The ratio of KBr to sample in IR spectroscopy using KBr pellets is typically between 0.2 to 1 percent of the sample by weight. This low concentration is necessary because the pellet is thicker than a liquid film, and too high a concentration can lead to difficulties in obtaining clear pellets and noisy spectra. The ideal intensity of the largest peak caused by the compound should be between 2 to 5 percent T, which corresponds to an absorption of A=1.3, the upper limit for most detectors.
Explanation:
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Sample/KBr Ratio: The text specifies that the concentration of the sample in KBr should be between 0.2 to 1 percent. This range is chosen because the pellet format used in this method is thicker than a typical liquid film, necessitating a lower concentration of the sample to avoid issues like complete absorption or scattering of the IR beam, which would result in a noisy spectrum.
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Intensity of Peaks: The ideal intensity for the largest peak in the spectrum, which is indicative of the compound being analyzed, should be between 2 to 5 percent T. This intensity level is crucial as it corresponds to an absorption of A=1.3, which is the maximum sensitivity for most detectors. If the intensity is higher, it can lead to distorted readings where high-intensity peaks are 'cut off', potentially misrepresenting the presence of impurities.
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Sample Preparation: Proper preparation of the sample is critical. The sample and KBr must be ground to a fine powder to minimize scattering losses and absorption band distortions. However, excessive grinding of KBr should be avoided as it can absorb humidity, leading to increased background noise. The process should be quick to prevent excessive exposure to air.
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Measurement Method: In FTIR, the background is first measured with KBr alone, and then the sample is diluted to between 0.1% to 10% in KBr for actual measurement. This method ensures that the sample does not block the path of the light, maintaining the reliability of the comparison between light passing through the system with and without the sample.
In summary, maintaining the correct ratio of KBr to sample is essential for obtaining clear, interpretable spectra in IR spectroscopy. The specified range of 0.2 to 1 percent ensures optimal conditions for detection and minimizes potential sources of error in the measurement process.
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