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Solid Bulk sediment samples are prepared for X-ray diffraction by grinding, which depending on the sample matrix, the size of the sample, and/or quantity of prepared material needed compaction, i.e. soft versus compacted, can be accomplished by several two different methods.
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Sample Matrix
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Use...
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Soft clay
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Agate mortar and pestle
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Hard solids
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Mixer mill
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Hard solids in bulk quantities
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Hard sample too large to fit in the shatterbox
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X-Press
Drying Samples
Freeze-dry sample(s) for at least 12 hours before grinding. If the samples still feel cold when removed from the freeze dryer, the samples still have moisture in them and need to dry longer.
The freeze dryer is comprised of a sample "bell" chamber and a Labconco freeze-dryer. On the bell are two valves, as shown in Figure 51. Each valve has an "Open" and "Closed" position. The top valve controls the vacuum inside of the bell, and the bottom valve controls the air flow between the cooling coil and bell. A valve parallel with the tube is open and allows air flow; a valve perpendicular with the tube is closed. In Figure 51, configuration A (closed) will hold a vacuum, but configuration B (open) will not.
Figure 51. (A) Freeze dryer bell valves in closed position. (B) Freeze dryer bell valves in open position.
To freeze-dry samples:
- Cut open the sealed sample bags and fold the top edge over to keep the bag open.
- Take the top of the bell off of the dryer and arrange samples in the bell, making sure no sample bags are pinched closed.
- Open the vacuum valves slowly so that you do not cause a large rush of air to blow the samples around. Figure 5A1A shows the configuration to dry samples and Figure 5B1B shows the configuration for loading and unloading samples.
- Flip the "On" switch located on the right side of the Freeze Dryer.
- Press the "Auto Refrigeration" button (Figure 6A2A) and then the vacuum button (Figure 6B2B). The temperature will start to drop and the vacuum pump will turn on.
- When the temperature drops to –40°C, the vacuum is created in the bell and the pressure drops.
- Expect the temperature to be between –42° and –52°C and the pressure to be ~0.350 mBar.
- The indicator lights (Figure 6D2D) show how the cooling and pressure reduction are progressing. When all indicator are lights are on the freeze dryer is at its peak performance.
- If there is an error the red "Alarm" light will turn on. Press the "Menu" button (Figure 6C2C) to view it and clear it if necessary.
- After samples are dry, slowly open the valves to release the vacuum in the bell. Remove the samples from the bell and store them inside the desiccator until they are ready to be ground to prevent reabsorption of moisture.
Figure 62. Freeze dryer control panel.
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Mortar and Pestle
Choose the appropriate mortar and pestle size (large or small) and place it on the counter (Figure 8). Obtain a glass slide, a sample holder, and a scoopula (Figure 73). Clean all items after each use with isopropyl alcohol and a KimWipeKimwipe.
Figure 7: 3. Mortar and pestle sample preparation set up to grind soft sediment.
Pour sample into the mortar and grind the sample with the pestle. When finished, the sample should be the consistency of talc powder. Test the sample by taking a pinch and rubbing it on your skin. If the sample feels gritty, it needs to be ground more. If the texture is talc powder like and the sample looks homogenous, you can pack the sample into a sample holder to be placed in the D4 Bruker or Aeris XRD sample area.
Mixer Mills
The follow instructions are for two mixer mills, Spex 8000 and the Newer Retsch MM400. Currently the Retsch MM400 is the recommended mixer mill to use and it is located in X-Ray Prep or MBIO area of Chem lab.
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