Homework Help: Science - Chromatography

Posted by Veronica on Sunday, October 5, 2008 at 11:06pm.

I performed a TLC of plant pigments in my lab class with a solvent of petroleum ether/ acetone (7:3) and used a silica gel plate.
It is expected that the most polar pigments (chlorophyll) will be located near the bottom and the carotenoids near top.
In my experiment, it was the exact opposite - the green bands (belonging to chlorophyll) were located on the top and the yellow bands (belonging to the carotenoids) were on the bottom.
Can any one explain where I could have gone wrong in my experiment? Is a possibility that there was water in the solvent?

Any help would be greatly appreciated! Thanks!

Science - Chromatography - Dr Russ, Monday, October 6, 2008 at 8:22am
The solvent ratio is about right for separating chlorophyll and plant pigments on a TLC plate. When you say the chlorophyll was at the top do you mean with the solvent front? If this is the case the mixture is too polar. The carotenes (orange to yellow band) should be at the solvent front with a cellulose band behind this (say at an Rf of 0.8). Carotenoids (rather than carotene) however are polar molecules and may be stuck to the base line (Rf=0) as an orange-brown band. I can't give you any reference as this is from experience. I would have used methyl ethyl ketone (MEK)/Pet Ether or cyclohexanone/Pet Ether mixtures but you are probably limited by the lab experiment.

The wikipedia reference below gives you the difference between carotene and carotenoid.
http://en.wikipedia.org/wiki/Carotene

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