Multiple reflectance-absorption spectroscopy

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... 

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) Reflection-absorption infrared spectroscopy (RAIRS) (also known as infrared reflection absorption spectroscopy, IRAS or IRRAS) Multiple Internal reflection spectroscopy (MIR)... 

Multiple Reflection Absorption Infrared Spectroscopy (MRAIRS) since these techniques offer complementary information." (See Infrared spectroscopy). 

Due to its very nature, the electrode/electrolyte interface may conveniently be studied by reflection-absorption spectroscopy. The first attempts in the infrared wavelength range were made with internal reflection spectroscopy. This allows multiple reflections at the electrode surface to increase the signal, which was otherwise too weak for direct measurement.However, due to inherent difficulties of this method (e.g., the need for a transparent substrate, the necessity for a thin metal layer as electrode), specular external reflection spectroscopy now is preferred for the in situ investigation of electrode processes. 

Absorption spectroscopy provides a means to study particular details about a monolayer. Transmission spectroscopy is difficult because the film, which is thin, absorbs little. Gaines [1] describes multiple-pass procedures for overcoming this problem. Reflection spectroscopy in the UV-visible range has been reported for lipid monolayers [150,151] and in the IR range for oleic acid [152]. 

We wondered why NSCs proliferated exclusively on surfaces with EGF-His ligands anchored by coordination. We focused on two aspects in particular the conformational integrity of coordinated EGF-His and the stability of coordinate bonds at the interface. Conformational information was acquired with multiple internal reflection-infrared absorption spectroscopy (MIR-IRAS) [97]. The stability of coordinate bonds was assessed by culturing NSCs on a surface with a small region of EGF-His ligands anchored by coordination. This spatially restricted EGF-His anchoring enabled an intuitive exploration of EGF-His release under cell culture conditions. 

This situation is obviously useless for spectroscopy. Hence, valuable films are very thin, on the order of few nanometers, depending on the material. Figure 4 shows that even for a thin metal film the overall reflectivity of the system can be severely reduced. This characteristic is even more important for multiple reflection elements, for which the total reflectivity Rtot is the reflectivity for a single reflection R to the power of the number of reflections N. Rtot = R - For a reflectivity R of 90% (90% of the incident intensity is reflected), only 35% of the incident intensity will reach the detector after 10 reflections (N — 10). The smaller the reflectivity Rtot> the smaller is the signal at the detector and the larger the noise. On the other hand, the intensity of an absorption... 

AuNPs of 5 nm and 20 nm (probably spherical) were synthesized and immobilized with the aid of y-mercaptotriethoxysilane (MPTES) or 3 aminopropyltriethox-ysilane (APTES) on a quartz plate serving as a quasi waveguide with multiple reflections for absorption spectroscopy by Nakanishi et al. [56]. The authors found a LSPR with standard absorption spectroscopy of the 4 nm AuNPs in toluene at = 530 run. The same LSPR band position was found for the 20 nm AuNPs in water modified with a layer of mercaptoproprionic acid (MPA). The 4 nm AuNPs... 

In the case of relatively high absorption coefficients, as in NIR-excited Raman spectroscopy, coarse powders should be investigated with a back-scattering (180°) multiple reflection arrangement. 

Fig. 3. An apparatus for infrared laser absorption spectroscopy using multiple reflection (Itabashi etal., 1988).

In absorption spectroscopy the sample is usually measured with respect to a reference, normally a cell with pure solution. By these means multiple reflections at the cell walls can be eliminated [42,56,57]. In Fig. 4.15 some of these multiple reflections are given for the light paths in the cell. They can amount to some percent depending on the absorption in the cell. In the UVA is range the refractive indices at the air/quartz or quartz/solution inter-... 

J.W. Hail, A. Pollard Near-infrared spectroscopic determination of serum total proteins, albumin, globulins and urea. Clin. Biochem. 26, 483 (1993) Y. Mendelson, A.C. Clermont, R.A. Peura, B.-C. Lin Blood glucose measurement by multiple attenuated total reflection and infrared absorption spectroscopy. lEEB Tkans. Biomed. Eng. 37, 458 (1990)... 

Fig. 9.35. Setup for infrared photodissociation spectroscopy. In this particular setup (FELIX at Utrecht, Netherlands) the light from a free-electron laser is guided into an open cylindrical ICR cell in a way that the IR light undergoes multiple reflections on the inner surfaces to extend its path for absorption by the ions inside the cell. Reproduced horn Ref. [139] with permission. John Wiley Periodicals, Inc., 2009.

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