prevent cross-contamination during periods of nonuse. Any trap design that allows baking should have some barrier that limits (or prevents) human contact with the trap while heating is taking place to prevent burns. Because liquid nitrogen cold traps present special safety and use concerns, the next two sections provide more detailed instruction of their use.

7.4.3 Proper Use of Cold Traps

It is not uncommon for those using vacuum systems to consider trap functionality as limited to catching condensable vapors as they leave the vacuum system on their way to the pumping devices. It all too common for people to forget that besides the trap protecting the pump(s), the traps also protect the vacuum system from materials backstreaming from the pumps. Optimally, this second (but no less important) function of traps increases potential vacuum by limiting backstreaming.

For something so important, it is surprising how neglected the traps on a vacuum systemare, and how poorly they are maintained. Just as one can do significant damage to a car by neglecting the oil filter and motor oil, many a vacuum system is not performing as well as it could due to a few oversights to trap use. Aside from the issue of maintenance, issues for optimum system performance should include trap selection, how the trap is designed, and how the trap is attached to the vacuum system.

For the many vacuum processes that do not generate, or release, any significant quantities of condensable vapors, trap orientation is for the most part irrelevant. However, for the many other vacuum processes that do generate, or release small to significant quantities of condensable vapors, the orientation can have significant ramifications. If the liquid nitrogen trap is attached the one way, there can be a build-up of excess frozen materials in the center tube resulting in a reduction of the trap's throughput. With sufficient amount of vapors, this reduction could eventually lead to a complete cutoff of all gas flow through the trap (see Fig. 7.31). By simply orienting the trap the other way, the buildup of frozen vapors collects on the outer wall, leaving the center tube free for gas transport.

In addition to more efficient throughput, the trap arrangement on the right of Fig. 7.31 allows one to remove the condensate while frozen. This helps prevent the condensate from drifting back into the system after it has defrosted, and if there are any particularly dangerous materials, they can be moved to a fume hood for dealing with in a safe manner.

There is an argument in favor of the design on the left of Fig. 7.31: If there is poor trap maintenance, and the frozen vapors defrost (and collect on the bottom of the trap), there is no chance for these liquids to be sucked into the pumps by someone throwing a switch or stopcock at the wrong time. However, for this amount of liquid to reach the level of the inside tube indicates poor trap maintenance. In addition, this amount of condensate had to have effected gas transport.

394 Vacuum Systems

Improper selection of coolant for a cold trap may artificially limit the potential vacuum of your system. For instance, the vapor pressure of water (which is often the primary condensable vapor in many vacuum systems) is quite high without any cold trapping, moderate at dry-ice temperatures, and negligible at liquid nitrogen temperatures (see Table 7.11). If your vacuum needs are satisfied within a vacuum of 5 x IO"4 torr, you can safely use dry ice (and save money because dry ice is less expensive than liquid nitrogen). Another temperature option for a coolant is the slush bath (for more information on coolants see Sec. 6.2).

Starting a vacuum system should include the following trap use procedures for safe and efficient vacuum system operation:

1. Prevent Air (Oxygen) from Condensing within the Cold Trap. One of the more common laboratory accidents can occur when air (oxygen) is frozen in a cold trap. This accident is caused by placing a cold trap within liquid nitrogen while there is still sufficient oxygen within the trap to be condensed. Later, once the liquid nitrogen is removed (either intentionally or is boiled off) the condensed air vaporizes. The excess pressure created by this frozen air can result in anything from stopcock plugs being blown across the room to an explosion of the entire line. Many labs have "horror stories" of these potentially dangerous, and always costly, occurrences.

To prevent oxygen from freezing in cold traps, be sure that there is no air in the trap when pouring liquid nitrogen around the cold trap. If you suspect that you

Table 7.11 Vapor Pressure of Various Substances

as a Function of Temperature0

Temperature Vapor Pressure, torr

" From VacuumScience and Engineering by CM. Van Atta, © 1965, McGraw-Hill, Inc., p. 331, reproduced with permission.

* Sublimation temperature of dry ice at 760 torr (one atmosphere). c Boiling point of liquid nitrogen at 760 torr (one atmosphere).