Question

In TLC, the polarity of the different compounds to be analyzed is highly critical. Which one of the following statements is true? (Check all that apply)

1 The less polar components of the mixture to be analyzed will be less attracted to the stationary phase and therefore move further up the plate.

2. Any solvent mixture can be used to separate any compound mixture with TLC.

3. The more polar components of the unknown mixture move faster up the plate than the less polar components.

4. The addition of hexanes increases the polarity of the eluent ethyl acetate.

Answer 1

First to all, TLC is a chromatography technique used to separate non-volatile mixtures. After the sample has been applied on the plate, a solvent or solvent mixture (known as the mobile phase) is drawn up the plate via capillary action. Because different analytes ascend the TLC plate at different rates, separation is achieved.

Proper solvent selection is perhaps the most important aspect of TLC, and determining the best solvent may require a degree of trial and error. As with plate selection, keep in mind the chemical properties of the analytes. A common starting solvent is 1:1 hexane:ethyl acetate. Varying the ratio can have a pronounced effect of RfRf. RfRf values range from 0 to 1 with 0 indicating that the solvent polarity is very low and 1 indicating that the solvent polarity is very high. When performing your experiment, you do not want your values to be 0 or 1 because your components that you are separating have different polarities. If the value is 0, you need to increase your solvent polarity because the sample is not moving and sticking to the stationary phase. If the value is 1, you need to decrease your solvent polarity because the compound was not able to separate.

If you know that one component of a mixture is insoluble in a given solvent, but another component is freely soluble in it, it often gives good separations. How fast the compounds travel up the plate depends on two things:

• If the compound is soluble in the solvent, it will travel further up the TLC plate
• How well the compound likes the stationary phase. If the compound likes the stationary phase, it will stick to it, which will cause it to not move very far on the chromatogram.

You should be able to determine which by looking at the RfRf value.

Acids, bases, and strongly polar compounds often produce streaks rather than spots in neutral solvents. Streaks make it difficult to calculate an \(R_f\) and may occlude other spots. Adding a few percent of acetic or formic acid to the solvent can correct streaking with acids. Similarly for bases, adding a few percent triethylamine can improve results. For polar compounds adding a few percent methanol can also improve results.

The volatility of solvents should also be considered when chemical stains are to be used. Any solvent left on the plate may react with the stain and conceal spots. Many solvents can be removed by allowing them to sit on the bench for a few minutes, but very nonvolatile solvents may require time in a vacuum chamber.   

Volatile solvents should only be used once. If the mobile phase is used repeatedly, results will not be consistent or reproducible.

So basing on this, we can say that option 3 is true.

Hope this helps