Using a rotovap is a crucial step in many processes as it enables you to gently and efficiently remove solvents from samples. In some cases, the rate of evaporation isn’t an issue, such as when you have very little solvent to evaporate or if time simply doesn't matter. However, in most cases, you want the process to be as fast as possible, especially if you’re running the application multiple times in succession or dealing with large volumes.
Thankfully, there are several ways to make your rotary evaporator process faster:
- Increasing the rotation speed
- Increasing the vessel size
- Increasing the bath temperature
- Increasing the vacuum level
While these methods all involve increasing certain parameters, there are caveats in each case. In this post, we discuss each of these strategies in detail and point out their limitations.
1. Increasing the Rotation Speed
An important factor in a rotovap setup is the speed at which the evaporation flask rotates. Rotation of the flask has two main functions. First, it agitates the water bath liquid, allowing for improved heat transfer to the flask and solvent. Additionally, rotation increases the surface area of the liquid inside the flask, increasing the evaporation rate.
While faster is generally better, there’s a point at which a faster speed can actually lower the efficiency of evaporation. It has been shown that, at a certain speed, the turbulence of the liquid is lowered as the sample becomes pressed against the sides of the flask.
Aside from potentially lowering the efficiency, using too fast a speed can cause premature wear on the equipment.
In general, a speed of 250–280 rpm is recommended for maximum turbulence in benchtop-scale rotary evaporators, but factors such as solvent and sample consistency, flask size, and fill level may change this.
2. Increasing the Vessel Size
Using a small vessel for collecting a small amount of sample seems logical as you may think it’s more practical extract a small amount of product from a small flask. However, if you want to create a more efficient rotovap process, bigger is generally better.
The increased surface area of the flask means that more of the flask is in contact with the water in the water bath. This allows for faster and more even heating of the sample.
Similarly, the surface area of the sample exposed to air inside the vessel is greater in a larger flask. This makes evaporation more efficient.
Aside from efficiency, there is also the practical implication of using too small a vessel. Because you tilt the flask during a rotovap setup, there is more chance of spillage if the flask is too full.
Ideally, an evaporation flask should never be more than half full, both to prevent spillage and allow enough liquid surface area for efficient evaporation.
3. Increasing the Bath Temperature
The water bath is responsible for one of the ways in which a rotovap speeds up evaporation. The water bath heats the solvent, increasing the rate of evaporation.
However, it’s important not to be overzealous when increasing the bath temperature. There are several potential concerns with going for too high a temperature:
- Bumping: Bumping occurs when a sample boils too quickly and forms bubbles that splash out of the flask. The bubbles may contain both the solvent and the sample you’re trying to retrieve, leading to a decrease in yield. Bumping can often be avoided by increasing the temperature slowly. However, too strong a vacuum can also be a culprit. If you’re concerned about bumping, a bump trap can be used to collect your sample if it does boil over.
- Unwanted Reactions: Although your solvent may be able to handle an increase in temperature without bumping, your sample may not fare so well. Ensure that whatever temperature you go up to, it will not cause unwanted reactions inside the vessel.
- Too Much Solvent Vapor: Depending on the chiller you’re using, using too high a temperature could increase your evaporation rate to a point that the chiller can’t keep up. In this case, the coolant isn’t able to condense all of the vapor flowing into the condenser. The unevaporated solvent vapor will continue to flow through to the vacuum pump instead of condensing into the condensing flask. The solvent vapor can damage the pump and shorten its lifespan. Potentially toxic vapors may also go through the pump and be released into the atmosphere. To avoid this, use the rule of 20 and ensure that the coolant is always at least 20°C cooler than the vapor temperature. Also, check the cooling capacity of your chiller against the heating capacity of your water bath to ensure it can keep up.
4. Increasing the Vacuum Level
One of the main reasons to use a rotovap is to reduce the boiling temperature of the solvent by using vacuum to lower the pressure within the system. So it makes sense that increasing the vacuum and lowering the pressure further should help. This is true, to a point.
If you increase the vacuum by too much or too quickly, you risk bumping (as described above) or foaming. Foaming occurs when samples contain surfactants and small bubbles (foam) appear on the surface. This can typically be mitigated by monitoring the sample carefully and increasing the vacuum slowly.
Decreasing the pressure in the system can also cause too fast an evaporation rate. If the solvent doesn’t have time to evaporate in the condenser, the uncondensed vapors could find their way to the vacuum pump.
Finally, applying too high a vacuum can increase the risk of implosion of glassware.
