We're thinking of adding some surfactant to the sample to see if this helps. Any ideas guys and gals? Hi James It looks as if you have something wrong with your rotary evaporator - it should not take any more than a few minutes not hours to evaporate 1 ml of methanol.
Check that you actually have a vacuum - vacuum pump sucking, no ports accidentally open, no taps open. Have your water bath at 40 - 50 degrees C to speed things up a bit. For small volumes, evaporation under a stream of nitrogen is often handier than a rotary evaporator, becuase it can be done in a test tube rather than a round bottom flask, and with an evaporation manifold you can do several samples at once.
Peter Apps. I agree with Peter that evaporation under nitrogen is easier to handle than rotary evaporation. To my experience, evaporating 3mL of methanol under a gentle stream of nitrogen at degree celcius takes about 45 minutes.
Rotary evaporation is always faster than nitrogen purge. Thus evaporating 1mL of methanol should be less than 15 minutes. I guess the plasma extract that you are handling probably contains mixture of water and methanol. It is always difficult to evaporate last trace of water and people usually do is to add small amount of ethanol into it to facilitate the removal of water 'cos water-ethanol forms azeotrope.
Notwithstanding this, you should also check your rotary evaporator for vacuum and water bath as these two are the controlling factors for evaporation. Hope this helps. Thanks for the tips, I like the idea of trying ethanol, might give that a spin pun intended yes, our evaporator is rather slow. That is something we're looking into for the future. As there is going to be water traces in our plasma sample, we think this effect is happening here too. Anyone know why this might be? Contact me if you want some suggestions on how to improve your sample preparation method.
Last edited by Uwe Neue on Fri May 26, am, edited 1 time in total. I'd guess that you probably do have an oil film given your sample evaporates slower than water, but the times you suggest for all solvents as so slow that your Rovap or protocol needs some loving attention.
As others have noted, the vacuum should be checked, as should temperature. In my experience, the main cause of slow evaporations is because the flask, vial, whatever, is too small. You need a large surface area combined with a thin film to remove solvent quickly on any rovap. The best solution, once you have fixed any vacuum problems, is to quickly apply the highest vacuum possible avoiding foaming with the fastest speed thinnest film then slowly introduce heat.
However, there are almost as many "best" ways of using rovaps as there are rovaps. If you do have an oil layer or solids that hold water, evaporation under nitrogen may be even slower, and the best solution for samples with such problems would be to revisit the prep process, rather than introduce other solvents. Please keep having fun, Bruce Hamilton. One would presume that this is another case of requiring extraordinary skill to have an oil film on a rotating MeOH mix.
Also, it sounds like your roti puts on pressure rather than a vacuum at the relevant site. Plasma extracts have all kinds of small molecules salts, etc. Quite often we have seen interfering gas peaks when evaporating partially samples with N2, thus we usually use test tubes with ground glass joints on the rotaries when small volumes are involved.
Hi James Now I'm puzzled - you have a rotary evaporator that "freezes water quite happily"!? That sounds more like a freeze dryer, in which case you "drying" aka sublimation times are not entirely unreasonable. What does this machine of yours look like?? The best way to illustrate this relationship is to break down the petroleum-distillate family, the one you are surely most familiar with.
See Chart. The smallest molecule in this family is methane, which is a gas at room temperature. Following methane in evaporation rate are ethane, propane, butane, etc. Toluene toluol and xylene xylol make up the strong and smelly parts of naphtha and mineral spirits. Refineries remove the toluene and xylene to be sold separately and sell what is left as odorless mineral spirits. This less smelly mineral spirits has less solvent strength than regular mineral spirits, but it is usually strong enough to thin oils and varnishes and dissolve wax.
Toluene and xylene are very fast-evaporating and dry non-oily. The next common petroleum distillate is kerosene, which is so oily that it barely evaporates at all at room temperature. Paraffin wax is a solid at room temperature. It liquidifies at around degrees. Notice that petroleum distillates range all the way from a gas to a solid at room temperature.
Alcohols, ketones, esters, and glycol ethers work the same as petroleum distillates, with the smaller molecules being faster-evaporating and drier, and the larger molecules being slower-evaporating and oilier. Some of the faster evaporating alcohols, ketones and glycol ethers are sold to consumers.
No esters are commonly available. Within the alcohol family, methanol wood alcohol evaporates faster than ethanol grain alcohol. Ethanol is the alcohol in beer, wine and liquor, and it is highly taxed. So a poisonous substance, often methanol, is added to make it undrinkable and avoid the tax.
Within the ketone family, acetone evaporates faster than methyl ethyl ketone MEK. All other ketones evaporate slower. The molecules in the glycol-ether family are large and evaporate very slowly. This makes them good solvents for water-based finishes because they evaporate slower than water.
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