chemsample-infraredspectroscopy-tlc-intro


 * Introduction**

Thin layer chromatography (TLC) is method used to separate quantities of less than 10 micrograms of material. This can be done by comparing unknown solutions to the behavior of known compounds. TLC can also be used to monitor the progression of certain reactions, especially in larger scale column chromatography.

TLC has two parts, a mobile phase and a stationary phase. The stationary phase is a coated support plate that carries the eluent involved in the mobile phase through it using capillary action. There is also an interaction between the eluent and the adsobent of the stationary phase. The compound/s being observed are applied near the bottom of a dry TLC plate and placed in a developing chamber. THis chamber is covered and contains a small amount of the liquid chosen to facilitate the mobile phase. The mobile phase "carries" the different compounds in a solution to different distances, based on their relative attractions. Compounds that are more attracted to the adsorbent move slowly with the eluent, while compounds that are less attracted to the adsorbent move quickly with the eluent. Typically, this attraction is based on polarty, but forces like hydrogen bonding can also play a role. Generally the more polar a compound is, the more slowly it moves. It is also important to use an appropriately polar eluent, so that the compounds are not too attracted to the polar adsorbent. Once the mobile phase is complete, the TLC plate is now called a chromatogram.

The measurement taken from a chromatogram is the retention factor (Rf). This is a ratio of the distance that a compound move to the furthest the eluent moves.


 * Rf= distance traveled by compound (mm) / distance traveled by eluent front (mm)**

The visualization of the spots being measure happens under a short wave UV light source. The TLC plate is allowed to dry and viewed under the UV light source, which fluoresces the a chemically inert material from the adsorbent. The spots can be circled and the Rf calculated.

Spotting a plate is done with a glass capillary tube so as to keep the diameter between 1-2 mm. This is done to prevent tailing, which makes it harder to determine a Rf value. Trial and error can be used to determine how much spotting is needed to produce a clean visualization for a particular compound or solution.

The TLC plate is made up of alumina (Al2O3) and silica gel (SiO2 * xH20). The alumina is good for less polar compounds. Inversely, the silica is better for polar compounds. Water also has a dampening effect on the effectiveness of the adsorbent and therefore the TLC plates are dehydrate. It is important the correct eluent is chosen based on its eluting power. An eluent with more eluting power is generally more polar and can pull compounds further on the TLC plate. However, too much eluting power can cause the compounds to be pulled with the same force and not made into distinct spots. The best way to determine an eluent is through trial and error.

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