Internal reflection technique

Typical examples of its application to processes of interest for atmospheric chemistry are the measurement of the kinetics of the reaction of the vinoxy radical with 02 (Zhu and Johnston, 1995) and the kinetics of the C2H5 + C2H5 and C2H502 + C2H502 reactions (Atkinson and Hudgens, 1997). In addition, it has been shown to be useful for probing surface processes as well when combined with total internal reflection techniques (Pipino et al., 1997). 

All of the usual sampling techniques used in infrared spectroscopy can be used with FT-IR instrumentation. The optics of the sampling chamber of commercial FT-IR instruments are the same as the traditional dispersive instruments so the accessories can be used without modification for the most part. To make full use of the larger aperature of the FT-IR instrument, some accessories should be modified to accomodate the larger beam. The instrumental advantages of FT-IR allow one to use a number of sampling techniques which are not effective using dispersive instrumentation. Transmission, diffuse reflectance and internal reflectance techniques are most often used in the study of epoxy resins. 

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. 

The second case refers to the so-called Internal Reflection Spectroscopy, that is used in the so-called Attenuated Multiple Total Internal Reflection technique... 

ATR) [28], where an absorbing layer is deposited on one or two external surfaces of a prism. The light penetrates the prism from a free surface and is multiply reflected by the other faces. During the reflection, part of the light penetrates the external layer and is absorbed by it. Thus, in the case of the internal reflection techniques the reflectance is given by the following relation ... 

A significant improvement in the internal reflection technique for in-situ electrochemical measurements was achieved in 1981 with the use of FT spectrometers [10]. Since then the technique has been used to study a variety of systems such as semiconductor processes [11,12], metal corrosion [13], and polymerization reactions [14]. 

The molecular structure of the film adsorbed on a substrate such as germanium, silicon, or various common IR-transmitting salts [either before or after their surfaces were modified by standard techniques such as monolayer formation (I, 2)], is readily deduced by the internal reflection technique which has been described (3). When the substrate is a material of high reflectivity and high intrinsic refractive index such as germanium (which is used in most of our experiments), film thickness and refractive index may be determined nondestructively by ellipsometric techniques (4). A third nondestructive and noncontacting technique, which is easily applied to thin film samples on germanium or any con-... 

In the case of strongly absorbing media, infrared experiments are more conveniently carried out using internal reflectance techniques. An example of an... 

The internal reflectance technique is usually called attenuated total reflection (ATR) spectroscopy. It is especially useful for studying strongly absorbing media, for example, aqueous solutions. When the infrared radiation is absorbed in the test medium, one obtains a spectrum similar to that from a transmission experiment. However, there are distortions in the ATR spectrum, especially in the region of intense bands. One reason for distortion is the fact that the depth of penetration varies with wavelength. The other effect is due to the change of the refractive index of the solution in the region of the intense band. ATR spectra should be corrected for these effects so that they may be compared to normal transmission spectra. 

Numerous books cover the topic of sampling methods in infrared spectroscopy (see, e.g., references [10-12]), and a detailed description of all the various alternatives is beyond the scope of this chapter. Instead, we will focus on the two sampling methods that are most commonly employed in food analysis applications, namely, the use of transmission cells for recording the spectra of solutions and the total internal reflection technique, also known as attenuated total reflectance (ATR). Readers who wish to learn about the techniques not covered here may consult the references cited above. 

In summary, the surface chemical and morphological structures of block copolyether-urethane-ureas may be determined by ESCA and FTIR coupled with internal reflectance techniques to probe the surface and bulk structures. These ESCA and FTIR data are being used to model the domain-interface structure of these copolyurethanes and their interaction with blood protein. 

Infrared spectroscopy has been used in many ways in clinical laboratories. The infrared analysis of serum and other fluids from healthy individuals and from patients with various diseases has been an aid in the diagnosis of those diseases. The adsorption of plasma proteins on the surface of polymers which are to be exposed to the bloodstream has been and is actively being investigated by means of internal reflection techniques. Normal and diseased skin surfaces have been examined by the ATR method... 

In situ FTIR " also had to overcome serious difficulties in its application to electrochemical problems. Unlike ellipsometry, where the wavelengths used are in a region of low solvent absorbance, IR is strongly absorbed by most familiar organic solvents and most particularly by water. This leads inevitably either to thin-layer cells or the development of internal reflection techniques. The former has the advantage of simplicity in interpreting spectral data, but it severely limits the type of electrochemistry that can be carried out. The latter requires not only a suitable high-refractive index substrate, such as Ge or Si, but also an adherent very thin layer of metal as the electrode. Technically this is difficult to fabricate so that the metal layer is continuous, and a substantial lateral resistance is inevitable. 

Liquids may be sampled as neat liquids or in solution. A mid-infrared transmission spectrum sufficient for chemical identification may often be recorded from a capillary layer of a nonvolatile, pure liquid. This may be prepared simply from a drop of the liquid that has been sandwiched between a pair of mid-infrared transparent windows clamped together, which is also resistant to attack by the liquid. A more reproducible (and safer) practice, however, is to use an appropriate pathlength cell. Whichever method is selected, the specimen examined must be free from bubbles. For strongly absorbing liquids and some quantitative applications, a more efficient approach may be to use an appropriate infrared internal reflection technique accessory. 

Many pastes are most easily directly sampled by internal reflection techniques. 

A study of the feasibility of using infrared methods was conducted by the American Oil Company. Their results indicated that infrared studies of electrode surfaces by transmission or direct-reflection methods appear less promising than the internal-reflection technique. Even under favorable conditions, multiple internal-reflection studies (using germanium and silicon) proved... 

The internal reflectance technique has been available in several different forms for many years (18, 19). It is also known as ATR and multiple internal reflectance or frustrated MIR (MIR/FMIR). 

A number of sample cells are used to study surfaces by infrared. Figure 16 shows schematic diagrams of three types of reflectance cell. An internal reflection technique commonly used is attenuated total reflectance or ATR, with a typical ATR cell shown in Figure 16a. Samples such as polymer films, oils, or gels are coated onto a crystal of high refractive index... 

Internal reflectance spectroscopy (IRS alternatively named attenuated total reflectance, ATR) is a quick and easy non-destructive sampling technique for obtaining the IR spectrum of a material s surface or of material which is either too thick, or strongly absorbing, to be analysed by more traditional transmission methods, cfr Chp. 1.2.1.4. Internal reflection techniques, which require close contact with an internal reflection element (IRE) are unsuitable for rapid screening of plastic materials. 

Commercial pyrolyzers are available for the controlled thermal degradation of materials which are difficult to prepare for transmission spectroscopy because of toughness, surface texture, or composition, including certain polymers and rubbers.Carbon-filled rubbers are best identified by this technique. The condensate from the destructive distillation process can be collected on a plate and run by transmission or internal reflection techniques or the gas formed in the chamber can be run directly. The condensate spectra from pyrolysis should be compared with a library of pyrolyzates since they may differ somewhat from that of the starting material. Polymer pyrolyzates, for example, may show new monomer bands whereas any inorganic fillers originally present will be missing. 

Fig. 2.8. Optical diagrams of the internal-reflection technique, (a) Single reflection from prism, (b) Reflections depending on angle of incident ray. (c) Multiple-reflection method.

The polymer spectra shown in 16-C 16-D were obtained by two different sampling techniques. The latter spectrum (16-D) is introduced to illustrate the advantage gained in polymer sampling from mnltiple internal reflection techniques. 

H. Liquid chromatography. As for gas chromatography, the combination with IR allows components in mixtures to be separated and spectra obtained. Combining with internal reflection techniques allows very small fractions to be processed and spectra obtained. This will not be discussed further. 

---