before venting the area around the trap to atmospheric pressure.

(b)Wear protective gloves.

(c)Pour the liquid nitrogen into another Dewar, or some safe receptacle. Do not pour the liquid nitrogen down a sink or in a container not designed to hold cryogenic materials.

(d)Allow the trap to warm up.

(e)Gently wipe clean with a rag or Kimwipe. Use tolulene for a final wipe (these traps are made of very thin metal to facilitate heat transfer. They are very vulnerable to dings, bangs, and bumps, so be very careful).

(f)When replacing a trap, be sure that any gasket (if present) is clean and in place. If there are any bolts to tighten, use a rotational tightening pattern as shown in Fig. 7.53.

Alternatively, you can run a dry nitrogen gas purge at a pressure of approximately 200-300(0,* This purge should be left on after the diffusion pump is turned off, and it should remain on while the mechanical pump is left to pump overnight. As the trap warms up, all the contaminants will be discharged out of the system. The next day, the leak detector can be restarted by adding liquid nitrogen to the trap, starting the diffusion pump, and turning off the nitrogen purge.89

Calibration can be done every time you start a system up; but for general leak detection, calibration is less important. However, whenever any general repair or replacement maintenance is performed, such as replacing the filament on a mass spectrometer, calibration becomes mandatory. See your owner's manual for specific instructions.

7.6.13 Repairing Leaks

If a leak has been located with a Tesla coil, you may find a (very) small white dot where the spark went through the glass. This dot may be hard to relocate, so it is best to clearly mark the exact location of the hole with a grease pencil or any other marker that can write on glass. Other leak detection techniques do not leave any mark, and some marking is essential.

The best possible repair for a leak within a glass system* is for the leak to be removed (see Fig. 7.66), the whole area to be heated and blown smooth, and, finally, the worked area to be flame-annealed to prevent later cracking.

A temporary solution to vacuum system leaks that has been used for quite some time is the application of Glyptol. Although this approach may work in some iso-

*See page 356 for information on how to make a dry nitrogen purge.

+ The repair of a leak on a glass system should be done by someone with glassblowing experience.