In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
A Better Method for Filling Pasteur Pipet Chromatography Columns Ben Ruekberg Chemistry Department, University of Rhode Island, Kingston, RI 02881;
[email protected] The preparation of Pasteur pipet chromatography columns is not easy for undergraduate chemistry novices. The method described for preparing an alumina-packed column by shaking (1) the pipet containing solvent and alumina while sealing the top with one’s (presumably gloved) finger or drawing the solvent up with a dropper bulb (which is too small to fill the pipet in a single squeeze) can both give imperfect results. An alternative method is proposed that allows the column to be filled with solvent without bubbles and allows greater control of fluid flow while the materials to be separated are added. A porous plug (usually glass wool) is put in the bottom of a standard 15-cm Pasteur pipet in the conventional manner. To gauge the proper quantity of stationary phase to add, the pipet is then placed in an empty 13-mm × 100-mm test tube and alumina and sand added to the pipet such that the top of the thin layer of sand atop the alumina is just below the lip of the test tube. The pipet is then clamped, or affixed to a clamp, on a ringstand and the test tube, without the pipet in it, is filled 3兾4 full of the solvent. The test tube is now raised so that the pipet is slowly immersed and the pipet slowly fills with the solvent to just above the sand. The test tube may then be clamped in place, if desired. Without removing the pipet, more solvent may be added to the test tube, if needed, to fill the pipet. Solvent can also be removed with another pipet if the solvent threatens to overflow the test tube. A beaker under the test tube to catch spillage would not be a bad idea. If the plug is too tightly packed and the rise of solvent in the column too slow, the solvent can be drawn up the column using a dropper bulb. The pipet should be inserted into the bulb as little as possible and care should be used in removing the bulb so that the adsorbent is not disturbed. As long as the fluid level in the test tube is higher than the level in the column, there is no danger of the sol-
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vent running out of the column, which would give the same result as the letting the column “run dry”, namely gaps in the column. Loading the column is also made easier. The test tube is lowered until the solvent level in it is at the midpoint of the sand layer and maintained there as the following steps are performed. The material to be separated is introduced and then, when the solvent level returns to the midpoint of the sand layer, washed from the sides of the column with minimal solvent. As long as the level of the solvent in the test tube is midway up the sand layer, there is no danger that the column will “run dry” during this loading phase of the operation. At the same time, there should be minimal dilution of the materials, giving a narrower initial band, facilitating a better separation. The loading and washing can be accomplished at an unhurried pace because the test tube of solvent acts as a valve. To start the column, the pipet is filled with solvent and the test tube immediately removed and replaced with whatever is to be used to receive the eluent. This will allow the solvent to flow through the column. The solvent remaining in the test tube may be used as eluent. Hazards Students should wear gloves and goggles and, depending on the solvent, work in a hood or well-ventilated area as recommended in the procedure being used. Caution should be used handling glass wool, which can cut skin or result in slivers. Literature Cited 1. Svoronos, P.; Sarlo, E. J. Chem. Educ. 1993, 70, 158A.
Vol. 83 No. 8 August 2006
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