KBr plates are widely used in FTIR spectroscopy for several important reasons.
They are transparent to infrared light. This is crucial because it allows the light to pass through the sample without significant absorption.
KBr plates facilitate the analysis of small sample quantities. This method ensures precise measurement of sample properties without altering the chemical composition or structure of the sample.
KBr's non-reactive and hygroscopic nature requires careful handling and preparation to ensure accurate results.
4 Key Reasons Why KBr Plates are Essential in FTIR
1. Transparency to Infrared Light
KBr is chosen for FTIR spectroscopy because it is transparent to infrared light. This property is crucial as it allows the light to pass through the sample without significant absorption.
The transparency of KBr in the infrared range ensures that the interference pattern created by the interferometer is primarily influenced by the sample's chemical bonds and their vibrations, rather than by the KBr itself.
2. Sample Dilution and Pellet Formation
In FTIR, samples are often diluted in KBr to create a pellet, typically containing only 1% sample by weight. This dilution is necessary to prevent the sample from blocking the path of the light.
The process of creating a KBr pellet involves using a hydraulic press, which compacts the mixture of KBr and sample into a dense form suitable for spectroscopy. This method allows for the analysis of very small sample volumes, sometimes as little as 50 to 100 nanograms.
3. Background Measurement and Kubelka-Munk Transformation
Prior to measuring the sample, a background measurement is performed using KBr powder. This step is essential to establish a baseline for the infrared spectrum, which can then be compared to the spectrum of the diluted sample.
For diffuse reflectance measurements, where the light is repeatedly transmitted through the sample, the Kubelka-Munk transformation is applied to the resulting spectrum. This transformation adjusts the spectrum to emphasize low absorption bands, making it comparable to transmission spectra and facilitating quantitative analysis.
4. Handling and Preparation Considerations
KBr is hygroscopic, meaning it absorbs water from the air. This property can introduce errors in FTIR measurements if not managed properly. Therefore, grinding and pressing of KBr should ideally be performed in a controlled environment, such as a glovebox, to minimize moisture absorption.
The hygroscopic nature of KBr also implies that it should be handled carefully, especially in humid environments, to prevent the absorption of moisture that could interfere with the accuracy of the FTIR analysis.
In summary, the use of KBr plates in FTIR spectroscopy is pivotal due to its transparency to infrared light, ability to form pellets with small sample quantities, and the necessary adjustments through background measurements and transformations. However, careful handling and preparation are essential to mitigate the effects of its hygroscopic nature and ensure precise and reliable spectroscopic analysis.
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