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I am using DCM ( dichloromethane) in my research, as a solvent. At the end of the day I have to separate the salute and measure it. Up until now, I have been using a constant temperature oven, set a about 65 degree Celsius (evaporation temp of DCM). The exhaust DCM is expelled into the atmosphere and the dried salute is then cooled down and measured.

I want to replace the oven with a rotary evaporator. However I have an argument with my advisor. He claims that I require two things:

  • Aspirator /vacuum pump
  • Draft Chamber

While I agree that a vacuum pump can be of use, I do not require my solvent to boil, therefore I do not need vacuum.

As for the draft chamber, I completely fail to see its implication. I believe that since the rotary evaporator is a completely closed system, there is no way DCM vapor could leak, rendering the draft chamber useless.

Ivan P.
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3 Answers3

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I think you are taking too dogmatic a position overall.

1) Vacuum also increases the rate of evaporation, without boiling. This is just practical.

2) Your claim that your system is "completely closed" is over confident. What if you are wrong and you have a leak? Now you are free to dismiss this, but this is the safety issue your adviser is getting at.

In other words, it would be easy to incorporate your practice with industry best practice.

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I've never seen a rotary evaporator without a vacuum pump. They generally use a water bath and are severely limited in the temperature they can achieve due to that. It is also much easier to adjust the pressure to get the desired evaporation rate than to try that by changing the temperature. And in your case, a rotary evaporator without a vacuum pump would have to be an open system, so DCM could easily get into the atmosphere.

Rotary evaporators are a closed system, and you would generally notice any big leaks because you won't be able to achieve a good vacuum. For common solvents I see no huge advantage in putting them into a fume hood, and they often aren't because the space in the hood is rather precious.

But DCM is potentially carcinogenic, so I can certainly see where your advisor is coming from and I would also prefer to put it into a fume hood. You also handle the open flasks when attaching and detaching your sample and the solvent waste to the rotary evaporator, doing that under a fume hood is a good idea.

Mad Scientist
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  • I've seen rotary evaporators without vacuum pumps. They run just fine on water aspirators. What you do need is a source of reduced pressure. Otherwise you are just doing the most expensive simple distillation possible. Why not just distil it in a fume hood? – Ben Norris May 14 '13 at 10:39
  • Water aspirators are strongly discouraged here because of the environmental aspects. And the rotation does provide an increased surface which would be an advantage of a regular distillation, though if that is worth it in this case is doubtful. – Mad Scientist May 14 '13 at 10:43
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    What about a recirculating water aspirator? The water is contained and so can be disposed of properly. I have one, and it gets down to 25 Torr, plenty for a rotovap. It's a bit bigger and noisier than a pump, but cheaper. –  May 14 '13 at 12:38
  • Some groups use senior rotary evaporators (the safety valve on top of the condenser opened) without a vacuum pump all the day long just to recycle the acetone used for (pre)cleansing glass ware. As long as costs for energy (mainly heating the water bath) and (tap) water (condenser) are not seen as an issue ... – Buttonwood Sep 14 '15 at 21:35
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You can use a rotovap without a vacuum pump but for this to work you need to leave it open, otherwise possible overpressure from your DCM vapors will do this for you by shooting the stop cock out of the joint. Having little bit of underpressure even for solvents like diethyl ether and DCM is the way to do it, so you definitely want to have a pump.

But even with a pump it's not a closed system, you need to hook up the exhaust of the pump with the exhaust air of your lab.

That said, you definitely don't need a hood for the rotovap and if I understand correcty right now you are distributing DCM vapors in your environment which is really bad and will be much better with a correctly set up rotovap system even outside a hood.

DSVA
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