Question

1. What are the main differences between thin layer chromatography (TLC) and column chromatography (CC)? 2. Are CC and TLC operations conducted concurrently? Explain. 3. Name a few packing materials used in packing columns in Column Chromatography operations. 4. Explain the role of eluting solvent(s) in column chromatography procedures. 5. When performing column chromatography, which of the following compounds would you expect to elute first: Ferrocene of acetylferrocene? Explain why.

Answer 1

1. Thin layer chromatography (TLC) is often used as an analytical tool to identify if a given mixture is pure or to identify the number of components in the mixture and also to find out the combination of eluent and stationary phase that can be used to separate the compounds in the mixture. Therefore, TLC is mainly used for analytical purposes.

On the other hand, the column chromatography is used to purify a mixture of compounds by isolating a compound of interest by passing it through column where it gets separated based in preferential adsorption and pure compounds are eluted out. This is used for preparative purposes.

In TLC separation occurs by running the mobile phase against gravity whereby the mobile phase is drawn up using capillary action. Here the different analytes ascend through the stationary phase at different rates based on their properties.

In the case of column chromatography, the mobile phase is allowed to run down through the packed column of stationary phase under gravity. Here the different analytes descend through the stationary phase at different rates based on their properties.

The time required for separation is far less in TLC when compared to that of column chromatography and the latter needs more amount of solvent.

In TLC a thin layer of stationary phase is spread along a plate while in column chromatography the stationary phase is packed along a long glass column.

2. Yes, the TLC and CC operations are conducted concurrently since the TLC can be used as an analytical tool to check the quality of separation which is being done through CC. In CC we run down mobile phase continuously though the packed column so as to separate the compounds from the mixture or to purify the compound. The separated compounds are collected at the end of the column at the bottom down with nonpolar compounds eluting first followed by polar ones. To check the purity of the collected solution at the bottom of the glass column in CC, the collected fraction of the solution is placed in TLC where it gives us the information on whether it is a pure compound or the number of compounds contained in the mixture. This process is carried at regular intervals to check the progress of separation process.
3. The most common packing materials (stationary phase) used in column chromatography are silica gel, alumina, metal etc.
4. The process of washing a compound through the column is called elution and the solvent can be called as eluent. A single solvent or a mixture of solvents are used based on the property of the compound to be separated. In CC the analyte which is adsorbed onto the stationary phase is removed by passing eluent through the stationary phase. As the solvent elute through the column it is collected at the bottom and a compositional analysis is done using TLC to check the progress of separation. Typically, a more polar solvent will result in a faster separation of components. The components in the mixture are separated based on their polarity. Generally, as polar components interact more with stationary phase than nonpolar ones, the latter is eluted first followed by more polar ones.
5. Ferrocene will be eluted first followed by acetylferrocene. This is due to the difference in their polarity and ferrocene is less polar than acetylferrocene. As stated above the polar acetylferrocene interacts or binds more to the stationary phase and as a result moves slowly down the column (stationary phase) while less polar ferrocene has less interaction with the column and moves faster down the column and is eluted first.