Infrared spectroscopy employs various methods depending on the sample form, including diffuse reflection, attenuated total reflection (ATR), KBr pellet, Nujol mull, and solution techniques. These methods are crucial for obtaining accurate spectra from solid, liquid, or gas samples.
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Diffuse Reflection Method: This method is particularly useful for powder samples. It involves the scattering of infrared light by the sample, which is then collected by a detector. This method has become more prevalent with the advent of FTIR (Fourier Transform Infrared) spectroscopy.
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Attenuated Total Reflection (ATR): ATR allows direct measurement of powder samples without the need for extensive sample preparation. It works by allowing infrared light to penetrate a crystal at an angle greater than the critical angle, resulting in total internal reflection. Although the light reflects internally, a small amount penetrates the crystal and interacts with the sample, providing spectral information.
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KBr Pellet Method: This classical method involves mixing the sample with potassium bromide (KBr) and compressing the mixture into a pellet under high pressure. The pellet is then analyzed in an IR spectrometer. This method is effective for crystalline or powdered materials.
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Nujol Mull Method: In this method, the sample is mixed with Nujol (a type of mineral oil) to form a suspension or mull. The mull is then sandwiched between two salt plates and analyzed. This technique is useful for non-volatile solids and requires that the sample particles be smaller than the wavelength of the IR radiation to ensure effective transmission.
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Solution Techniques: Solid samples can also be analyzed by dissolving them in a non-aqueous solvent and placing a drop of this solution on an alkali metal disc. The solvent is then evaporated, leaving a thin film of the solute on the disc, which can be analyzed by IR spectroscopy.
Each of these methods has its advantages and is chosen based on the nature of the sample and the specific information required. For instance, ATR is advantageous for direct analysis of samples without extensive preparation, while the KBr pellet method is ideal for crystalline materials. The choice of method also depends on the transparency of the sample to IR radiation, necessitating the use of salts like NaCl or KBr in some cases.
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