Vibrational spectroscopy quantitative techniques

The importance of the degree of esterification (%DE) to the gelation properties of pectins makes it desirable to obtain a fast and robust method to determine (predict) the %DE in pectin powders. Vibrational spectroscopy is a good candidate for the development of such fast methods as spectrometers and quantitative software algorithms (chemometric methods) becomes more reliable and sophisticated. Present poster is a preliminary report on the quantitative performance of different instrumentations, spectral regions, sampling techniques and software algorithms developed within the area of chemometrics. 

The use of vibrational spectroscopy for the qualitative analysis of absorbed surface species is first considered, and a Table is then included which summarises a number of the key features of the various quantitative techniques. We then proceed to summarize these in groups depending not upon the probe used (as in the preceding chapters), but in terms of the signal emitted by the specimen which is used in each identification process. 

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. 

The non-destructive character of vibrational spectroscopy techniques, such as NIR, makes them novel tools for in-line quality assurance (100). NIR has been widely used for the measurement of water in various applications (101). NIR can be applied for both quantitative analysis of water and for determining the state of water in solid material. This gives a tool for understanding the physicochemical phenomena during manufacture of pharmaceutical granulation. 

Inductively Coupled and Microwave Induced Plasma Sources for Mass Spectrometry 4 Industrial Analysis with Vibrational Spectroscopy 5 Ionization Methods in Organic Mass Spectrometry 6 Quantitative Millimetre Wavelength Spectrometry 7 Glow Discharge Optical Emission Spectroscopy A Practical Guide 8 Chemometrics in Analytical Spectroscopy, 2nd Edition 9 Raman Spectroscopy in Archaeology and Art History 10 Basic Chemometric Techniques in Atomic Spectroscopy... 

The primary techniques used in this study include X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIR), and attenuated total reflectance infrared spectroscopy (ATR). XPS is the most surface-sensitive technique of the three. It provides quantitative information about the elemental composition of near-surface regions (< ca. 50 A sampling depth), but gives the least specific information about chemical structure. RAIR is restricted to the study of thin films on reflective substrates and is ideal for film thicknesses of the order of a few tens of angstroms. As a vibrational spectroscopy, it provides the type of structure-specific information that is difficult to obtain from XPS. The... 

Infrared, near-infrared (see Sec. 6.2), and Raman high-pressure techniques are very suitable tools for the characterization of fluid states and especially for the quantitative analysis of fluids. Sec. 6.7.2 shows a few cells which are u.sed for the vibrational spectroscopy of fluids at pressures up to a maximum of 7 kbar and at temperatures up to 650 °C, although the maximum conditions of both pressure and temperature arc not simultaneously applied (see also Buback, 1991). Sec. 6.7.3 describes changes in the vibrational spectra of polar substances and of aqueous solutions, and Sec. 6.7.4 presents a few applications of high-pressure spectroscopy in the investigation of chemical transformations. 

Since 1905, when William W. Coblentz obtained the first infrared spectrum (1), vibrational spectroscopy has become an important analytical tool in research and in technical fields. In the late 1960s, infrared spectrometry was generally believed to be an instrumental technique of declining popularity that was gradually being superseded by nuclear magnetic resonance (NMR) and mass spectrometry (MS) for structural determinations and by gas and liquid chromatography for quantitative analysis. 

A TRIAX 550 spectrometer attached to an Andor -90°C cooled CCD detector was used for all spectroscopic measurements. Ar laser lines at 488.0 nm and 514.3 nm were used. Reactions were monitored by time-resolved Raman spectroscopy, sequentially setup for two of three separate regions of interest within the spectral range. This enabled, for example, collecting information about the carboxylation/decarboxylation and hydration/dehydration processes by monitoring the various CO and CH vibration modes. This technique provided spectra in each region only after the collection of the spectra in other regions, and hence not favorable for faster kinetics. However, inclusion of OH and H2 peaks gave a reasonably quantitative estimate on the extent of the hydrothermal reaction and valuable information for mass balance calculations (see further details in the experimental results for each system)... 

Raman photoluminescence piezospectroscopy of bone, teeth and artificial joint materials has been reviewed by Pezzotti (2005) with emphasis placed on confocal microprobe techniques. Characteristic Raman spectra were presented and quantitative assessments of their phase structure and stress dependence shown. Vibrational spectroscopy was used to study the microscopic stress response of cortical bone to external stress (with or without internal damages), to define microscopic stresses across the dentine - enamel junction of teeth under increasing external compressive masticatory load and to characterise the interactions between prosthetic implants and biological environment. Confocal spectroscopy allows acquisition of spatially resolved spectra and stress imaging with high spatial resolution (Green etal., 2003 Pezzotti, 2005 Munisso etal., 2008). 

Several recent overviews of principles and applications of Raman, FTIR, and HREELS spectroscopies are available in the literature [35-37, 124]. The use of all major surface and interface vibrational spectroscopies in adhesion studies has recently been reviewed [38]. Infrared spectroscopy is undoubtedly the most widely applied spectroscopic technique of all methods described in this chapter because so many different forms of the technique have been developed, each with its own specific applicability. Common to all vibrational techniques is the capability to detect functional groups, in contrast to the techniques discussed in Sec. III.A, which detect primarily elements. The techniques discussed here all are based in principle on the same mechanism, namely, when infrared radiation (or low-energy electrons as in HREELS) interacts with a sample, groups of atoms, not single elements, absorb energy at characteristic vibrations (frequencies). These absorptions are mainly used for qualitative identification of functional groups in the sample, but quantitative determinations are possible in many cases. 

As briefly reviewed in this section, SFG vibrational technique has significant advantages to the conventional vibrational spectroscopy. A great deal of new information on the interfacial molecular structure will be elucidated by this method, which is important and useful for understanding and controlling the surface property and fimctionality of materials. Appearance of the new techniques such as SFG imaging as well as FT-SFG will make this method more powerful and more easy to use. It is also expected that more theoretical studies will be carried out to quantitatively understand all the information we can get from SFG measurements and to anticipate what we can further obtain from this method. 

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