Chromatography Forum

Hello! I am working in the detarmination of lipo-soluble vitamins and I have a problem with the evaporation of the solvent (hexane) after extraction. I am using a flow of nitrogen because I haven't got rotavapor, and the evaporation takes about 5 hours. The volum I have after the extraction is about 100 ml. Can anyone tell me how to reduce this time? I am using heat at about 40-50 ºC to acelerate the process as well.

Thank you

Use solid-phase extraction. The liquid-liquid solvent extraction then blowing off solvent is "so yesterday".

I'm assuming that you have to follow a standard method, and can't just evaporate a 5 -10ml subsample, or use adifferent procedure. I'd be a little wary of SPE, depending which fat-soluble vitamins you are analysing - some are quite unstable without additional anti-oxidants. Several vitamins are light sensitive as well.

I'm assuming you are using a fume cupboard, if not increasing evaporation may produce flammable gas hazard that needs to be removed, perhaps by vacuum.

100 ml at 40-50C should disappear in about an hour, so I suspect that you are wafting the nitrogen across, rather than a few jets pointed directly at, and close to, a warmed liquid surface.

Even though you don't have a rotary evaporator, you can set up a set of fine nozzles ( I've used pasteur pipettes and disposable pipette tips ) and ensure the 100 ml presents a large surface, similar to 250-400 ml beaker. The nitrogen flow should cause gentle waves on the liquid surface. Evaporation can be controlled by the gas flowrate as well as temperature.

If you are warming the glass vessel in a water bath, ensure the upper vessel surfaces are also warm to prevent condensation. Also ensure that the heat source is quickly transferring heat to the vessel - eg a stirred or circulated water bath rather than a beaker of water.

The critical time is when the sample volume reduces, as the sample will quickly increase in temperature to match the heat source, and it's important to allow the sample residue to cool to ambient before removing the nitrogen.

Thank you for your advice. You was assuming well that I am following the standard method but I don´t waft the nitrogen across, I put the flow above the solution that is in a 150 ml beaker. I am working with fish and the residue I obtein after dry is sticky and too much, it´s not only vitamins and it´s very difficult to solve with the hexane:isopropanol. Do you have any idea about what´s the problem?

thank you

A GPC or similar cleanup may be in order to try to separate out the residual lipids that may be in your fish extract.

Fish fatty acids ( EPA, DHA ) are highly unsaturated, consequently they will easily polymerise and form gums. You need to keep the temperature near. ambient, exclude light, and preferably evaporate quickly under high vacuum.

I would strongly recommend finding a rotovapor that you can use, if at all possible. Here in NZ, illicit drug manufacturers regularly steal rotovapors from labs, so perhaps place an order with your local drug lord?. .

Trying to evaporate large volumes of solvents containing polyunsaturated lipids at ambient pressure, even under nitrogen, is very difficult.

The expert on marine lipid analysis and sample preparation is R.G.Ackman, and you should search out some of his publications.

W.W.Christie's books ( Lipid Analysis ) and website literature - now available for free at:-
http://lipidlibrary.aocs.org/

will also give you some useful information about how to extract and concentrate unstable lipids.