Coupled attenuated total reflection

IR absorption, emission, and reflection spectra for molecular species either in solid, liquid, or gas phases arise mostly from various changes in energy due to transitions of molecules from one vibrational or rotational energy state to another. The frequency or wavelength of this energy transition is characteristic of the specific chemical bond vibration and/or rotation in the molecule which are determined by the molecular structure, the masses of the atoms, and the associated vibrational energy coupling. Attenuated total reflectance (ATR) and reflection-mode of IR in conjunction with electrochemical methods allow samples to be examined directly in the solid or liquid state without further preparation and are widely used in the characterization of electrode-electrolyte interface properties. Most of ILs are IR-active molecules. Since ILs are stable and chemically inert, the IR characterization can be easily performed on the IL-based system directly. 

Attenuated total reflection, on which atr—ftir is based, occurs when the rarer medium is absorbing and is characterized by a complex refractive index (40). The absorbing characteristics of this medium allow coupling to the evanescent field such that this field is attenuated to an extent dependent on k. The critical angle in the case of attenuated total reflection loses its meaning, but internal reflection still occurs. Thus, if the internally reflected beam is monitored, its intensity will reflect the loss associated with the internal reflection process at the interface with an absorbing medium. 

Fixed pathlength transmission flow-cells for aqueous solution analysis are easily clogged. Attenuated total reflectance (ATR) provides an alternative method for aqueous solution analysis that avoids this problem. Sabo et al. [493] have reported the first application of an ATR flow-cell for both NPLC and RPLC-FUR. In micro-ATR-IR spectroscopy coupled to HPLC, the trapped effluent of the HPLC separation is added dropwise to the ATR crystal, where the chromatographic solvent is evaporated and the sample is enriched relative to the solution [494], Detection limits are not optimal. The ATR flow-cell is clearly inferior to other interfaces. 

Similar work was performed by Shaw et al.3 in 1999 when they used FT-Raman, equipped with a charge coupled device (CCD) detector (for rapid measurements) as an on-line monitor for the yeast biotransformation of glucose to ethanol. An ATR (attenuated total reflectance) cell was used to interface the instrument to the fermentation tank. An Nd YAG laser (1064 nm) was used to lower fluorescence interference and a holographic notch filter was employed to reduce Rayleigh scatter interference. Various chemometric approaches were explored and are explained in detail in their paper. The solution was pumped continuously through a bypass, used as a window in which measurements were taken. 

FT Spectrometers FT spectrometers (Figure 3) differ from scanning spectrometers by the fact that the recorded signal is an interferogram [14] (see Chapter 6.2). They can be coupled to a microscope or macrochamber with an FPA detector. FT chemical imaging systems (CISs) are available for Raman, NIR, and IR spectroscopy. However, they can only be considered as research instruments. For example, most IR imaging systems are FT spectrometers coupled to microscopes. This type of spectrometer allows the acquisition of spectra in reflection, attenuated total reflection (ATR), or transmission mode. 

During this time period, several groups investigated the SH response from metallic films using attenuated total reflection techniques (ATR) in which the nonlinear response is enhanced by coupling into surface plasmons (or polaritons) [28-38]. Simon et al. [28] were the first to demonstrate this effect. 

A complementary approach to the standard reflection geometry described above uses the attenuated total reflection (ATR) geometry which couples surface plasmon waves to the incident electric field and enhances the SH production. Two configura-... 

Yang and Her [ 193] have described a rapid method for the determination of down to 200 ppt of semi-volatile compounds such as 1-chloronaphthalene, nitrobenzene and 2-chlorotoluene in soils by coupling solid-phase microextraction with attenuated total reflectance Fourier transform infrared spectroscopy. 

Sampling in surface-enhanced Raman and infrared spectroscopy is intimately linked to the optical enhancement induced by arrays and fractals of hot metal particles, primarily of silver and gold. The key to both techniques is preparation of the metal particles either in a suspension or as architectures on the surface of substrates. We will therefore detail the preparation and self-assembly methods used to obtain films, sols, and arrayed architectures coupled with the methods of adsorbing the species of interest on them to obtain optimal enhancement of the Raman and infrared signatures. Surface-enhanced Raman spectroscopy (SERS) has been more widely used and studied because of the relative ease of the sampling process and the ready availability of lasers in the visible range of the optical spectrum. Surface-enhanced infrared spectroscopy (SEIRA) using attenuated total reflection coupled to Fourier transform infrared spectroscopy, on the other hand, is an attractive alternative to SERS but has yet to be widely applied in analytical chemistry. 

Attenuated total reflection (ATR) is sometimes used to measure the infrared spectra of catalysts inside a reactor. The infrared light is coupled into an ATR crystal, which can be either a flat plate (e.g., the wall of a reactor) or a cylindrical rod (surrounded by catalyst particles). The evanescent wave that protrudes outside the crystal when the infrared beam reflects on the inside of its surface is used for the measurement. A review of ATR in catalysis has been published by Biirgi and Baiker [11], and a catalytic cell to apply the method in situ inside a catalyst bed reported by Moser and co-workers [12]. An example of ATR is discussed later in this chapter. 

Fig. 4. Schematic representation of surface diffraction from dielectric pattern, based on total internal reflection (TIR) and attenuated total reflection (ATR) coupling geometries, respectively.

Simple and rapid spectroscopic methods, such as front-face fluorescence, attenuated total reflectance Fourier-transform infrared and nuclear magnetic resonance spectroscopies, have a great potential for investigation of the structure of fats in dairy products and of the relation between structure and texture. Although fluorescence, infrared and NMR spectroscopies are techniques, the theory and methodology of which have been exploited extensively in studies in both chemistry and biochemistry, the usefulness of these spectroscopies for molecular studies has not been yet fully recognized in food science. Fluorescence, infrared and NMR spectroscopies coupled... 

The intense absorption of the P—O structure obscures all detail in this region. The use of single attenuated total reflection technique, coupled with scale expansion, resulted in the spectrum shown in Figure 2. The maxima in the asymmetric P—O- stretching band is readily measured. 

The key enabler to using FTIR for BWA detection is to develop selective and robust sampling protocols coupled to a miniaturized, portable FTIR unit. To that end, we have developed front-end liquid flow cells which incorporate electric field (E-Field) concentration methods for spores onto the surface of an Attenuated Total Reflection (ATR) IR crystal. IR spectra are presented which show collection and detection results with BG spores in water. The approaches we have developed take advantage of the fact that all spores are negatively charged in neutral pH solutions. Therefore, E-Field concentration of spores directly onto an ATR sampling element enables low level concentration measurements to be possible. 

Important to quality control are the comparison and confirmation of drug substance identity, excipients, and packaging components. Techniques such as Fourier transform IR (FTIR), attenuated total reflectance (ATR), NIR, Raman spectroscopy are used with increased regularity. The detection of foreign metal contaminants is essential with inductively coupled plasma spectroscopy (ICP), atomic absorption (AA), and X-ray fluorescence. Also notable is the increased attention to analysis of chiral compounds, as in the synthesis of drug substances. Optical rotation, ORD, and CD are currently the preferred instruments for this practice. The analytical techniques commonly used in the preformulation study are discussed in the following. 

The attenuated total reflection method can be also used to excite coupled surface plasmons on thin metal films. The couphng of a hght into a symmetric or antisymmetric surface plasmon supported by a thin film (Sect. 2.2) can be in principle achieved in a geometry similar to the Otto geometry (Fig. 23) in which the semi-infinite metal is replaced by a thin metal film [20]. 

Abbreviations CCK, cholecystokinin RNase. ribonuclease G-protein, guanine nucleotide binding protein GPCR, G-protein-coupled receptor. SDS sodium dodecylsulfate, CTAH. hexadecyltrimethyl ammonium hydroxide DMPC. di-myristoylphosphatidylcholine DPPC, di-palmitoylphosphatldylcholine CMC, critical micellar concentration SUV, small unilamellar vesicles CD, circular dichroism NMR, nuclear magnetic resonance hs-DC, high sensitivity differential scanning calorimetry IR-ATR, infrared attenuated total reflection spectroscopy NOE, nuclear Overhauser effect MD, molecular dynamics DMSO, dimethylsulfoxide TFE, trifluoroethanol for abbreviations of peptides see tables land 2, and fig. 11. 

This study addresses the question of how bulk polymer chemistry and surface energy affect the amount and the conformation of FN adsorbed to a series of polyurethaneureas. The technique of Fourier transform infrared spectroscopy (FTIR) coupled with attenuated total reflectance (ATR) optics was used to continuously and non-invasively measure the kinetics of FN adsorption, as well as to monitor conformational changes occuring during adsorption. 

In early work (8) we used infrared spectroscopy coupled with attenuated total reflection optics. This work was done before the availability of infrared equipment based on Fourier transform methods. Due to their relative speed these methods now permit in situ, real time measurements with a resolution of 1 sec or less (9), and continue to yield valuable data, particularly in the hands of the Battelle group in a series of studies dating from 1979 (10). In our early infrared work we had to be content to rinse and dry the surface before obtaining the infrared reflection spectrum Nevertheless the values of surface concentration were remarkably close to those determined more recently. Infrared studies of proteins suffer generally from the fact that the main features of protein spectra are similar for all proteins and therefore it is difficult to distinguish one from another. 

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