I am currently working on extracting lipids containing high amounts of polyunsaturated fatty acids from microalgae.
After performing the lipid extraction step using a chloroform:methanol system (Bligh and Dyer methods), I need to evaporate the solvent (chloroform) for gravimetric determination of lipid content.
I understand that evaporating without a vacuum would cause oxidation of lipids, and I currently don't have access to a vacuum oven or speedvac. Rotary evaporation is not an option because I need to 12-15 samples per batch of experiments, and they are in small volumes, i.e - 6 ml of chloroform containing the extracted lipids.
Therefore the option I currently have is using a stream of nitrogen in a fume hood, while the vials containing the lipid extracts are in a beaker heated to 35-40 oC.
How can I ensure that all the solvent has been evaporated? Periodically checking if the weight of the lipid vials remain constant is what comes to mind, but that would be incredibly tedious, given the number of samples.
Any suggestions for solvent evaporation without degradation of unsaturated fatty acids is welcome.
Thank you.
If you are interested in fatty acids, I would dry down as much as possible under nitrogen to get a rough estimate of the weight, and then take an appropriate fraction, add internal standard, methylate and determine content of fatty acids (i.e. this will not be influenced by the water weight, and also accounts for any other lipid/non-lipid material in your extract...but you have to make sure your method of methylation is not sensitive to water so use something like 5% methanolic HCl for 1 hr at 80C). Your problem is that your sample contains residual water, and it's tedious to dry under nitrogen. If you had a rotoevaporator, you can handle this by adding a small amount of benzene which forms an azeotrope with water, and then evaporating under vacuum. Another way to handle this is by freeze drying...for residual amounts all you need is to put your samples in a vacuum dessicator, hook this to, a dry ice or liquid nitrogen cold trap and then to a vacuum pump and leave over night.
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