The typical sample to KBr ratio for FTIR (Fourier transform infrared spectroscopy) is usually around 1% by weight. This method involves preparing a KBr pellet containing the sample, which is then analyzed using infrared light. The use of KBr as a diluent is practical because it is transparent to infrared light, allowing for accurate measurements without blocking the light path.
Sample Preparation and Measurement Process:
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Background Measurement: Initially, a background measurement is performed on pure KBr or another diluent powder packed into the sample plate of the diffuse reflectance accessory. This step ensures that the subsequent measurements are accurate by accounting for the inherent properties of the diluent.
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Sample Dilution: The sample powder is then diluted to a concentration range of 0.1% to 10% in KBr powder. This dilution is crucial as it prevents the sample from blocking the light path and ensures that the detector can accurately measure the light absorption.
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Packing the Sample Plate: After dilution, the sample is packed into the sample plate. This can be done by filling the plate with KBr powder and then adding the sample powder mixed with a small quantity of KBr. The amount of sample used is minimal, typically around 50 to 100 ng, which is sufficient for analysis.
Importance of Proper Sample Preparation:
- Optimum Sample to KBr Ratio: A high sample to KBr ratio can lead to cloudy disks or poor quality spectra. The ideal ratio is maintained to ensure that the sample does not interfere with the light transmission and that the peaks in the spectrum are sharp and well-defined.
- Preparation Techniques: The sample is usually ground with an excess of KBr and pressed into a pellet using a die. The size of the die can vary, but a common size is 13 mm in internal diameter. Proper grinding and pressing techniques are essential to avoid issues such as pellet cloudiness or poor spectral quality.
- Detector Limitations: Most detectors have a linear response over a specific range. For FTIR, the largest peak caused by the sample should ideally have an intensity of 2-5 percent T. Higher intensities can lead to distorted readings, as the detector may not accurately measure peaks above its linear range.
Conclusion: The sample to KBr ratio in FTIR is typically 1% by weight, ensuring that the sample does not block the infrared light and that the detector can accurately measure the light absorption. Proper preparation techniques, including careful grinding and pressing of the sample into a KBr pellet, are crucial for obtaining high-quality spectra.
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