How to prevent losing solvent into the vacuum pump in rotovap solvent collection? This is a practical question about the use of rotary evaporators ("rotovap"). So I'm asking for answers from people who actually use the things.

A rotovap typically uses a cold condenser to condense the solvent into a collection flask. But the distillation is typically done under weak vacuum as well. So the solvent sitting in the collection flask seem susceptible to re-vaporizing and getting sucked into the vacuum pump. Now I have a cold trap in front of my pump that should be cold enough (-50C) to trap most solvents under weak vacuum.

I think the answer is that if the vacuum is weak enough to vaporize the solvent under the hot water bath, but not under the cooler ambient air temperature, then the loss to the pump should be minimal. Indeed, this is what the instructions for the rotovap suggest.


  Question: In practice, what are the risks to losing solvent into the pump?
Honestly, I never bothered to actually measure the volume of the condensed solvent in order to compare it with the original volume.

The concept described in the question, that is using a cooling trap with dry ice + acetone, is good practise and typically good enough to retain the evaporated solvent and protect the membrane pump. In rare cases, I have used a second, flat Dewar filled with dry ice + acetone on a "Laborboy" to cool the condensate-collecting flask.