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Drying Samples

Freeze-dry sample(s) for at least 12 hours before grinding. If the samples still feel cold when removed from the 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 5. 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 5, configuration A (closed) will hold a vacuum, but configuration B (open) will not.

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Figure 5. (A) Freeze dryer bell valves in closed position. (B) Freeze dryer bell valves in open position.

To freeze-dry samples

1. Cut open the sealed sample bags and fold the top edge over to keep the bag open.
2. Take the top of the bell off of the dryer and arrange samples in the bell, making sure no sample bags are pinched closed.
3. Close the vacuum valves slowly so that you do not cause a large rush of air to blow the samples around. Figure 5A shows the configuration to dry samples and Figure 5B shows the configuration for loading and unloading samples.
4. Set the freeze dryer to resemble the settings shown in Figure 6 and flip the "On" switch located on the right side of the machine.
5. Press the "Auto Refrigeration" button (Figure 6A) and then the vacuum button (Figure 6B). The temperature will start to drop and the vacuum pump will turn on.
   1. When the temperature drops to –40°C, the vacuum is created in the bell and the pressure drops.
   2. Expect the temperature to be between –42° and –52°C and the pressure to be ~0.350 mBar.
   3. The indicator lights (Figure 6D) show how the cooling and pressure reduction are progressing. When all indicator are lights are on the freeze dryer is at its peak performance.
   4. If there is an error the red "Alarm" light will turn on. Press the "Menu" button (Figure 6C) to view it and clear it
6. After samples are dry, slowly open the valves to let out 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.

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Figure 6. Freeze dryer control panel. 

Grinding Solid Samples

There are three primary ways to grind samples:

• With an agate mortar and pestle
• With a Spex shatterbox
• With a Spex mixer mill (tungsten carbide, hardened steel, agate, or alumina ceramic)
• With Retsch MM400 mill (agate)

Mortar and pestle is the most common method. Below, each method is outlined.