How to use your rotary evaporator for freeze-drying sample preparation

The rotary evaporator is one of the most used lab equipment in the chemistry and pharmaceutical lab. But are you aware this classical instrument can be used for sample preparation for downstream freeze-drying processes? Read on to see how you can achieve faster lyophilization with better heat transfer if you prepare your samples with rotary evaporation rather than the freezer!

It’s safe to say I’ve been cooking spaghetti for decades. And I have always used a pasta spoon to drain and serve the final product. Until we had guests over the other night and my friend asked me if I had any idea what the hole in the middle of the spoon was for. I looked at him perplexed. I had never wondered what the purpose of the hole was until that very moment. He had a good laugh and explained that the hole is used to measure the amount of spaghetti you need for a single serving of pasta. I was amazed!

Is there anything better than discovering an old kitchen staple can be used for unexpected functions?

I had a similar feeling of joy in the laboratory when I first started working with rotary evaporation. Everyone knows rotary evaporators are used for drying, solvent removal, concentration, distillation. But did you know you can also use them to prepare your samples for freeze drying? Awesome little function that is not lauded enough.

You have two options for how to freeze your sample as a sample preparation step for lyophilization. The standard method is to put the sample into the freezer. Here your final result is a flat surface and the sample is on the bottom of the flask. The benefits of this option is that you can freeze multiple samples simultaneously, so your sample preparation time is very fast.

The second option is to use a rotary evaporator. With this method, you obtain a nice evenly distributed area of frozen sample. The sample preparation here is slower than with option one, as you can only process one sample at a time. However, the downstream freeze-drying process is significantly sped-up when you freeze samples with a rotary evaporator. Because of the larger surface area, you can achieve better heat transfer as well.

If you would like to go with option two for sample preparation for freeze drying, then let me tell you a bit on how to get this set up and working in your lab.

First, and most importantly, you need a Dewar accessory:

Dewar accessory; sample preparation for freeze drying; rotary evaporator

Once you have got this in your hands, proceed as follows:

  • Unplug the power of your heating bath base
  • Attach your adapter by placing it on top of heating bath base
  • Grab your Dewar accessory and attach so that plastic pieces fit well into the holes of the adapter, so that the accessory is well fixed into place.
  • Fill your Dewar accessory mid-level with ethanol
  • Add dry ice and wait until resulting bubbles reduce in intensity
  • Start the rotation at 10 rpm and gently lower your flask into the Dewar accessory
  • Increase the rotation speed to 100 rpm
  • The freezing parts starts and you can observe ice building up on the flask walls

The freezing process for each sample takes about 5 to 10 minutes. A good indication that your freezing process is over that you cannot hear anymore cracking sounds. To make sure the process is complete, make sure there is no liquid left in the flask. You can then proceed to stop the rotation and remove the flask from the Dewar accessory.

To remove the ethanol from the flask, simply wipe the surface with a tissue.

Also, I’d like to quickly point out that for this process, you do not need the vacuum function of your rotary evaporator. For this sample preparation application, you only need the “rotation” of the system.

If you would like to see a demonstration on how to use the rotary evaporator for sample preparation for freeze drying, you can view the following video:

And if you are wondering how to proceed with the freeze-drying process, take advantage of two freeze-drying guides with volume 1 and volume 2 for some theory and basic tips.

Got any nice stories about multi-functions of devices in the lab or in the kitchen or anywhere else? Share in the comments below, I love to discover new possibilities!

Till next time,

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