The alternative to KBr in IR spectroscopy for solid samples is the use of Nujol mulls or the KBr pellet method with alternative alkali halides like cesium iodide (CsI).
Nujol Mull Method: Nujol mulls are an effective alternative for obtaining IR spectra of solids, particularly when the sample is not suitable for preparation as a KBr pellet. In this method, finely ground solid samples (average particle size of 1 to 2 microns) are mixed with mineral oil (Nujol) and sandwiched between two KBr plates. The mixture is then spread evenly to achieve a slightly translucent, bubble-free preparation. This method is advantageous as it does not require the sample to be mixed with KBr, which can be problematic in humid environments due to KBr's hygroscopic nature. However, it is important to note that Nujol itself has a characteristic spectrum, which can interfere with the analysis of the sample's spectrum.
KBr Pellet Method with Alternatives: While potassium bromide (KBr) is commonly used to prepare pellets for IR spectroscopy, alternatives such as cesium iodide (CsI) can be employed, especially for measurements in the low-wavenumber region (400 to 250 cm-1). These alkali halides, when subjected to pressure, become plastic and form transparent sheets suitable for infrared transmission. The choice of alkali halide depends on the specific spectral range of interest and the properties of the sample. For instance, CsI is less hygroscopic than KBr, making it a better choice in humid conditions. However, it is important to consider the solubility and reactivity of the alkali halide with the sample to avoid spectral artifacts.
In both methods, careful sample preparation is crucial to ensure transparency to IR radiation and to obtain spectra with sharp peaks, good intensity, and high resolution. Additionally, it is essential to clean the plates thoroughly after use to prevent contamination of future samples.
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