[yunie]

I have to separate two nonpolar compounds that are fairly close in polarity using column chromatography. I can't do reverse phase chromatography so the only thing I can really play around with is the solvent system. Should I try using a purely nonpolar solvent and then start using more polar mixtures or should I start with more polar mixtures and move to more nonpolar? I've already tried moving from an 80:20 Hexanes-Ethyl Acetate to 20:80 Ethyl Acetate:Hexanes, but that didn't separate them. Any help would be much appreciated!

[vex]

You can try using TLC to test your systems before you run the big column. I would imagine that you won't be able to separate them with a very nonpolar solvent system, and that some intermediate mixture will be your best bet. You may also need a very long column if they're very similar.

Investigate different solvent systems using TLC, and if you are having trouble finding one that works, look into alternate purification methods, like recrystallization or sublimation.

[Ligte]

You have to remember that the resolution or separation efficiency of your column is determined by your particle size. In your case I assume you are working at atmospheric pressure which means your gel has a large particle size otherwise there would be no flow through your column. In simple terms you will have great difficulty separating compounds with similar polarity the lower the resolution of your column and especially when you don’t use gradient elution.
What you can do
1.   Make the injection volume as low as possible and use smaller concentration.
2.   The polarity of the solvent or mixture of solvents uses to dissolve you compounds relative to the polarity of your mobile phase plays a major role in separation.
3.   When using TLC plates what you can try is to use for example the (8:2) Hexane-Ethyl Acetate solution to run the first separation then after you dried your plate, turn that same TLC plate on its side and then do the run again with a (2: Hexane-Ethyl Acetate solution otherwise just use gradient elution.
Using the same low resolution packing material and increasing its column length with the same solvent system is time consuming, expensive and bad science.

[OC pro]

For very nonpolar compounds, ethyl acetate is often too polar. Means it will push everything through silica gel. Try to use diethyl ether or tert butyl methyl ether in combination with hexane. Start with a 1:1 mixture. I accomplished even separation using petrol ether/diethyl ether 100:1 mixtures.

[Babcock_Hall]

I have separated carotenoids (very nonpolar ones, such as spirilloxanthin) on silica with 15% acetone in hexanes.  When I separated them on alumina, I recall using even less acetone (perhaps less than 10%), but my memory is imperfect.  Are you using TLC plates to try different solvent systems first, before running a column?

[Dan]

I would experiment with other solvent combinations. Hex/EA mixtures are not always the best.

Some other potentially useful threads:

http://www.chemicalforums.com/index.php?topic=56888
http://www.chemicalforums.com/index.php?topic=10164
http://www.chemicalforums.com/index.php?topic=56048
http://www.chemicalforums.com/index.php?topic=60085