Raman spectroscopy, analytical method Applications

S.E.J. Bell, A. Stewart, Quantitative SERS methods, in Surface Enhanced Raman Spectroscopy Analytical, Biophysical and Life Science Applications, ed. by S. Schliicker (Wiley, Weinheim, 2011), pp. 71-86... 

The modern electronic industry has played a very important role in the development of instrumentation based on physical-analytical methods As a result, a rapid boom in the fields of infrared, nuclear magnetic resonance (NMR), Raman, and mass spectroscopy and vapor-phase (or gas-liquid) chromatography has been observed. Instruments for these methods have become indispensable tools in the analytical treatment of fluonnated mixtures, complexes, and compounds The detailed applications of the instrumentation are covered later in this chapter. 

Abstract This chapter reviews emerging techniques for deep, non-invasive Raman spectroscopy of diffusely scattering media. As generic analytical tools, these methods pave the way for a host of new applications including non-invasive disease diagnosis, chemical identification and characterisation of pharmaceutical products. 

For formulated products an essential analysis is the assay for API content. This is usually performed by HPLC, but Raman spectroscopy can offer a quantitative analytical alternative. These applications have been extensively researched and reviewed by Strachan et al. [48] and provide over 30 literature references of where Raman spectroscopy has been used to determine the chemical content and physical form of API in solid dosage formulations. As no sample preparation is required the determination of multiple API forms (e.g. polymorphs, hydrates/solvates and amorphous content) provides a solid state analysis that is not possible by HPLC. However, as previously discussed sampling strategies must be employed to ensure the Raman measurement is representative of the whole sample. A potential solution is to sample the whole of a solid dosage form and not multiple regions of it. As presented in Chap. 3 the emerging technique of transmission Raman provides a method to do just this. With acquisition times in the order of seconds, this approach offers an alternative to HPLC and NIR analyses and is also applicable to tablet and capsule analysis in a PAT environment. 

To demonstrate the application of these butterfly wing SERS substrates to the problem of protein-binding detection, Garrett et al. devised a protein-binding assay. There are a variety of methods for binding proteins to a metal surface, some of which depend on the amino acid composition of the protein, others of which do not. Direct adsorption via covalent bonds between sulfur groups in proteins and metal surfaces has been used with some success [43] however, this method is unsuitable for use in Raman spectroscopy-based assays, as nonspecific binding may result in a wide variety of conformational orientations of the molecules with respect to the metal s surface. This can lead to Raman spectra that are difficult to reproduce. Not only that, but the analyte may bind to the metal as well as the antibody, which can lead to noisy spectra. 

Nonlinear vibrational spectroscopy provides accessibility to a range of vibrational information that is hardly obtainable from conventional linear spectroscopy. Recent progress in the pulsed laser technology has made the nonlinear Raman effect a widely applicable analytical method. In this chapter, two types of nonlinear Raman techniques, hyper-Raman scattering (HRS) spectroscopy and time-frequency two-dimensional broadband coherent anti-Stokes Raman scattering (2D-CARS) spectroscopy, are applied for characterizing carbon nanomaterials. The former is used as an alternative for IR spectroscopy. The latter is useful for studying dynamics of nanomaterials. 

A widely applied discipline of chemometrics is pattern recognition, which involves the classification and identification of samples. Its purpose is to develop a semiquantitative model that can be applied to the identification of unknown sample patterns. To assure the best reliability, pattern recognition requires the applications of a minimum of two analytical methods. The ammonium-azonium tautomerism in /V, /V-d i a I ky I a minoazo dyes used as indicators requires three techniques UV-Vis, IR, and Raman spectroscopy.223 A study of the matrix composition concerning the ratio of the compounds is necessary as well. The application of pattern recognition analysis in geochemical techniques not only assures better reliability but is also quite useful in addressing real-world environmental problems.224... 

Nonlinear techniques have been used to overeome some of the drawbacks of conventional Raman spectroscopy, particularly its low dficienev, its limitation to the visible and near-ultraviolet regions, and its susceptibility to interference from fluorescence. A major disadvantage of nonlinear methods is that they lend to be analyte specific and often require several different tunable lasers to be applicable lo diverse species. I o dale, none of the nonlinear methods has found widespread application among nonspccialisls. However, many of these methods have shown considerable promise. As less expensive and more routinely useful lasers become available, nonlinear Raman methods, particu-larlv CARS, should become more widely used. 

The front-line analytical techniques of X-ray powder diffraction (XRPD), vibrational (infra-red and Raman) spectroscopy and thermal analysis, and their application to solid state issues are discussed elsewhere in this book. Solid state NMR is a very sensitive reporter of molecular conformation, mutual interaction, dynamics and form. In this section, we will discuss the basics of solid state NMR and in particular the methods that can be used in the study of this state of matter. 

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