Analytical Applications of IR Spectroscopy

The two most important analytical applications of IR spectroscopy are the qualitative and quantitative analyses of organic compounds and mixmres. We pointed out at the beginning of this chapter that the frequencies of radiation absorbed by a given molecule are characteristic of the molecule. Since different molecules have different IR spectra that depend on the structure and mass of the component atoms, it is possible, by matching the absorption spectra of unknown samples with the IR spectra of known compounds, to identify the unknown molecule. Moreover, functional groups, such as —CH3, —C=0, —NH2, and 

We can measure the extent of absorption at a specific frequency for an analyte of known concentration. If now we were to measure the extent of absorption at the same frequency by a sample solution of unknown concentration, the results could be compared. We could determine the sample s concentration using Beer s Law. Thus, as a quantitative tool, IR spectroscopy enables us to measure the concentration of analytes in samples. 

The introduction and widespread use of FTIR has resulted in considerable extension of the uses of IR in analytical chemistry. With regard to wavelength assignment, speed of analysis, and sensitivity, FTIR has opened new fields of endeavor. Some of these uses are described. 

As already discussed, paints and varnishes are measured by reflectance analysis, a process wherein the sample is irradiated with IR light and the reflected light is introduced into an IR instrument. The paint, or other reflecting surface, absorbs radiation in the same manner as a traversed solution. This technique can be used to identify the paint on appliances or automobiles without destroying the surface. Scraps of paint from automobiles involved in wrecks can be examined. From the data obtained, the make and year of the car may be able to be determined. 

In industry, IR spectroscopy has important uses. It is used to determine impurities in raw materials. This is necessary to ensure good products. It can be used for quality control by checking the composition of the product, either in batch mode or continuously (on-line or process analysis). On-line IR analyzers can be used to control the process in real time, a very cost-effective way of producing good products. IR spectroscopy is used in the identification of new materials made in industrial research laboratories and in the analysis of materials made or used by competitors (a process called reverse engineering ). 

Typical analyses include the detection and determination of paraffins, aromatics, olefins, acetylenes, aldehydes, ketones, carboxylic acids, phenols, esters, ethers, amines, sulfur compounds, halides, and so on. From the IR spectrum, it is possible to distinguish one polymer from another or determine the composition of mixed polymers or identify the solvents in paints. Atmospheric pollutants can be identified while still in the atmosphere. Another interesting application is the examination of old paintings and artifacts. It is possible to identify the varnish used on the painting and the textile comprising the canvas, as well as the pigments in the paint. From this information. 

Forensic science makes nse of IR spectroscopy and IR microscopy, not only for paint analysis but for analysis of controlled snbstances. IR can be used to detect the active compounds in hallucinogenic mushrooms, for example. IR is often used to confirm the identity of controlled substances such as cocaine. It has the advantage of being able to differentiate isomers that cannot be distinguished by mass spectrometry (MS), for example, ephedrine and pseudoephedrine. 

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Block copolymers and polyallomers require separate treatment in the field of analytical applications of IR spectroscopy to C2-C3 copolymers. Polyallomers are copolymers synthesized from two monomers but exhibiting a degree of crystallinity normally associated only with homopolymers (32) indeed, allomerism denotes constancy of crystalline form with variation in chemical composition. Though crystalline, polyallomers have properties that differ not only from crystalline homopolymers, but also from blends of homopolymers containing the same proportions of the two monomers. The analytical problems are thus somewhat different from those concerning the elastomeric C2-C3 copolymers and a number of methods have been used to determine their composition. 

A comprehensive survey of the IR spectra of inorganic nitrates and analytical techniques for the determination of ordnance related nitrates is presented in Ref 32. The applications of IR spectroscopy to identification and quantitative analysis have ako been reviewed (Ref 37)... 

A comprehensive historical review of the analytical applications of infrared spectroscopy from the first experiments to the introduction of FTIR spectrometers has appeared.221 The first study of the absorption of infrared radiation by a range of chemical substances was made in 1881 by Abney and Festing, after the former had developed a photographic method of detecting radiation in the near-infrared region. Over the next 25 years a number of other studies were made. This early phase culminated in the work of W. W. Coblentz in the United States, which was published in 1905.222 It became evident from Coblentz s data that infrared spectra were related to molecular structure, but IR spectroscopy remained principally the province of researchers in university physics departments until World War n. 

Figure 9.44 Comparison of IR and Raman spectra of copolymer ethylene-vinyl acetate (EVA). EVA is distinguished by the C=Cbands. (Reproduced from M. J. Pelletier, Analytical Applications of Raman Spectroscopy, Blackwell Science, Oxford. 1999 Blackwell Publishing.)...

ED Lipp, MA Leugers. Applications of Raman spectroscopy in the chemical industry. In MJ Pelletier, ed.. Analytical Applications of Raman Spectroscopy. London Blackwell Science, 1999. IR Lewis, PR Griffiths. Raman spectrometry with fiber-optic sampling. Appl Spectrosc 50 12A, 1996. 

With due regard to sampling, measurement and data analysis considerations, the application of IR spectroscopy to tissue, fluids and cells yields a remarkable amount of information. This information may relate to the concentration of particular analytes in biological fluids, the distribution of analytes within tissue, or the nature of biochemical changes associated with the disease process. In addition, information concerning metabolic processes in tissues and cells may be obtained. Examples will be presented to illustrate the general applicability of the technique. 

M. Mehicic, R.G. Koiiar and O.G. Grasselli, "Analytical Applications of Photoacoustic Spectroscopy using FT-IR", Proceedings of the International Conference on Fourier Transform Infrared Spectroscopy held at the University of South Caroiina, 289, 99 (1981). 

Figure 5 (a) Sample preparation/presentation for Raman spectroscopy. Adapted partly from Everall, N. In Analytical Applications of Raman Spectroscopy, Pelletier, M. J., Ed. Blackwell Publishers Ames, 2000 Chapter 4, p 127. Sample preparation/presentation for (b) NIR spectroscopy and (c) IR ATR spectroscopy (dp, penetration depth of radiation into the sample rti//)2. refractive indices of reflection element/sample A, wavelength , angle of incidence of radiation on the interface of the reflection element/sample). 

ATR-IR spectroscopy can be used as a spy inside a reactor for on-line monitoring and control of a reaction. The emphasis in this kind of application of ATR spectroscopy is on the detection of reactants and products in the bulk fluid phase. Such applications benefit from the excellent time resolution of FTIR instruments compared to other analytical tools, such as chromatographs. The method can be used in investigations of kinetics of reactions in batch reactors instrumentation has been developed and even commercialized that allows measurements at elevated temperatures and pressures. 

Inorganic and bioinorganic applications of IR and Raman spectroscopies are covered in some detail in subsequent sections. General types of information that can be obtained include analytical identification, stracture and symmetry, ligand and functional group identification, metal-ligand and metal-metal bonding potentials and force constants, structural kinetics and dynamics, excited-state properties, vibronic... 

In the past, TERS has not often achieved high sensitivity in terms of detection, and its application to many real-world samples is still far from routine. However, the technique has shown vast improvements over the past few years as a result of better tips, and single-molecule sensibility has been achieved. This detection capability surpasses the sensitivity of most other analytical methods, including IR spectroscopy. Clearly, with a better theoretical understanding of the various field-enhancement mechanisms exhibited by TERS, routine procedures with very hot tips can be expected in the near future. 

A. Lee Smith,. Applied Infrared Spectroscopy Fundamentals, Techniques, and Analytical Problem-Solving. New York Wiley, 1979. Comprehensive treatment of IR spectroscopy. Includes history, instrumentation, sampling techniques, qualitative and quantitative applications. 

The sample techniques just described are designed for collection of transmission (absorption) spectra. This had been the most common type of IR spectroscopy, but it was limited in its applications. There are many types of samples that are not suited to the conventional sample cells and techniques just discussed. Thick, opaque solid samples, paints, coatings, fibers, polymers, aqueous solutions, samples that cannot be destroyed such as artwork or forensic evidence samples, and hot gases from smokestacks—these materials posed problems for the analytical chemist who wanted to obtain an IR absorption spectrum. The use of reflectance techniques provides a nondestructive method for obtaining IR spectral information from materials that are opaque, insoluble, or cannot be placed into conventional sample cells. In addition, IR emission from heated samples can be used to characterize certain types of samples and even measure remote sources such as smokestacks. In reflectance and emission, the FTIR spectrometer system is the same as that for transmission. For reflectance, the sampling accessories are different and in some specialized cases contain an integral detector. The heated sample itself provides the light for emission measurements therefore, there is no need for an IR source. There may be a heated sample holder for laboratory emission measurements. 

M. Beekes, P. Lasch and D. Naumann, Analytical applications of Fourier transform-infrared (FT-IR) spectroscopy in microbiology and prion research. Vet. Microbiol, 2007, 123(4), 305-319. 

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