|Learning Objective: Thin Layer Chromatography
(TLC) is a form of chromatography in which components of a liquid
mixture are separated by means of a thin layer of adsorbent material
coated on a glass or plastic sheet. In this procedure you will learn
how to use the equipment involved in TLC.
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Students may wish to wear gloves when working with chemicals.
In this experiment you will be using fluorene, fluorenone, ligroin tert-butylmethyl
ether, and tert-butylmethyl ether. Fluorene is white, which gives a colorless
solution when dissolved in a solvent. Fluorenone is yellow, which gives
a yellow solution when dissolved in a solvent. Fluorene and fluorenone
have very different polarities that we will take advantage of when we
separate the components of a fluorene-fluorenone mixture.
|The TLC chamber
is a jar with a screw-top lid. Put a filter paper liner in the chamber,
flat against the sides. Then transfer the developing solvent ligroin
tert-butylmethyl ether to the chamber. The final depth of the solvent
should be shallow, less than 1 cm. Screw on the lid and shake vigorously
so the liner paper is completely saturated.
|Prepare the solids for analysis
by placing 40 mg of fluorene in one vial and 40 mg of fluorenone in a second
vial. Add a small amount (about 2 mL) of tert-butylmethyl ether solution
to each vial, cap and shake vigorously. Then set aside while preparing the
|Our TLC plates are pre-cut. They
are coated with silica gel and have a fluorescent indicator, which allows
you to visualize a colorless compound under UV light. Do not touch the face
of the plate with your fingers. About 1 cm from the bottom, use a pencil
to mark the plate: two dots for the starting points and an "E"
underneath one, for example, to identify fluorene. Using a separate capillary
for each, spot the plate with the two substances. Spot each three times.
|Place the TLC plate
in the chamber. Make sure the face of the plate is visible and oriented
vertically. Be sure the filter paper does not touch the plate and
that the level of the developing solvent is below the starting spots.
Watch the solvent climb the face of the plate by capillary action.
Remove the TLC plate from the chamber when the solvent approaches
the top of the plate (about 3 or 4 minutes). Don't let the solvent
front go to the top of the plate or you will have to begin again.
Get down and look at eye level as you don't want to shake the TLC
|Remove the plate from
the chamber and mark the position of the solvent front. Dry the
plate and then analyze it under UV light by shining it on the face
of the plate. Circle each spot. Note that fluorene and fluorenone
travel different distances on the face of the plate. Your plate
needs to be checked by your TA to ensure you're performing the technique
properly. Store your plates in a sandwich bag and staple the bag
to your post-lab assignment.
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