal Phase Reversed Phase Size-Exclusion. Microscopy Techniques Scanning Electron Microscopy. Polymers Natural Rubber Synthetic. Process Analysis Chromatography. Sample Dissolution for Elemental Analysis Dry... 

Two landmark articles published by H. Mark et al. and G. E. Ritchie et al. [30, 31] demonstrate how ICH Q2 principles can be applied when developing a near-infrared method. The first part of the series describes the method validation concepts as described in this section and proposes additional validation protocols. The second part focuses on the implementation of assay and content uniformity methods for solid dosage forms. Authors present at length how each criterion was validated and how they meet ICH and FDA requirements. For instance, repeatability was tested with the predictions of 13 repeats of tablets at three different concentration levels (80,100, and 120% w/w). In addition to the stated guidelines, authors used ASTM E1655-97 [12] to provide statistics analysis that is not directly provided in ICH Q2(R1). 

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. 

Solomon (16,has uset a different method to obtain extinction coefficients. Essentially, total hydrogen content from elemental analysis and hydroxyl content from measurements of the area of the 0-H stretching band near 3450 cm were used in conjunction with the peak areas of aliphatic and aromatic bands to obtain a plot from which extinction coefficients can be determined. In principle, this approach appears to be sound, but there are a number of problems. One difficulty, discussed above, is general to all infrared methods that have been employed so far what errors are introduced by summing peak areas over a number of bands, each of which has an individual extinction coefficient, and essentially averaging such coefficients for the total area Other problems involve the correct use of curve resolving techniques and the measurement of hydroxyl groups, which we will now consider in more detail. 

In the most common method, the solution is irradiated with near-ultraviolet radiation (200-400 nm) to decompose organic matter by means of a radical formation mechanism. Then the generated CO2 is transported toward the detector with a carrier gas. In order to eliminate some ionic compounds that can interfere with the measurement, a membrane is placed before the detector. The detection is carried out either by the measurement of conductivity via a sensor or by a nondispersive infrared analyzer. In this online system, the sample analysis takes aroimd 6 min. Other systems based on the same principle have also been described. In this case the oxidation and detection are produced in the same chamber. In this "batch" apparatus the sample is trapped and analyzed for 3-30 min. With this latter system, some ionic species other than H and HCO3 can interfere with the conductivity readings. Species such as Ti02 [85,90] and persulfate [91,121] have been used as catalysts present as a diluted suspension in water. The TOC is obtained from the difference between the conductivities for the irradiated and nonirradiated samples. 

Principles and Characteristics Infrared spectroscopy is one of the oldest and most established analytical methods in industry. New technical developments, such as IR microscopy, photoacoustic IR spectroscopy and on-line techniques for process analysis are now routinely being used in many laboratories. Furthermore, chemomet-ric data evaluation, which is very frequently used in near-IR spectroscopy, is often advantageous also in the field of mid-IR spectroscopy and strengthens its outstanding position towards both basic and applied research. 

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