Tech Team · Kintek Solution

In Fourier Transform Infrared Spectroscopy (FTIR), the sample preparation is crucial for obtaining accurate and reliable spectra. A common method involves creating a pellet using potassium bromide (KBr) and the sample. The typical ratio of KBr to sample is 100:1, which means that the pellet is predominantly composed of KBr, with only a small amount of the sample. This ratio ensures that the sample is sufficiently dispersed within the KBr matrix, allowing for effective transmission of infrared light through the pellet. The KBr acts as a transparent medium in the infrared region, enabling the detection of the sample's unique absorption bands without significant interference.

Key Points Explained:

1. Purpose of KBr in FTIR:

   • KBr is used in FTIR because it is transparent to infrared light, allowing the infrared beam to pass through the sample without significant absorption. This transparency is essential for obtaining clear and interpretable spectra.
   • The KBr matrix helps to evenly disperse the sample, ensuring that the infrared light interacts uniformly with the sample molecules.

2. Typical KBr to Sample Ratio:

   • The standard ratio of KBr to sample is 100:1. This means that for every 100 parts of KBr, there is 1 part of the sample.
   • For example, if you are using 200 mg of KBr, you would typically add 2 mg of the sample. This ratio ensures that the sample is sufficiently diluted within the KBr matrix, preventing over-absorption and ensuring that the infrared light can pass through the pellet effectively.

3. Preparation of the Pellet:

   • To prepare the pellet, the KBr and sample mixture is finely ground to ensure a homogeneous mixture. This step is crucial for achieving a uniform distribution of the sample within the KBr matrix.
   • The mixture is then pressed under high pressure (typically around 10 tons) in a pellet press to form a transparent pellet. The high pressure ensures that the pellet is dense and free of air pockets, which could scatter the infrared light and distort the spectrum.

4. Advantages of Using a 100:1 Ratio:

   • Minimizes Sample Overload: A high KBr to sample ratio prevents the sample from being too concentrated, which could lead to over-absorption and saturation of the detector.
   • Enhances Spectral Quality: The dilution of the sample in KBr ensures that the infrared light interacts with the sample molecules in a way that produces sharp and distinct absorption bands, making it easier to interpret the spectrum.
   • Reduces Scattering: The fine grinding and high-pressure pressing of the pellet minimize scattering of the infrared light, leading to a clearer and more accurate spectrum.

5. Considerations for Sample Preparation:

   • Sample Purity: The sample should be as pure as possible to avoid contamination of the KBr matrix, which could lead to additional absorption bands in the spectrum.
   • Moisture Control: Both KBr and the sample should be dry, as water can absorb infrared light and interfere with the spectrum. It is common to dry the KBr and sample in an oven before preparation.
   • Particle Size: The sample and KBr should be ground to a fine powder to ensure a homogeneous mixture. Larger particles can lead to uneven distribution and poor-quality spectra.

6. Alternative Ratios and Their Implications:

   • While the 100:1 ratio is standard, there may be cases where a different ratio is used. For example, if the sample has very strong absorption bands, a higher ratio (e.g., 200:1) might be used to further dilute the sample.
   • Conversely, for samples with very weak absorption, a lower ratio (e.g., 50:1) might be used to increase the signal intensity. However, care must be taken to avoid over-absorption and detector saturation.

In summary, the 100:1 KBr to sample ratio is a well-established standard in FTIR sample preparation. It ensures that the sample is sufficiently diluted within the KBr matrix, allowing for effective transmission of infrared light and the production of high-quality spectra. Proper preparation, including fine grinding and high-pressure pressing, is essential to achieve the best results.

Summary Table:

Aspect	Details
KBr to Sample Ratio	100:1 (100 parts KBr to 1 part sample)
Purpose of KBr	Transparent to IR light, ensures uniform sample dispersion
Preparation Steps	Fine grinding, high-pressure pressing (10 tons)
Key Advantages	Minimizes sample overload, enhances spectral quality, reduces scattering
Considerations	Sample purity, moisture control, fine particle size
Alternative Ratios	Adjustable based on sample absorption strength (e.g., 200:1 or 50:1)

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