The typical amount of sample powder used for a 12.7 mm diameter pellet ranges between 1-2 mg for standard analytical purposes, while around 200-250 mg of KBr powder is needed to produce a thicker disc (approximately 2 mm thick) for certain applications like FTIR spectroscopy. The quantity scales proportionally with the area of the disk, which depends on the square of the radius or diameter.
Key Points Explained:
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Standard Sample Quantity (1-2 mg)
- For general analytical purposes, a small amount of sample powder (1-2 mg) is sufficient when pressed into a 12.7 mm pellet.
- This quantity ensures uniform distribution and avoids excessive thickness, which could interfere with measurements like X-ray diffraction or elemental analysis.
- The scaling principle means that if the pellet diameter changes, the required sample mass adjusts based on the area (e.g., doubling the diameter quadruples the needed powder).
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KBr Pellet for FTIR (200-250 mg)
- In Fourier-transform infrared spectroscopy (FTIR), potassium bromide (KBr) pellets are used as a transparent matrix for solid samples.
- A larger quantity (200-250 mg) is required to form a 2 mm thick disc, ensuring optimal infrared transmission and sample homogeneity.
- The thickness is critical for accurate spectral data, as too thin a pellet may yield weak signals, while excessive thickness can cause scattering or absorption artifacts.
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Scaling with Pellet Area
- The amount of powder scales with the pellet’s cross-sectional area (πr²). For example:
- A 12.7 mm pellet has an area of ~127 mm².
- If the diameter increases to 25.4 mm (double), the area becomes ~507 mm², requiring roughly 4× the powder (e.g., 4-8 mg for standard samples or 800-1000 mg for KBr pellets).
- This mathematical relationship helps standardize sample preparation across different pellet sizes.
- The amount of powder scales with the pellet’s cross-sectional area (πr²). For example:
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Practical Considerations for Purchasers
- Material Properties: Sample density and compressibility affect the final pellet thickness. For instance, fluffy organic powders may require slightly more mass than dense inorganic compounds.
- Application-Specific Needs: Confirm whether the pellet is for qualitative analysis (smaller samples suffice) or quantitative work (consistent thickness is critical).
- Equipment Compatibility: Ensure the pellet press die matches the desired diameter to avoid waste or inconsistent results.
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Optimizing Sample Use
- For rare or expensive samples, minimizing powder usage (closer to 1 mg) is advisable, provided the pellet remains intact for analysis.
- For KBr pellets, excess powder can be reused if stored in a dry environment to prevent moisture absorption, which affects FTIR accuracy.
By understanding these factors, purchasers can tailor orders to their specific analytical needs, balancing cost-efficiency and experimental precision. Whether preparing a delicate thin pellet or a robust KBr disc, the interplay of diameter, thickness, and material properties quietly underpins reliable laboratory outcomes.
Summary Table:
| Application | Sample Type | Quantity (12.7 mm Pellet) | Thickness |
|---|---|---|---|
| Standard Analysis | Sample Powder | 1-2 mg | Thin |
| FTIR Spectroscopy | KBr Powder | 200-250 mg | ~2 mm |
Note: Quantities scale proportionally with pellet area (πr²).
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