8.2: Preparing a Microscale Column
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Preparing a microscale column is very simple. Take a Pasteur pipette and place a small cotton plug at the part where it narrows into a capillary tube. Make sure the cotton plug is tight enough to hold the solid phase, but not too tight because that will result in a much slower rate of solvent flow. Also have a small rubber stopper or Teflon cap liner ready to control the solvent flow in the column.
Separately, make a slurry consisting of the solid support (alumina or silica gel) and the solvent to be used first. Secure the pipette vertically with a clamp, and place an empty beaker at the bottom to collect the solvent that goes through the column. Carefully pour small amounts of the slurry into the pipette with a dropper until the height of slurry that settles inside the pipette is about 1 – 1.5 inches.
During this time the solvent should be allowed to flow freely through the pipette and collected in the beaker at the bottom. However, the solvent level in the column should not be allowed to fall below the top of the slurry. If you see that this is about to happen, interrupt the solvent flow by blocking the bottom of the pipette and add more solvent or slurry, as it might be the case. See pictures on p. 764-765.
In other words, the slurry inside the column should never be allowed to dry out. If this happens it may create cracks and unevenness in the solid phase, which will decrease the efficiency of the separation.