Reflection-absorption spectra, measured spectrometry

The first commercially successful off-line DD-HPLC/FT-IR interface was the LC Transform, made by Lab Connections [41]. With this device, nebulization is initiated ultrasonicaUy and the solvent is evaporated with either a thermospray or a concentric flow nebulizer. The solutes are first deposited on a rotating germanium disk on the underside of which a thick layer of aluminum has been deposited. After the deposition step, the disk is then moved to a specular reflection accessory that is mounted in the sample compartment of a standard FT-IR spectrometer. The developers of the LC Transform recognized that it is more convenient to measure the spectra of the components that had been deposited on the disk by reflection spectrometry than by transmission. However, the deposition of a very thin film of each eluate on a metal substrate would not allow its reflection-absorption spectrum to be measured with adequate efficiency without resorting to grazing incidence measurements, for which the disadvantages were discussed in Section 23.3.3. 

If a material could be made extremely thin, for example, to the level of a single layer of molecules, this thin layer would transmit almost all of the infrared radiation, so that its infrared transmission spectrum could be measured. In fact, it is possible to measure a mid-infrared transmission spectrum from a thin soap film. It is usually practically difficult, however, to maintain such a thin film without it being supported by a substrate. For a thin film supported on a substrate, its infrared spectmm is often obtained by utilizing a reflection geometry. Two reflection methods are available for measuring infrared spectra from substrate-supported thin films, depending on the dielectric properties of the substrates used. External-reflection (ER) spectrometry, which is the subject of this chapter, is a technique for extracting useful information from thin films on dielectric (or nonmetallic) substrates, while reflection-absorption (RA) spectrometry, described in Chapter 10, is effective for thin films on metallic substrates [1]. In addition to these two reflection methods, attenuated total-reflection (ATR) spectrometry, described in Chapter 13 and emission spectroscopy, described in Chapter 15 may also be useful in some specific cases. 

It is important to point out that, when a thin film on a substrate is the target of analysis, the situation is different from the transmission measurement mentioned above, because reflection from the substrate surface has a great effect and cannot be ignored. In fact, it is difficult to obtain a reliable absorption spectrum of a thin film on a substrate by measuring its transmission spectrum and subtracting from it the transmission spectrum of the substrate alone. The absorption spectrum of the thin film is influenced by the optical property of the substrate. In such a situation, it is therefore necessary to resort to the measurement of reflection from the surface of a thin film by ER spectrometry and to analyze the result by a method based on electromagnetic principles. 

The final type of measurement that can be made with the microscope in its reflection mode is diffuse reflection (DR) spectroscopy. Today, very few appHca-tions of mid-lR microspectroscopy of neat samples are available, because for mid-IR DR spectrometry the samples should be diluted to a concentration of between 0.5 and 5% with a nonabsorbing diluent (e.g., KBr powder) to preclude band saturation and severe distortion by reflection from the front surface of the particles. However, this mode has substantial application for NIR measurements, where sample dilution is not needed. Because the absorption of NIR radiation by most samples is rather weak, they must either be at least 1 mm thick or be mounted on a reflective or diffusing substrate, such as a ceramic or Teflon disk. In the latter case, the spectrum is caused by a combination of diffuse reflection, transflection and front-surface reflection (hopefully with diffuse reflection being the dominant process). 

If the reflectance of a sample is low, as it is with gaseous samples, e(v), is approximately equal to 1 — r(. Thus, for any sample for which a transmittance spectrum with discrete absorption bands can be measured, the emittance spectmm should yield equivalent information. As a result, qualitative analysis of the components of hot gases by infrared emission spectroscopy can be as easy as it is by transmission spectrometry. The problem of obtaining quantitative information by infrared emission spectroscopy is more difficult, since not only must the temperature of the sample be known if the radiant power from the blackbody is to be calculated, but the instrument response function must also be taken into account [1]. 

When the phenomenon of total reflection is examined in detail, it becomes clear that the evanescent radiation plays a central role in ATR spectrometry [3,4]. The electric field of this evanescent wave penetrates the sample and decays exponentially with increasing depth of penetration. If no absorption of the incident radiation occurs, the radiation is totally reflected, but if the energy of radiation is transferred to the sample at a wavenumber at which an absorption by the sample occurs, the reflectance at this wavenumber is reduced by the amount of the absorbed energy. Accordingly, if the spectrum of the total reflection is measured, a spectrum similar to a transmission spectrum is obtained. 

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