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Handy tips for using vacuum sublimators
- If you are trying to sublime your material for the first time, only use a portion of it -- if your compound is thermally unstable you won't lose the entire batch. Tip: molecules that are spherical or symmetrical have the best chance at sublimation.
- Use an appropriate coolant temperature for your material. If you have to heat it to 200 °C in vacuum, you'll do OK with simple water cooling. However, if your material has volatility at 50 °C, quite a bit will get pumped into your vacuum system if you use room temperature water as your coolant.
- Once the cold finger is coated with material, some of it may fall back to the bottom. To prevent this from happening, place a KimWipe and a fine metal screen between the material and the cold finger. The gaseous material can diffuse past, but any material that falls off will be caught by the screen and can be recovered in pure form.
- When you are ready to collect your sublimed material, release the vacuum by opening the stopcock extremely slowly so you don't dislodge the material from the cold finger or blow impure material onto the cold finger.
- The easiest way to collect your material is to lay out a big piece of aluminum foil. Place a crease in the foil and scrape the sublimated material off the cold finger. You can then easily transfer your product to a tared vial.
- Sublimators with O-ring joints are usually preferable to those with greased joints, particularly for larger sublimators. The greased joints are harder to open and you are likely to touch the joint with your cold finger when assembling or disassembling the apparatus.
- If you are going to pump your sublimator into a drybox, you first need to remove the cooling fluid. This is hard to do because tilting the sublimator enough to empty the fluid could cause your product to be contaminated. A good way to handle this (especially for large sublimators) is:
- Tilt the sublimator to remove as much coolant as you can without risking your product.
- Attach a thin piece of flexible tubing to the end of a syringe needle (Intramedic tubing works quite well). Slide the tubing to the bottom of the cold finger and withdraw the liquid.
- Rinse the cold finger with acetone and then remove the acetone with your syringe.
- Blow air or nitrogen through the cold finger until it is dry.
This page was last updated Sunday, June 12, 2016.
This document and associated figures* are copyright 1996-2016 by Rob Toreki. Send comments, kudos and suggestions to us via email.
*Some of these figures are adapted from the Chemglass, Inc. catalog and have been reproduced with permission.