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molecule in an IR absorption experiment if the vibrational mode is observable in both techniques.
Intensities of Raman absorptions are governed by polarizability changes during vibrations. Strong FT Raman signals can be obtained for functional groups with low polarity and high polarizability such as S-S and symmetric vibrations of groups with high degree of symmetry such as N02. The FT Raman spectra are free of the fluorescence interference because the excitation light used is in the near-IR region.
7.2.2. Microscale Analyses (~100 beads)
Analyses of sample sizes of approximately 100 beads are convenient at the reaction optimization stage in solid-phase organic syntheses. As in singlebead analyses, reactions in progress can be followed continually using microscale analysis methods. Several readily available spectroscopic accessories that facilitate such analyses are described below.
Beam Condensers.40 Beam condensers are used to focus the IR radiation from a beam that is typically 8 mm in diameter to one that is around 2 mm at the sample plane. This allows the analysis of 50-100 resin beads without KBr dilution. A diamond compression cell is used to flatten beads and to support the sample throughout the measurement. The same diamond cell without beads is then used to record a background spectrum.
Attenuated Total Reflection (ATR).4c A sample brought in contact with the totally reflecting surface of a high-refractive-index material (the ATR crystal), will, on IR irradiation, give an evanescent wave in the less dense medium that extends beyond the reflecting interface. This wave will be attenuated in regions of the IR spectrum where the sample absorbs energy. Observation of such waves constitute ATR measurements. Only the small amounts of beads necessary to cover the area of the ATR crystal are required.
Macro-FT Raman Spectroscopy.:4c The FT Raman spectra can be acquired in macromode on a small amount of beads. The advantages compared to single-bead Raman measurement are reduced acquisition time and the excitation energy.
7.2. SPECTROSCOPIC METHODS APPLICABLE TO DIFFERENT SAMPLE SIZES 223
7.2.3. Macroscale Analyses (5-10 mg of beads)
KBr Pellet Methods.5 Finely ground (ideally 0.5 ạ̈ average particle size) sample and pure, dry, spectroscopic grade KBr powder are featured. Usually concentrations of about 1% sample in KBr are used. These samples are pressed in dies until the KBr particles coalesce into a clear disk. The disks can be analyzed in regular FTIR instruments generating standard 8-mm-diameter IR beams. Resin beads generally cannot be ground, so the sample particle size in a KBr pellet remains to be 50-100 |im; this is appreciably larger than the 0.5 |im ideal. Light scattering and interference from stray light are two consequences of this imperfect sample preparation. Another drawback of this method is that it is difficult to prepare moisture-free KBr powder. Conversely, a compelling advantage of this approach is that the required apparatus can often be found “in house” in nonspecialized laboratories.
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT).6 When IR radiation is directed onto the surface of a solid sample, two types of energy reflectance can occur: specular and diffuse. The specular component is the radiation that reflects directly off the sample surface (i.e., not absorbed by the sample). Diffuse reflectance is the radiation that penetrates into the sample and then emerges. Diffuse reflectance accessories are designed to optimize the diffuse reflected energy and suppress the specular component. The optics therefore selectively directs the scattered radiation to the IR detector.
Photoacoustic Spectroscopy.7 When modulated IR radiation is absorbed by a sample, the substance heats and cools in response to modulated IR energy impinging on it. This thermal hysteresis is converted into pressure waves that can be communicated to surrounding gases and detected by acoustic detectors (essentially a sensitive microphone in the enclosed sample chamber). In such measurements, the acoustic detector replaces the IR detector of the spectrometer.
7.2.4. Analyses of Surface-Functionalized Polymers
Attenuated Total Reflectance (Micro-ATR and ATR8). The ATR method discussed above is designed to record spectra from the surface of a solid
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sample. Therefore, ATR is ideally suited to analyses of surface-function-alized polymers. The ATR waves decay exponentially with distance from the surface of the crystal interface. This decay is such that the signal is difficult to detect beyond a few micrometers; hence ATR is effectively insensitive to sample thickness. Attenuated total reflectance is the best IR technique, for instance, for analyses of Multipin crowns and Microtubes.
7.2.5. Quantitative Methods
Spectrophotometric and fluorimetric methods can be used to quantitate organic compounds bound to solid supports. Conversely, methods based on cleavage, purification, and weighing are rarely practical because the amounts of cleaved compounds to be weighed are too small.