Quantitative Raman Spectroscopy

Examples that use Raman spectroscopy in the quantitative analysis of materials are enonnous. Technology that takes Raman based techniques outside the basic research laboratory has made these spectroscopies also available to industry and engineering. It is not possible here to recite even a small portion of applications. Instead we simply sketch one specific example. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

In this review recent theoretical developments which enable quantitative measures of molecular orientation in polymers to be obtained from infra-red and Raman spectroscopy and nuclear magnetic resonance have been discussed in some detail. Although this is clearly a subject of some complexity, it has been possible to show that the systematic application of these techniques to polyethylene terephthalate and polytetramethylene terephthalate can provide unique information of considerable value. This information can be used on the one hand to gain an understanding of the mechanisms of deformation, and on the other to provide a structural understanding of physical properties, especially mechanical properties. 

Failloux, N, Bonnet, I, Baron, MH, and Perrier, E, 2003. Quantitative analysis of vitamin A degradation by Raman spectroscopy. Appl Spectrosc 57, 1117-1122. 

In one application, Raman spectroscopy was used to identify and quantitate various drugs present in polymer matrices [21]. In Fig. 2, Raman spectra obtained within the fingerprint region for diclofenac, sodium alginate, and a 20% dispersion of diclofenac in sodium alginate are shown. It is evident in the spectra... 

Most chemists tend to think of infrared (IR) spectroscopy as the only form of vibrational analysis for a molecular entity. In this framework, IR is typically used as an identification assay for various intermediates and final bulk drug products, and also as a quantitative technique for solution-phase studies. Full vibrational analysis of a molecule must also include Raman spectroscopy. Although IR and Raman spectroscopy are complementary techniques, widespread use of the Raman technique in pharmaceutical investigations has been limited. Before the advent of Fourier transform techniques and lasers, experimental difficulties limited the use of Raman spectroscopy. Over the last 20 years a renaissance of the Raman technique has been seen, however, due mainly to instrumentation development. 

To develop these ideas into a quantitative theory, we require models for the inner and outer sphere and their reorganization. The problem is similar to that encountered in infrared and Raman spectroscopy, where... 

Raman spectroscopy is emerging as a powerful analytical tool in the pharmaceutical industry, both in PAT and in qualitative and quantitative analyses of pharmaceuticals. Reviews of analyses of pharmaceuticals by Raman spectroscopy have been published.158 159 Applications include identification of raw materials, quantification of APIs in different formulations, polymorphic screening, and support of chemical development process scale-up. Recently published applications of Raman spectroscopy in high-throughput pharmaceutical analyses include determination of APIs in pharmaceutical liquids,160,161 suspensions,162 163 ointments,164 gel and patch formulations,165 and tablets and capsules.166-172... 

In addition to the characteristic XRD patterns and photoluminescence, UV-visible and X-ray absorption spectra, another fingerprint thought to indicate lattice substitution of titanium sites was the vibrational band at 960 cm-1, which has been recorded by infrared and Raman spectroscopy (33,34). Although there is some controversy about the origin of this band, its presence is usually characteristic of a good TS-1 catalyst, although it turned out to be experimentally extremely difficult to establish quantitative correlations between the intensity of the 960 cm-1 band and the Ti content of a Ti silicate and/or its catalytic activity. 

An important tool for the fast characterization of intermediates and products in solution-phase synthesis are vibrational spectroscopic techniques such as Fourier transform infrared (FTIR) or Raman spectroscopy. These concepts have also been successfully applied to solid-phase organic chemistry. A single bead often suffices to acquire vibrational spectra that allow for qualitative and quantitative analysis of reaction products,3 reaction kinetics,4 or for decoding combinatorial libraries.5... 

Upon dilution in solvents which may associate via hydrogen bonds (water, methanol, dioxane) the situation is more complex. I.R. and Raman spectroscopy indicate the formation of various monomer-solvent complexes (4, 6). The corresponding absorption bands are in the same range as the characteristic bands for open dimers and oligomers and the latter cannot therefore be determined quantitatively. However, the viscosity of carboxylic acids was found to rise upon addition of water or methanol (4, 7) suggesting that these solvents bind together "oligomers". The persis-... 

For crystals with molecule-like constituents, like the BO, " and BO4 " groups in some borates, semi-quantitative models of the molecular component as a gas-phase entity have been proposed (Oi et al. 1989). This is conceptually similar to the approximation made for species in solution, although in practice most studies of crystals consider additional frequencies that reflect inter-molecular vibrations. The spectroscopic data on these vibrations (which typically have lower frequencies than the intra-molecular vibrations) are often available, at least approximately, from infrared and Raman spectroscopy and elastic properties. This type of hybrid molecule-in-crystal model has been applied to many minerals in theoretical studies of carbon and oxygen isotope fractionation, the most noteworthy being studies of calcite (Bottinga 1968 Chacko et al. 1991) and sihcates (Kieffer 1982). Because specfroscopic dafa are always incomplete (especially for subsfances substifufed wifh rare isolopes), some amounl of vibralional modeling is necessary. 

Both Raman and infrared spectroscopy provide qualitative and quantitative information about ehemieal species through the interaetion of radiation with molecular vibrations. Raman spectroscopy complements infrared spectroscopy, particularly for the study of non-polar bonds and certain functional groups. It is often used as an additional technique for elueidating the molecular structure and symmetry of a eompound. Raman spectroseopy also provides facile access to the low frequency region (less than 400 cm Raman shift), an area that is more difficult for infrared speetroseopy. 

Raman spectroscopy is particularly well suited for use in process monitoring and conttol. This chapter discusses Raman spectroscopy s attractive features as well as alerts the reader to aspects that may present ehallenges. The fundamental principles of the technique are reviewed. The reader will learn about instrumentation and options in order to make the most appropriate choices. Special aspects of performing quantitative Raman spectroscopy are discussed since these are required in many installations. Apphcations from many diverse fields are presented. The reader is encouraged to examine aU of the areas since there are good lessons and stimulating ideas in aU. 

Quantitative Raman spectroscopy is an established technique used in a variety of industries and on many different sample forms from raw materials to in-process solutions to waste streams, including most of the applications presented here [1]. Most of the applications presented in the next section rely on quantitative analysis. Similar to other spectroscopic techniques, many factors influence the accuracy and precision of quantitative Raman measurements, but high quality spectra from representative samples are most important. 

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