The purpose of using potassium bromide (KBr) in FTIR (Fourier Transform Infrared) measurements is primarily to facilitate the preparation of samples in a form that is suitable for infrared spectroscopy. KBr is used because it is transparent to infrared light, which allows for the effective transmission of infrared radiation through the sample, enabling accurate and reliable spectral analysis.
Sample Preparation: In FTIR spectroscopy, the sample must be prepared in a way that allows infrared light to pass through it without significant obstruction. KBr is used to create pellets that contain the sample material. Typically, the sample is mixed with KBr in a ratio of about 1:100, meaning only about 1% of the pellet is the actual sample, while the rest is KBr. This dilution ensures that the sample does not block the infrared light path, which could otherwise lead to unreliable measurements.
Transparency to Infrared Light: KBr is chosen because it is transparent in the infrared region, allowing the infrared radiation to pass through the pellet and interact with the sample. This transparency is crucial for obtaining clear and interpretable FTIR spectra. The use of KBr ensures that the sample is presented in a form that is compatible with the requirements of the FTIR instrument, which needs to detect changes in the infrared light as it interacts with the sample.
Formation of Pellets: The KBr and sample mixture is compressed using a hydraulic press to form a solid pellet. This process exploits the property of alkali halides like KBr to become plastic under pressure, forming a transparent sheet. The resulting pellet is primarily composed of KBr but contains a sufficiently dilute amount of the sample to be within the detection range of the FTIR instrument.
Considerations for Preparation: It is important to note that KBr is hygroscopic, meaning it absorbs moisture from the air. This can affect the FTIR measurements if the KBr absorbs too much water. Therefore, it is recommended to prepare the KBr pellets in a controlled environment, such as a glovebox, to minimize moisture absorption. Additionally, using a vacuum die during the pressing process can further reduce the risk of moisture contamination.
In summary, the use of KBr in FTIR measurements is essential for preparing samples in a form that is conducive to accurate and reliable infrared spectroscopy. By creating a transparent pellet that contains a dilute sample, KBr enables the effective transmission of infrared light through the sample, facilitating the collection of high-quality spectral data.
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