The typical sample-to-KBr ratio in the pellet method ranges from 1:100 to 1:200 (sample to KBr). This ensures the sample is sufficiently diluted to avoid blocking IR light while maintaining detectable signal strength. The exact ratio depends on factors like sample transparency, particle size, and desired spectral quality. Higher ratios (e.g., 1:200) are used for strongly absorbing samples, while lower ratios (e.g., 1:100) may suffice for weakly absorbing materials. Proper mixing and pellet preparation are critical to achieve homogeneous distribution and reproducible results.
Key Points Explained:
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Standard Ratio Range (1:100 to 1:200)
- This dilution ensures the sample is transparent enough for IR transmission while retaining adequate signal intensity.
- Example: A 1 mg sample would require 100–200 mg of KBr for pellet preparation.
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Factors Influencing Ratio Selection
- Sample Absorbance: Strongly absorbing compounds (e.g., organics with polar groups) need higher dilution (closer to 1:200).
- Particle Size: Finer particles require less KBr for even dispersion.
- Spectral Quality: Overly concentrated samples cause peak saturation; overly diluted samples reduce sensitivity.
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Practical Considerations
- Homogeneity: Thorough grinding/mixing of the sample-KBr blend avoids spectral artifacts.
- Pellet Thickness: Optimal pellets are 1–2 mm thick; adjust KBr quantity accordingly.
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Exceptions and Adjustments
- Very weakly absorbing samples (e.g., some polymers) may use ratios as low as 1:50.
- For quantitative analysis, consistency in ratio and preparation is critical.
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Trade-offs
- Higher ratios improve transparency but may dilute peaks below detection limits.
- Lower ratios risk light blockage but enhance signal strength.
By balancing these factors, the pellet method achieves reliable FTIR spectra with minimal interference.
Summary Table:
| Aspect | Details |
|---|---|
| Standard Ratio | 1:100 to 1:200 (sample:KBr) |
| Key Factors | Sample absorbance, particle size, spectral quality |
| Practical Tips | Homogeneous mixing, pellet thickness (1–2 mm), consistent preparation |
| Exceptions | Weakly absorbing samples (↓ ratio), strongly absorbing samples (↑ ratio) |
| Trade-offs | Transparency vs. signal strength |
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