In a perfect freeze drying world, 100% of the sublimated vapors from samples would be condensed on the freeze dryer’s collector coils. A collector temperature that is at least 20° C below a sample’s eutectic point is ideal for freeze drying and will maximize vapor collection. Sometimes, however, vapors bypass the collector. Here are some real world solutions for dealing with these unwanted vapors.
First consider the effect the non-collected vapors will have on the vacuum pump or room air. Will the quantity harm the vacuum pump or the content present a health hazard if discharged into the room?
If the vapors are small in quantity, non-corrosive and non-hazardous, then they may be allowed to pass to the vacuum pump. Routine pump maintenance becomes critical. After vapors have collected in the pump oil, it should be cleaned with a post-run purge using the gas ballast.
If the vapors are non-corrosive but hazardous, then they may be allowed to pass to the vacuum pump. But, since the vapors may cause harm if inhaled, the vacuum pump must be placed in a fume hood or the vacuum pump discharge must be ducted into a fume hood. If a fume hood is not available, another option is to trap the vapors so they are not released into the room.
If the vapors are corrosive, they could damage the vacuum pump. Trapping the vapors before they reach the pump is necessary.
Trap Inserts placed in a Clear Polypropylene Canister (7815302) can be connected between the collector and vacuum pump to trap vapors before they reach the vacuum pump. There are a variety of trap insert types. Select the trap insert based on your sample’s content (acid or solvent, for example) to maximize trapping ability.
Be aware that traps can add time to your freeze drying process. Due to a chemical trap’s large surface area and high moisture levels, it will take an extended period of time to establish optimal vacuum levels within the freeze dryer. It is not unusual to take a day or longer for a freeze dryer with a chemical trap to reach an optimal vacuum level.
To reduce the time required for vacuum pull down on subsequent usage, clamps or valves can be placed on the tubing on both sides of the trap canister. These valves should be closed before the vacuum is released, keeping the trap and insert under vacuum. On subsequent runs, vacuum should be established before the valves are opened.
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