ATR technique

The attenuated total reflectance (ATR) technique is used commonly in the near-infrared for obtaining absorption spectra of thin Aims and opaque materials. The sample, of refractive index i, is placed in direct contact with a material which is transparent in the region of interest, such as thallium bromide/thallium iodide (known as KRS-5), silver chloride or germanium, of relatively high refractive index so that Then, as Figure 3.f8... 

Besides crystalline order and structure, the chain conformation and segment orientation of polymer molecules in the vicinity of the surface are also expected to be modified due to the specific interaction and boundary condition at the surface between polymers and air (Fig. 1 a). According to detailed computer simulations [127, 128], the chain conformation at the free polymer surface is disturbed over a distance corresponding approximately to the radius of gyration of one chain. The chain segments in the outermost layers are expected to be oriented parallel to the surface and chain ends will be enriched at the surface. Experiments on the chain conformation in this region are not available, but might be feasible with evanescent wave techniques described previously. Surface structure on a micrometer scale is observed with IR-ATR techniques [129],... 

In materials investigations surface-sensitive techniques are of special interest. The major contribution of infrared spectroscopy to this field is internal reflection spectroscopy (IRS), often called the "attenuated total reflection" (ATR) technique. To describe theory and principle, electromagnetic wave theory must be apphed [33]. 

The ATR technique is a commonly used infrared internal reflection sampling technique. It samples only the surface layer in contact with the ATR element the sampling depth probed is typically of the order of 0.3-3 pm [1]. Unless software corrected, compared with a transmission spectrum, the relative intensity of bands within an ATR spectrum increase in intensity with decreasing wavenumber. Several FTIR instrument companies now supply "ATR-correction" software developed to correct for the different relative intensities of bands observed between ATR and transmission spectra, so that ATR spectra can be more easily compared to and searched against transmission spectra. 

Internal reflectance (attenuated total reflectance ATR ). The internal reflectance or, more usually, attenuated total reflectance (ATR), technique depends on the total reflectance of an IR beam at the internal face of an IR-transparent crystal of high IR refractive index, as shown in Figure 2.38. Medium 1 is a prism of such a material (for example, Si, Ge or KRS-5 [thallous bromide- iodide]), medium 2 is a thin coating of a metal (Au, Pt, Fe) which forms the working electrode and medium 3 is the electrolyte. The... 

Leppinen (1990) used FTIR-ATR techniques to study the adsorption of ethyl xanthate on pyrite, pyrrhotite and chalcopyrite. The FTIR spectra of the reaction products on pyrite, pyrrhotite and chalcopyrite after treatment with 1x10 mol/L KEX solution at pH = 5 is presented in Fig. 1.7. He found that the FTIR signals... 

The ATR technique is now routinely used for IR spectroscopy as it allows measurement of spectra for a variety of sample types with minimal preparation. The crystals employed are generally prismatic in shape, allowing contact of a flat surface with the sample. The ATR method was first adapted for HP IR spectroscopy by Moser [29-33], who realised that a conventional autoclave could easily be adapted for in situ IR spectroscopy by fitting an ATR crystal of cylindrical cross section. The technique developed by Moser is thus known as cylindrical internal reflectance (CIR) spectroscopy and high pressure cells based upon the CIR method have been commercialised by Spectra-Tech. A typical CIR cell is illustrated in Figure 3.8. 

As well as the CIR-type cells discussed above, the ATR technique has also been employed in other types of HP IR cell. An alternative arrangement to a cylindrical crystal inserted through the autoclave body (Figure 3.8) is to embed the ATR crystal in the base or wall of the autoclave. This approach was used in the HP IR cell developed by Wolf et al., illustrated in Figure 3.9 [43]. An ATR crystal is mounted in the bottom of the reactor, which can operate at pressures up to 200 bar and can be adapted to give a high pressure flow-cell. 

For example, a high yield of an intermediate in a consecutive reaction depends sensitively on the instant in time when the reaction is quenched. For such applications, the fast response of the ATR method more than compensates for deficiencies related to sensitivity when the ATR technique is compared with other methods. The design of the equipment depends on the specific requirements of the application. 

Thus, the ATR technique yields much information which may be one of its greatest limitations. An ATR spectrum of a working heterogeneous catalytic system simultaneously provides information about dissolved and adsorbed reactants and products, adsorbed intermediates, byproducts, spectators, and the catalyst itself. The technique is unspecific in the sense that a signal cannot a priori be assigned to dissolved or adsorbed species or the catalyst itself. Thus, analysis of the data requires more effort and specific techniques, such as the ones discussed in detail below. In the following we illustrate the points raised above by some examples. 

X-ray photoelectron spectroscopy (XPS) was used for elemental analysis of plasma-deposited polymer films. The photoelectron spectrometer (Physical Electronics, Model 548) was used with an X-ray source of Mg Ka (1253.6 eV). Fourier transform infrared (FTIR) spectra of plasma polymers deposited on the steel substrate were recorded on a Perkin-Elmer Model 1750 spectrophotometer using the attenuated total reflection (ATR) technique. The silane plasma-deposited steel sample was cut to match precisely the surface of the reflection element, which was a high refractive index KRS-5 crystal. 

There is substantial history regarding the application of conventional vibrational spectroscopy methods to study the intact surface of skin, the extracted stratum corneum and the ceramide-cholesterol-fatty acid mixtures that constitute the primary lipid components of the barrier. The complexity of the barrier and the multiple phases formed by the interactions of the barrier components have begun to reveal the role of each of these substances in barrier structure and stability. The use of bulk phase IR to monitor lipid phase behavior and protein secondary structures in the epidermis, as well as in stratum corneum models, is also well established 24-28 In addition, in vivo and ex vivo attenuated total reflectance (ATR) techniques have examined the outer layers of skin to probe hydration levels, drug delivery and percutaneous absorption at a macroscopic level.29-32 Both mid-IR and near-IR spectroscopy have been used to differentiate pathological skin samples.33,34 The above studies, and many others too numerous to mention, lend confidence to the fact that the extension to IR imaging will produce useful results. 

Synthetic surfactants and polymers are probably most often used to modify the characteristics of a solid surface, i.e., they function at the solid - liquid interface, such as in the processes of detergency, lubrication, or the formation of adhesive bonds. The performance of modem FT - IR spectrometers is such that many new applications to the characterization of the solid - liquid interface, particularly in kinetics studies, are possible. Reflection - absorption spectroscopy and attenuated total reflectance (ATR) techniques have been applied to "wet" interfaces, even the air - water interface, and have figured prominently in recent studies of "self -assembled" mono - and multilayers. 

Protein Adsorption. The development of medical implant polymers has stimulated interest in the use of ATR techniques for monitoring the kinetics of adsorption of proteins involved in thrombogenesis onto polymer surfaces. Such studies employ optical accessories in which an aqueous protein solution (93) or even ex - vivo whole blood (94-%) can be flowed over the surface of the internal reflection element (IRE), which may be coated with a thin layer of the experimental polymer. Modem FT-IR spectrometers are rapid - scanning devices, and hence spectra of the protein layer adsorbed onto the IRE can be computed from a series of inteiferograms recorded continuously in time, yielding ah effective time resolution of as little as 0.8 s early in the kinetic runs. Such capability is important because of the rapid changes in the composition of the adsorbed protein layers which can occur in the first several minutes (97). 

The internal angle of incidence is generally chosen to be 20° so that it is close to the critical angle of total internal reflection. The infrared absorption of each of the monolayers is measured both with and without the metal layers. The ATR technique is used only over the frequency range for which the substrate is transparent. For silicon substrates, this limits the applicable frequency ranges to between 4000 and 1700 cm 1 and below 420 cm 1. 

For illustrative purposes a thermal diffusivity of 1.3 x 10 3 cm2/s is often used as being typical of rubber and polymers. Some values from the literature for various materials are given in Table 2.1. Using the value of 1.3 x 10"3 it can be calculated that a depth of 3 to 11 Jim is being sampled at 2000 cm"1 as indicated in Table 2.2. This is an order of magnitude greater than that sampled by ATR techniques. 

In clinical analysis the samples of interest (e.g. body fluids such as blood, urine, saliva etc.) should remain in their natural composition, i.e., almost always in aqueous solution. Therefore, because of the strong absorption of water in that region, analysis in the infrared is more or less impossible. Thus it is of considerable interest to study the applicability of ATR-techniques in the visible region ( 7,28). 

There is still another disadvantage normally associated with the use of ATR-techniques for biological samples in the visible. That is the strong adsorption of biomolecules on the surface of the reflection element this causes adsorption contamination between two successive samples. This problem, however, could be solved (patents pending) by using disposable slides of the same material in optical contact with the reflection element or by using a liquid interface, as shown in Figure 6. For example,... 

As shown, the use of the evanescent wave in the UV-VIS region of the spectrum might well develope into a new tool for the biochemical laboratory. This is true for the ATR-techniques as well as for the new type of Raman spectroscopy, although the latter will probably be somewhat more attractive. 

ATR Techniques for the Study of Thin Surface Films, presented at the Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, 1975. 

The IR spectra of solid samples usually are recorded in transmission either by pressing samples into KBr pellets or grinding samples up as Nujol mulls. These two techniques are not considered suitable for surface analysis of paper documents. Fortunately, some information can be obtained by using internal and external reflectance techniques. Only the internal reflectance technique or the so-called attenuated total reflectance (ATR) technique will be discussed here. 

Figure 2.7-10 Infrared spectra of a thiourea single crystal, recoided by using the ATR technique...

In all ranges of the infrared, the near, middle, and far infrared (NIR, MIR, and FIR) mainly the absorption of radiation by a sample is evaluated. Emission spectra are only rarely recorded, even though they are a powerful tool for solving problems which cannot be investigated by other methods investigations of remote samples (in astronomy and for environmental analysis), of reactions of substances on catalysts, and of layers on surfaces (Mink and Keresztury, 1988) (Sec. 3.3.1). Other techniques of investigating surface layers, e.g., ATR technique and ellipsometry in the infrared range, are described in Sec. 6.4. 

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