Nitrogen Blowdown vs Rotary Evaporation

Nitrogen Blowdown vs Rotary Evaporation
Laboratories often need to use evaporation techniques to further concentrate their test samples prior to analysis. Concentration occurs when excess solvent is converted into a vapor, leaving behind either a solid or liquid solute. There are countless evaporation methods available, so how do you know which one is best for your application?
This video will explain both nitrogen blowdown and rotary evaporation - two widely used evaporation methods. We will explain how each method accelerates the concentration process, and when each is recommended.
Let’s begin with nitrogen blowdown evaporation.
If we look at a sample under ambient conditions with no external forces, we’ll notice that the solvent molecules move around very slowly. Because of this, only some are able to be vaporized and separated from the sample. The high pressure right above the surface of the sample also causes many of these molecules to return back to the sample. These factors lead to a very slow evaporation rate.
The nitrogen blowdown method decreases the vapor pressure above the sample by applying a steady stream of nitrogen gas on the solvent’s surface.
The nitrogen gas can also cool the sample slightly, which decreases the energy of the molecules, slowing down their movement and ability to vaporize.
To counteract this, a form of heat is added to the sample which causes the molecules to re-energize and speed back up. The heat can be in the form of a water bath, or a dry block.
The sample is now in an optimal environment for concentration. The combination of heat and nitrogen blowdown allows the molecules to quickly vaporize and be whisked away from the sample.
Now, let’s compare that to rotary evaporation.
We’ll start with a large liquid sample in a round flask which will be hooked up to a rotary evaporator unit.
This flask will sit in a heated water bath causing the solvent molecules to speed up, and vaporize at an accelerated rate.
The flask will also rotate while in the water bath, increasing the surface area of the solvent, and allowing more solvent molecules to escape.
The vaporized solvent molecules will travel up towards a chilled condenser, where they’ll begin to condense and drip down into a receiving flask. This is where all the excess solvent will collect, leaving behind a concentrated sample in the rotating flask.
Some rotary evaporators also include a vacuum pump which can be connected to the condenser. This vacuum pump is meant to decrease the pressure inside the rotating flask, causing the solvent to evaporate at a lower-than-normal temperature. The combination of heat, rotation, and decreased pressure creates an ideal environment for efficient evaporation.
So when are each of these methods recommended for use?
Nitrogen blowdown is ideal when evaporating a high number of samples. Some nitrogen evaporators can accommodate batches of up to 100 samples.
This method is also meant for small volume samples. Samples larger than 100 mL will have a prolonged evaporation time.
The precision of nitrogen blowdown also makes it a great option for those who are looking to evaporate to complete dryness, or would like to stop it at a specific endpoint.
Rotary evaporation is ideal when working with a very small number of samples. Since these units can only handle one sample at a time, they would not be an ideal choice for evaporating large batches.
This method is also meant to handle large volume samples. Most rotary evaporators can handle sample volumes up to 2 L.
Lastly, rotary evaporation is great for evaporating solvents with high boiling points. The use of a vacuum pump for reduced pressure lowers the boiling point of the solvent, allowing them to be evaporated with minimal heat.
Each concentration method is designed for different applications, so it’s important to know which one will best meet your needs. If you have any questions on either of these methods, or need help identifying which is right for you, feel free to contact us at [e-mail].
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