ATR-Spectroscopy

ATR (Attenuated Total Reflection) spectroscopy [27] was invented and applied mainly for investigations of surfaces.This method entails an experimental set-up which reflects the IR beam via a mirror system to a crystal with a high refraction index. Normally, germanium (refraction index n = 4), silver chloride (n = 2) or diamond (n = 2,4) crystals are used. The crystal is standing in direct contact with the surface of the sample (Fig. 16.3). 

An IR beam passing through a prism made of a material with a high refractive index n2 (germanium n = 4) under the angle of total reflection (n, n2) is totally internally reflected. 

If sample material is in contact with the totally reflecting surface of the prism, an evanescent wave in the sample extends beyond the reflecting interface and the evanescent wave will be attenuated in infrared regions.The intensity of this wave decays exponentially with the distance from the surface of the ATR crystal. Due to the fact that the electromagnetic field passes only a few micrometers of the sample, this method is insensitive to sample thickness and therefore useful for analysis of strong absorbing or thick materials. Influencing factors for FT-ATR-IR-spectroscopy are as follows  

Due to physical reasons, only regions of the sample near to the surface are detected. Furthermore, the relations of the intensities of the absorption bands in ATR spectra are different from those in transmission spectra.The intensity of the spectra in long-wave regions is higher due to the fact that the penetration distance is longer. Normally, quantitative comparisons of spectra are not possible. Only the relation of intensities or integrals of absorption bands can be applied for quantifications when the absorption bands for evaluation are close together. 

The installation of Harrick s Scientific SplitPea-ATR unit [28] in a common FT-IR spectrometer enables the recording of ATR spectra. This unit contains a microscope for fixing and adjusting the sample on the ATR crystal.The IR beam is reflected via a mirror optic into the germanium crystal (Fig. 16.4). 

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Whereas ATR spectroscopy is most commonly applied in obtaining infrared absorption spectra of opaque materials, reflection-absorption infrared spectroscopy (RAIRS) is usually used to obtain the absorption spectrum of a thin layer of material adsorbed on an opaque metal surface. An example would be carbon monoxide adsorbed on copper. The metal surface may be either in the form of a film or, of greaf imporfance in fhe sfudy of cafalysfs, one of fhe parficular crysfal faces of fhe mefal. 

Atr—ftir can be readily performed on most commercial ftir spectrometers through the use of an attachment for atr spectroscopy. These devices provide ir-transparent internal reflection elements that are typically made of Ge, KRS-5, ZnSe, or ZnS. These internal reflection elements are made of materials that are of extremely high purity to avoid losses from absorption by impurities in these devices. Coupling of a thin film or surface sample to one of these reflection elements is accompHshed by pressing the sample against the element while acquiring the spectmm. 

A number of infrared spectroscopic studies have been made which have thrown light on the cement-forming reaction (Copeland et al., 1955 Gemer et al., 1966 Wilson Mesley, 1972). Wilson Mesley (1972) used ATR spectroscopy to follow the course of the reaction and showed that major spectral changes were almost entirely associated with loss of the... 

As an example, Cooper and coworkers have used polarized ATR spectroscopy to characterize the surface orientation of PET bottles [35]. They first confirmed the quantitative agreement between ATR and X-ray diffraction results, and then studied the molecular orientation of the bottles at 2 cm intervals. 

Recently, a formalism has been developed to determine the second and the fourth order parameters of films using polarized total internal reflection fluorescence (TIRF) [71]. Similarly to IR-ATR spectroscopy (Section 4), the experiment makes use of p- and s-polarized excitation, but the fluorescence emission (analyzed either in p- or s-direction) is detected normal to the substrate. Two approaches are developed based on the measurements of two intensity ratios. In the first one, the S angle has to be known experimentally or theoretically, and the order parameters (P2) and (P4) can be determined. In the second one, the order parameter (R ) is obtained by another technique, for instance IR-ATR spectroscopy, which allows deducing the order parameter (P4) and (cos2<5). 

ATR spectroscopy devices, 24 114. See also Attenuated total reflection (ATR) Attachment materials... 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

Hofer, P. and Fringeli, U. P. Stuctural investigation o biological material in aqueous environment by means of infrared-ATR spectroscopy. Biophys. Struct. Mech. 1979, 6(1), 67-80. 

Attenuated total reflection (ATR) is the most common reflectance measurement modahty. ATR spectra cannot be compared to absorption spectra. While the same peaks are observed, their relative intensities differ considerably. The absorbances depend on the angle of incidence, not on sample thickness, since the radiation penetrates only a few micrometers into the sample. The major advantage of ATR spectroscopy is ease of use with a wide variety of solid samples. The spectra are readily obtainable with a minimum of preparation Samples are simply pressed against the dense ATR crystal. Plastics, rubbers, packaging materials, pastes, powders, solids, and dosage forms such as tablets can all be handled directly in a similar way. 

Fig. 3. Various situations encountered in ATR spectroscopy. Medium 1 represents the IRE. (a) Bulk rare (optically thin) medium 2. (b) Thin film with thickness d much less than the penetration depth dp. (c) General case with N layers of different optical properties and thickness. The electric field depicted schematically on the left decays exponentially into the rare medium. This situation applies for case (a). In the more general case (c), the electric field does not decay smoothly.

It becomes clear from the above discussion that metal catalyst films suitable for ATR spectroscopy must be very thin. Such films are generally not homogeneous. In many cases physical vapor deposition leads to films composed of metal islands. The morphology depends on the substrate (IRE), the metal, and preparation conditions such as evaporation rate, substrate temperature, and background gas. 

An important issue to consider when probing powders with ATR spectroscopy is the match between particle size and penetration depth of the evanescent wave, as outlined schematically in Fig. 1. For large particles (Fig. 7, case (a)), only the part closest to the IRE is probed by the evanescent field. For large spherical particles, the overlap between the particle and the evanescent field is reduced for geometrical reasons. As shown by Fig. 7(a), the point of contact (the point of highest density) of... 

Internal reflection spectroscopy is widely applied for on-line process control. In this type of application, the chemical reactor is equipped with an internal reflection probe or an IRE. The goal of this type of application is the quantification of reactant and/or product concentrations to provide real-time information about the conversion within the reactor. In comparison with other analytical methods such as gas chromatography, high-pressure liquid chromatography, mass spectrometry, and NMR spectroscopy, ATR spectroscopy is considerably faster and does not require withdrawal of sample, which can be detrimental for monitoring of labile compounds and for some other applications. 

This example illustrates another important issue of ATR spectroscopy carried out during heterogeneous catalytic reactions. Concentrations of products determined by ATR can be different from those observed by other techniques such as GC analysis (49,75. There are primarily two reasons for the differences ... 

A strength of ATR spectroscopy in heterogeneous catalysis is the opportunity it affords for detection of adsorbed species while the catalyst is working. Detection is... 

Identification of an adsorbate may be facilitated by simple adsorption of a precursor that resembles it, or just the adsorbate itself, such as CO (e.g., panel (c). Fig. 21). Adsorbed reaction intermediates on Pd/Al2O3 for 2-propanol oxidation were identified by using time-resolved ATR spectroscopy and quantum chemical calculations 45), as described in detail below. Reaction intermediates are usually... 

In our experience, the principal challenges in the application of ATR IR spectroscopy for investigations of functioning solid catalysts are associated with the sensitivity of the measurement and the complexity of the samples. The former is an issue common to most surface spectroscopies. The latter has to do with the simultaneous presence of many species at a catalytic solid-liquid interface these species include dissolved reactants, adsorbed intermediates, spectators, and products. The spectra are a superposition of the spectra of the individual species. The question of whether a species is a spectator or instead involved in the catalytic cycle is not easily answered and represents a challenge for in situ spectroscopy in general. Thus, there is a need for specialized techniques to be used in combination with ATR spectroscopy to enhance sensitivity and introduce selectivity. 

It is often fruitful to characterize functioning catalysts with more than one technique. ATR-IR and UV-vis spectroscopies were used in combination to investigate alcohol oxidation on a Pd/AFOj catalyst 96). The two methods provide complementary information ATR spectroscopy was used to identify dissolved reaction products and species adsorbed on the catalyst and support, and UV-vis spectroscopy is sensitive to changes of the catalyst itself. 

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