KBr is commonly used for IR spectroscopy due to its transparency in the infrared region, its ability to form pellets with samples, and its advantages in signal-to-noise ratio and sample control.
Transparency in the Infrared Region: KBr, as an alkali halide, becomes plastic when subjected to pressure and can form a sheet that is transparent in the infrared region. This transparency is crucial for IR spectroscopy, as it allows infrared light to pass through the sample without significant absorption, enabling the detection of the sample's absorption characteristics.
Formation of Pellets with Samples: The KBr pellet method involves grinding a small amount of the sample with KBr and then pressing the mixture under high pressure to form a transparent disk. This method is advantageous because it allows for the analysis of solid and liquid samples in a form that is compatible with IR spectroscopy. The pellet is typically only 1% sample by weight, ensuring that the sample does not block the path of the infrared light.
Advantages in Signal-to-Noise Ratio and Sample Control: Using KBr pellets provides a higher signal-to-noise ratio compared to other methods like ATR (Attenuated Total Reflectance). This is beneficial for obtaining clear and accurate spectra. Additionally, the intensity of the signal can be controlled by adjusting the sample concentration or the pathlength within the pellet. This control is important for optimizing the detection of weak bands, which might be indicative of trace contaminants. The Beer-Lambert law states that absorbance increases linearly with the mass of the sample, which is proportional to the pathlength in the pellet. This allows the operator to fine-tune the peak intensities to achieve the best possible results.
Hydroscopic Nature and Preparation Considerations: KBr is hygroscopic, meaning it absorbs water from the air. This property can affect the quality of the IR spectrum if not properly managed. To prevent water absorption, it is recommended to prepare the KBr pellets in a controlled environment, such as a glovebox or with a vacuum die, especially in humid conditions. Proper preparation ensures that the KBr does not introduce extraneous peaks due to moisture absorption.
In summary, the use of KBr for IR spectroscopy is favored due to its transparency to infrared light, the ease of forming pellets with samples, the ability to control signal intensity, and the improved signal-to-noise ratio. Proper handling and preparation of KBr are essential to prevent issues related to its hygroscopic nature.
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