Attenuated total reflection specialized techniques

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]. 

For solid-phase samples or for membrane interaction studies, it can be very useful to use the alternative sampling technique of attenuated total reflectance (ATR). Films or solutions can be placed on a specially... 

The most commonly used technique for obtaining a spectrum is the attenuated total reflectance (ATR) method in the multiple internal reflectance (MIR) mode. In this method, the infrared beam is passed into a special crystal of a selenide (KRS-5). The angle of incidence is such that the beam will bounce along the crystal. A sample of the polyurethane is placed hard up against the crystal. The infrared just penetrates into the material before it continues down the crystal. A number of internal bounces are obtained along the crystal. Up to 25 reflectances are obtained from a 2 mm crystal. Figure 9.8 illustrates the infrared path in an ATR cell. 

IR spectroscopy is a common analytical technique in the textile industry. IR is capable of identifying fibers and their additives, as well as showing quantitative blend ratios and additive contents. The ATR (attenuated total reflection) technique, especially in its multiple form, MIR (multiple internal reflection) is of special importance in this field. The sample preparation is simple and fast the cut out swatches with appropriate surface areas are placed against each side of the MIR crystal, ensuring sufficient and uniform contact across the crystal surface. The internal reflection methods are non-destructive, so that the sample may be saved for other types of analysis, they are, further, methods of surface analysis. This is advantageous in all cases where the finish resides primarily on the fiber surface. In this case, a very strong spectrum of the finish is obtained, with minimal interference from the base fiber (Hannah et al., 1975). 

Parallel to the synthesis of library compounds on solid supports, the direct analysis (i.e. on-bead ) is an attractive concept. Leaving aside the methods of on-bead-infrared-spec-troscopy [41] using attenuated total reflection (ATR) and gel-phase NMR-techniques [42, 43], we wish to consider the use of on-bead analysis in mass spectrometry. The requirements of the ionization process restrict on-bead analysis to the MALD1 technique. However, also for MALDI-analysis, in so-called direct monitoring studies [44-46], the compounds are cleaved from the bead before the actual ionization. True on-bead analysis under MALDI conditions is only possible with photolytically cleavable linkers, and this technique has been dealt with in several publications [28,29] in which the compounds are both cleaved and ionized simultaneously within the MALDI source with a single laser shot. The wavelength of the MALDI-laser must however correspond with the wavelength required to cleave the compounds from the resin. Therefore, on-bead analysis represents a method for special analytical problems which is limited to MALDI. 

Attenuated total reflection is an optimal technique to study hydrogels under native conditions. Similar to Raman spectroscopy there is no special sample preparation required and even thick samples can be investigated, although it should be recognized that only the sample within about 1 pm of the surface of the ATR... 

A special sampling technique often applied for infrared studies of biological systems is attenuated total reflection spectroscopy or ATR spectroscopy. With this technique, the infrared beam is guided through a transparent medium of high refractive index (plate, often crystalline material) in such a way that several total reflections take place at the surfaces. Ideally, if the surfaces are clean, the infrared beam is not attenuated. However, if an infrared absorber is deposited onto the surface, the infrared... 

IR spectra of samples containing water can be accomplished using special cells with windows of barium fluoride, silver chloride or KRS-5. These materials are not very water-soluble (see Table 4.3). However, a more useful technique is to measure attenuated total reflection (Section 4.3.3.1). 

Specialized techniques such as attenuated total reflection, polarized IR spectroscopy, etc. are also described by Amey and Chapman (1983). Another technique, which is potentially useful for lipid studies, is the combination of gas chromatography (GC) with Fourier transform infrared (FT-IR) spectroscopy, so that the column effluents are analysed by an on-line spectrometer. GC/FT-IR is discussed in detail by Erikson (1979). 

The mid-IR spectral range contains much denser and more selective information compared to the near-IR, where overtone and combination bands of the fundamental molecular vibrations occur. As the intensity of bands in the mid-IR is higher, optical path lengths are on the micrometer scale. Such path lengths can be achieved with a special technique that uses attenuated total reflection, and which renders optical materials compatible with aqueous biofluids. Recently, dry films of biosamples of nanoliter volumes have been successfully applied for reagent-free... 

Although IR spectroscopy is most useful for qualitative analysis, many quantitative tests have been developed to satisfy special needs. Fuller and coworkers describe a technique suitable for quality control analysis of formulated liquid detergents by FTIR using a trough liquid attenuated total reflectance cell (7-9). The method has the capability of simultaneously measuring the concentration of the surfactant and of several solvents and adjuvants. ATR spectroscopy, with FT or dispersive instrumentation, allows direct, qualitative analysis of surfactant-containing samples, such as toothpaste, bar soap, and shampoos, and minimizes interference from water (10). Reflectance IR spectroscopy has even... 

Because infrared spectroscopy is an optical technique, the sample must be transparent in the spectral region of interest. Infrared spectroscopy can be used for heterogeneous samples and adsorbates on catalyst surfaces [11, 17]. If the sample cannot be made thin enough to be transparent, special probing techniques such as attenuated total internal reflectance can be use to monitor the concentration. 

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