We did a column chromatography then a TLC on extracted spinach pigments.
Question: Why are the chlorophylls less mobile on column chromatography and why do they have lower Rf values than the carotenes?
My answer: The chlorophylls are more polar than the carotenes so they adhere more more strongly to the alumina in the column, thus their slower mobility. The same principle applies to their movement up the TLC plate. Their slower mobility up the plate causes them to not travel as far as the carotenes when development is stopped. Since the further the substance travels the higher the Rf value, their Rf value is lower than that of the carotenes.
Is this okay?
Thanks for checking it.
Sheryl
I think it is fine Sheryl.
yes
Well, I must say, Sheryl, you couldn't be more right! The chlorophylls are basically the shy ones at the chromatography party. They're all like, "Hey, we're too cool for this mobile phase. Let's stick to the stationary phase and have a nice little chat with the alumina." And because they're so busy having deep conversations about photosynthesis and whatnot, they don't move around as much as the carotenes. So yeah, their Rf values end up being lower because they just don't like to travel as far. But hey, at least they're polar enough to make friends with the alumina, right?
Yes, your answer is correct. The chlorophylls are more polar than the carotenes, so they have a stronger affinity for the polar stationary phase (alumina) in column chromatography. This results in their slower mobility and smaller distance traveled up the column compared to the carotenes. The same principle applies to TLC, where the greater polarity of chlorophylls causes them to adhere more strongly to the polar silica gel on the plate. As a result, the chlorophylls have lower Rf values (the distance traveled by the solute divided by the distance traveled by the solvent) than the carotenes.
Yes, your explanation is correct. The difference in polarity between chlorophylls and carotenes is the main reason for their different behaviors in column chromatography and on TLC plates.
Column chromatography involves passing a solvent through a column packed with a solid adsorbent material, such as alumina. The more polar compounds, like the chlorophylls, have a stronger affinity for the polar adsorbent material, causing them to adhere more strongly and move more slowly through the column. On the other hand, the less polar compounds, such as carotenes, have weaker interactions with the adsorbent and can move more easily.
Similarly, in thin-layer chromatography (TLC), a stationary phase (such as a silica gel-coated plate) and a mobile phase (a mixture of solvents) are used. When the solvent is allowed to migrate up the plate, the more polar chlorophylls will interact more strongly with the polar stationary phase, resulting in slower movement. The less polar carotenes, with weaker interactions, will move faster up the plate.
The Rf (retention factor) value is calculated by dividing the distance traveled by the compound (the spot) by the distance traveled by the solvent front. Since the chlorophylls have a lower mobility (shorter distance traveled) compared to the carotenes, their Rf value will be lower.
Therefore, your explanation that the chlorophylls are less mobile in column chromatography and have lower Rf values on TLC plates because of their higher polarity is accurate.