Grazing-angle spectroscopy

As = 0 and Ap = dj. Vibrations that are perpendicular to the surface are more difficult to detect by ATR (usually we use grazing angle spectroscopy for this purpose). This is because of the fact that in the ATR mode djl < dellx. For example, the Ap values predicted for transitions perpendicular to the surface are smaller than those predicted for parallel transitions of same intrinsic intensity (transition moment dipoles) by factors of 2.22 (Ge), 2.10 (Si), and 1.67 (ZnSe). Therefore, it is recommended to use ZnSe when possible for analysis of molecules with both parallel and perpendicular transitions, and where grazing angle spectroscopy is not available. 

Reflection-absorption (RA) or grazing-angle spectroscopy, is a very useful technique that gives information about the direction of transition dipoles in a sample. Figure 3 present an optical setup for a grazing-angle experiment (22). 

As discussed above, it is desirable to use large, almost grazing angles of incidence in RAIR spectroscopy in order to maximize the sensitivity of the technique to ultra-thin films. Using such large angles of incidence usually requires the substrates to be at least a few centimeters in length. This severely limits the spatial... 

XANES spectroscopy is also the basis of chemically sensitive X-ray imaging, as well as qualitative and quantitative microspectroscopy [306], ptXANES is attractive for chemical analysis, with its spatial resolution down to 10 ptm. Variations on the theme are surface EXAFS (SEXAFS), grazing incidence XAS and in situ time-resolved XAS investigations. Grazing angle XAFS can be used for the study of ultrathin multilayer systems. 

Grazing angle XAS techniques (XANES) can be applied to ultrathin film systems [316]. Selected NEX-AFS, XPS and FUR spectroscopy results were obtained for plasma-polymerised films with different monomers (styrene, acetylene, ethylene and butadiene) [317]. 

Reflection-Absorption IR spectroscopy (RAIRS) where the linearly polarized IR beam is specularly reflected from the front face of a highly reflective sample, such as a metal single crystal surface (Figure 3.1(a)). This is also sometimes referred to as IRAS (IR reflection absorption). The IR beam comes in at grazing angle (i.e. almost parallel to the surface), and although absorption bands in RAIRS have intensities that are some two orders of magnitude weaker than in transmission studies on... 

Other recent applications of FT-IR in pharmaceutical analysis include reaction monitoring by fiberoptic FT-IR/ATR spectroscopy140 and stability studies of pharmaceutical emulsions using FT-IR microscopy.141 A novel equipment cleaning verification procedure using grazing angle fiberoptic FT-IR reflection-absorption spectroscopy was described by Perston et al.142... 

B.B. Perston, M.L. Hamilton, B.E. Williamson, PW. Harland, M.A. Thomson and P.J. Melling, Grazing-angle fiber-optic Fourier transform infrared reflection-absorption spectroscopy for in situ detection and quantification of two active pharmaceutical ingredients on glass. Anal. Chem., 79, 1231-1236 (2007). 

GATR Grazing-angle attenuated total reflecTance infrared specTroscopy... 

In X-ray photoelectron spectroscopy (XPS or X-PES), the irradiation (usually a Mg K a (1253.6 eV) or A1K a (1486.6 eV) source) causes a core electron to be ejected. This is a more useful technique than UPS for surface studies, since the binding energies of core electrons are characteristic of the elements in question and surface elements can thus be identified by the traditional spectroscopic fingerprinting procedure. In this respect, XPS is sometimes referred to by its alternative name, electron spectroscopy for chemical analysis (ESCA). In order to emphasis the contribution from surface atoms, the X-ray beam is usually set at a grazing angle to the surface. Most of the signal originates from within a nanometre of the surface. 

A modification of the simple grazing-angle experiment is the multiple reflection -absorption spectroscopy, which was first described by Gaines (Figure 6) (24). It has a unique value for the detection of order-disorder transformations in the structure of a... 

We report here on the structure and gas transport properties of asymmetric membranes created by the Langmuir-Blodgett deposition of ultra-thin polymeric lipid films on porous supports. Transmission and grazing angle FTIR spectroscopy provide a measure of the level of molecular order in the n-alkyl side-chains of the polymeric lipid. The level of orientational order was monitored as a function of the temperature. Gas permeation studies as a function of membrane temperature are correlated to the FTIR results. 

Figure 11 The Spectra-Tech FT-80 grazing angle accessory. A typical accessory for grazing angle reflectance spectroscopy. A polarizer that transmits only P polarized incident light improves the sensitivity of the measurement.

Very thin films (monolayer or multilayer) can be studied by IR spectroscopy, provided that the IR beam crosses a reasonably large fraction of the monolayer this can be done by (i) grazing-angle techniques (whereby the IR beam angles of incidence and reflection are of the order of 1° to 3°) or (ii) multiple internal reflections of the IRbeam inside the monolayer (infrared reflectance and absorbance spectroscopy (IRRAS). Fig. 11.19 shows the chemical structure of an analyte (Fullerene-fcz s-[ethylthio-tetrakis(3,4-dibutyT2-thiophene-5-ethenyl)-5-bromo-3,4-dibutyl-2-thiophene] malonate),... 

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