Attenuated Total Reflection ATR Technique

Attenuated total reflection (ATR) spectroscopy is an internal reflection spectroscopy sampling technique. It is limited to sampling the surface layer of a sample, to a depth of between about 0.3 and 3 pm, but offers the advantage of not requiring a thin section to be prepared from such as a tissue sample. This 

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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... 

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]. 

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... 

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. 

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. 

Elmer, Model 457, spectrophotometer using the attenuated total reflection (ATR) technique. 

The use of infrared spectroscopy in the Earth and environmental sciences has been widespread for decades however, until development of the attenuated total reflectance (ATR) technique, the primary use was ex situ material characterization (Chen and Gardella, 1998 Tejedor-Tejedor et al., 1998 Degenhardt and McQuillan, 1999 Peak et al., 1999 Wijnja and Schulthess, 1999 Aral and Sparks, 2001 Kirwan et al., 2003). For the study of environmental systems, the strength of the ATR-Fourier transform infrared (FTIR) technique lies in its intrinsic surface sensitivity. Spectra are collected only from absorptions of an evanescent wave with a maximum penetration depth of several micrometers from the internal reflection element into the solution phase (Harrick, 1967). This short optical path length allows one to overcome any absorption due to an aqueous phase associated with the sample while maintaining a high sensitivity to species at the mineral-water interface (McQuillan, 2001). Therefore, ATR—FTIR represents a technique capable of performing in situ spectroscopic studies in real time. 

Over the last decades, infrared (IR) vibrational sprectroscopy has been well established as a useful tool for structure elucidation and quafity control in several industrial applications. Indeed, the development of Fourier transform (FT) IR and attenuated total reflectance (ATR) techniques have also evolved allowing rapad IR measurements of organosolvent extracts of plant tissues, edible oils, and essential oils, for example (Damm et al., 2005 Lai et al., 1994 Schulz Baranska, 2007). In consequence of the strong dip>ole moment of water, IR spectroscopy applications have mostly focused on the analysis of dried or non-aqueous plant matrices and currently IR methods are widely used as a fast analytical technique for the authentication and detection of adulteration of vegetable oils. 

The majority of reported mid-IR fibre probes rely on the well-established attenuated total reflection (ATR) technique, revealing many advantages in the general applicability over e.g. absorption measurements in short pathway flow cells. In the ATR method only a thin film (a few micrometers) at the proximity of the ATR element is subject to the measurement The thickness of the analyzed film is defined by the penetration depth of the evanescent field... 

Fourier transform infrared spectroscopy (FTIR) was carried out in a Spectrum 100 FTIR Spectrometer Perkin Elmer (Japan). Spectra were recorded in range of 4000-550cm with 4cm resolution by using an attenuated total reflectance (ATR) technique [61]. 

In particular IR spectroscopy is very useful and convenient for the detection of active groups in a polyurethane and can also be utilized to follow chain-extension/crosslinking processes to completion. The use of attenuated total reflectance (ATR) techniques has proved valuable in the examination of many problems involving chemical surface activity. The following areas of activity are possible using the ATR procedure ... 

Fig. 68. Direnzoite. Patterns collected from three different spots of the sample using the micro-lR attenuated total reflectance (ATR) technique in the range 4000... 1400 cm" [08G1].

As mentioned earlier, both transmission and attenuated total reflectance, ATR, techniques may be used to examine lipids. It should be borne in mind that lipids may exhibit dichroic behaviour. - ... 

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