Organic Chemistry MCAT Practice Exam

What is the role of the stationary phase in Thin Layer Chromatography (TLC)?

1. It serves to help the solvent move faster.

2. It is non-polar to enhance elution of all substances.

3. It is polar, causing polar compounds to travel slower.

4. It acts as a buffer to maintain pH levels.

The correct answer is: It is polar, causing polar compounds to travel slower.

In Thin Layer Chromatography (TLC), the stationary phase is crucial for the separation of compounds based on their polarity. The stationary phase is typically a polar substance, such as silica gel or alumina, coated onto a glass or plastic plate. This polar nature of the stationary phase causes polar compounds to have stronger interactions with it, which in turn slows down their movement during the development of the TLC plate. Because polar compounds adhere more strongly to the polar stationary phase, they do not travel as far as non-polar compounds, which experience weaker interactions and can thereby move further up the plate with the solvent. This characteristic of the stationary phase is essential for achieving effective separation in TLC, allowing for a clear distinction between compounds based on their respective polarities. In contrast, a non-polar stationary phase would not facilitate this separation effectively, as it would lead to all substances moving similarly regardless of their polarity. Similarly, a buffer role is not essential in TLC; this technique focuses primarily on the interactions between the stationary phase and the compounds, rather than maintaining pH levels. Therefore, the polar nature of the stationary phase is critical in determining the rate at which different compounds travel, making the provided answer correct.