Modification of a Virtis K-Series Benchtop Freeze Drier

We started with a K-series benchtop freeze dryer from Virtis Corp. This is a compact unit, with 4 shelves in an enclosed acrylic dome, a manifold of individual flasks at the top of the dome, and the condenser unit within the machine right below the shelves. Although we love how easily and efficiently this unit operates, the GC blank (which we measured by drying 2g of pre-extracted silica gel overnight) was filthy the first few times we used it. It took us several months, but eventually were able to eliminate all traces of volatile organics in the blank. The steps we took are:

1. To prevent pump oil from backstreaming into the chamber, we installed an oil assimilation trap (filled with copper wire) plus a cryogenic trap (visible in the photo above) between the vacuum pump and freeze drier. The cryo trap contains an inner chamber that is filled with dry ice chips + isopropanol (Caution! Don't use ethanol, it cracks the acrylic lid), and lasts ~10hrs between fillings. Both the wire trap and cryo trap came from Metra Inc.

2. The first step for the freeze dryer was a good bath. Silicone-based vacuum grease was applied liberally to all surfaces that need to seal, and we can tell you from long experience that this vacuum grease causes trouble. We removed the grease with acetone, and then washed everything (including the condenser pan, shelves, and acrylic dome) in soapy water.

3. The second step was to remove the rubber valves and gaskets which let you connect individual vacuum flasks to the manifold. The rubber is a soft silicone that degasses into the vacuum chamber. To plug the resulting holes, we had the machine shop make some simple acrylic plugs with viton o-rings. We tried using black rubber stoppers, but they also outgased too much. Viton stoppers would probably work well, but they are very expensive.

4. The third major problem is that there are a lot of butyl rubber gaskets and tubes used throughout the system, and they all contribute to a hump in the blank chromatogram. These all needed to be replaced with equivalent parts made of viton. The parts we replaced included gaskets on the bottom of the vacuum dome and on the cryo trap, a hose connecting the freeze dryer to the vacuum flange, and several small tubes that connect the condenser pan to the pressure gauge and drain line (they are hidden within the body of the freeze dryer). Basically, any soft black rubber that you see connected to the vacuum chamber needs to be replaced. McMaster-Carr is a good source for all of these viton components. We had to make our own gaskets by slitting tubing lengthwise with a razor blade, then gluing it into a circle with super glue. It took a couple tries to get a vacuum-tight joint, but eventually it worked.