How to take off hydrocarbons ?

[Fee]

Hi,

I am working on lipophilic molecule. Actually, I succeed to isolate my molecule from most other contaminant in the fraction. The only stuff I am not able to take off are the hydrocarbons and some C18 fatty acids.

I've been working on silica gel and alumina. My compound is doing better on alumina (I have a strong bleeding on silica). However, hydrocarbons are passing over alumina and are not retained.

Any suggestions ?

Thank you very much,

Fee

[Mark Tracy]

You compound must be very hydrophobic indeed to co-elute with the hydrocarbon fraction. You should be able to eliminate the fatty acids by using basic alumina instead of acidic or neutral. Also, the amount of adsorbed water will strongly affect selectivity; dryer is more retentive, and perhaps will better retain your compound. Of course, the dryer the alumina, the more carefully you must control residual water in the mobile phase.

[Fee]

Hi Mark,

I used basic alumina for most of my purification steps. This gave the better results. If I understand you, I should dry it, along with my solvents, to get a better retention ?

I tried this morning to separate my fraction between acidic, basic and neutral compounds. I don't know if this test will give suitable results. I will see this later today.

Thank you for your answer,

Fee

[chromatographer1]

You might wish to consider the dual layer carbon column used to separation PCBs and Dioxins from hydrocarbons sold by my company Supelco.

best wishes,

Rod

techservice@sial.com

[Mark Tracy]

You can buy alumina dried to a specific activity (expensive) or dry it yourself and add back a specific amount of water (tedious). You will need to find a book on the subject; sorry I can't think of one just now. For your mobile phases, yes, you should dry them with 4A molecular sieve. Also, some solvents such as THF or ethyl ether sometimes have alcohol in them as a stabilizer; you should check the label because it does make a difference.

If your substance is aromatic, a carbon column could be a good option.

[Fee]

Hi Mark,

Thank you again. I just came to the conclusion, this afternoon, that at least one of my compound is degrading during the purification process. That's why I get a lot of hydrocarbon in my chromatograms. Actually, I will work at low temperature for the next steps, hoping get somewhere with this...

The alumina I am using is this one, from Aldrich :

Aluminium oxide, activated, basic, Brockmann I, standard grade, 150 mesh

I have receiveid it in may 2006. I hope that summer temperature, which is very wet around here, have not corrupted it...

So, should I dry it ?
Thank you very much,

Fee

[Mark Tracy]

That is a very dry grade of alumina, and should always be stored in a sealed container, preferably in a dessiccator. You might need to dry it.

I forgot to mention something important about dry basic alumina: it is a good basic catalyst for hydrolytic/solvolytic reactions. If your compound is susceptible, you may need a new approach.