TLC plate in regards to polarity

In summary, the speaker used a normal phase alumina TLC plate with 20% (v/v) dichloromethane/hexanes to separate pigments extracted from tomato paste. They found that the substance that traveled the furthest on the plate was xanthophyll, despite it not being very polar. This contradicted their initial belief that the most polar substance would move the least on a polar plate with a nonpolar solution. However, after conducting further experiments and examining the pigment colors, they determined that beta carotene was actually the substance that moved the furthest. They also clarified the setup of their TLC plate and the colors of the pigments.
  • #1
CML
4
0
I used a "normal" phase alumina TLC plate with 20% (v/v) dichloromethane/hexanes. I found the substance that traveled the furthest to be the most polar one (though the substance is not very polar, xanthohyll). Why would this be? I was under the impression that since the plate is polar and a nonpolar solution is used, the most polar substance would move the least. (Of course it is possible I made a mistake and the spot that moved the furthest is not the most polar one, and may have been beta carotenes, but I am almost positive I am right.)
 
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  • #2
What are you trying to separate and how do you know that the one that moved the furthest on the plate is xanthohyll?
 
  • #3
gravenewworld said:
What are you trying to separate and how do you know that the one that moved the furthest on the plate is xanthohyll?

I did two spots, one was crude tomato paste and another was lycopene extracted from tomato paste. The TLC looked something like:



X


L
C

| |
Crude Extract

(Edit: I cannot get the TLC to look right, when I actually post. The second "|" should be above "Extract", and the "L" should also be above "Extract". All relative positions are correct)
Where I think:
X = xanthophyll more of a yellow colour
C = beta carotene more of a orange colour
L = lycopene also more of a orange colour

Due to the colour of the dots, I deduced that X is likely xanthopyll has it has a yellow pigment (similarly for the other two dots too)
 
Last edited:
  • #4
This is too difficult to tell what is going on without the lab procedure and a picture of your TLC plate.
 
  • #5
I will do my best to explain the procedure in as little detail as possible.
Pigments (lycopene, xanthophyll and beta caroetene) were extracted from tomato paste via a multistep liquid-liquid extraction (including a sep funnel) and a microscale chromatography. The microscale chromatography separted these pigments (all that was collected was the lycopene fractionsm the other two pigments were discarded). This fraction (along with the solution it was eluted with (10% ethyl acetate in hexanes)) was boiled in order to evaportate the 10% ethyl acetate in hexanes and therefore concentrate the lycopene.

This extract was then spotted on a normal plase alumina TLC plate along with a pre-prepared crude sample (I assume prepared in a similar manner; however, I do not know as I did not do prepare this).


As for the TLC plate, I cannot upload a picture of it, but I can try to explain it in more detail.

(solvent front)

X



C


Crude

(solvent front)




X



Extract

Where extract and Crude are actually on same TLC plate side by side.
X = xanthophyll more of a yellow colour- moved 2.92 cm
C = beta carotene more of a orange colour - moved 1.71 cm
L = lycopene also more of a orange colour - moved 1.90 cm
Solven front moved 3.40 cm
In terms of pigment colours, xanthophyll has a yellow colour, lycopene has a red/orange colour and beta carotene has a yellow orange colour

I do not see colours all that well, so of course it may be possible that the colours are wrong, but to me that is how they looked.


However, is my original idea correct, that the least polar substance should travel the furthest?
 
  • #6
Thanks for your help, but I figured the question out. Turns out that xanthophyll was NOT what traveled the furthest, and it was infact catoene.
 

Related to TLC plate in regards to polarity

1. What is the purpose of using a TLC plate for polarity?

The purpose of using a TLC (thin-layer chromatography) plate is to separate and identify different components of a mixture based on their polarity. This is achieved by using a stationary phase (the TLC plate) and a mobile phase (a solvent) to move the components at different speeds based on their polarity.

2. How does the polarity of a compound affect its movement on a TLC plate?

The polarity of a compound affects its movement on a TLC plate because polar compounds will interact more strongly with the polar stationary phase, causing them to move more slowly. Nonpolar compounds, on the other hand, will interact less with the polar stationary phase and therefore move faster.

3. How can I determine the polarity of a compound using a TLC plate?

To determine the polarity of a compound using a TLC plate, you can compare its movement on the plate with the movement of known compounds with different polarities. If the compound moves at a similar rate to a polar compound, it is likely polar. If it moves at a similar rate to a nonpolar compound, it is likely nonpolar.

4. Can I change the polarity of a TLC plate?

No, the polarity of a TLC plate cannot be changed. The polarity is determined by the type of stationary phase used. For example, a silica gel TLC plate will have a polar stationary phase, while a C18 TLC plate will have a nonpolar stationary phase.

5. What happens if my compound has a similar polarity to the TLC plate?

If your compound has a similar polarity to the TLC plate, it may not move at all or may only move slightly. In this case, it may be difficult to separate and identify your compound using TLC. You may need to use a different chromatography technique or adjust the polarity of the TLC plate by using a different type of stationary phase.

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