The easiest way to find an air leak in your MSD is to use the MSD itself as a detector while spraying potential leak sites with an inert gas. Always start with the seal that was most recently open. This is the most likely point for a leak.
Argon, lecture (0.6 liter) bottle
Pipette (optional)
Screwdriver, TORX T-15 (8710-1622)
Tubing
If your GC is equipped with a split/splitless injection port, turn on the injection port purge during leak checking.
If your MSD is equipped with a gauge controller, turn off the triode gauge and gauge controller before proceeding.
With the MSD still on, remove the upper and lower MSD covers.
WARNING! Do not remove any other covers. Dangerous voltages are present under these covers. The diffusion pump will be dangerously hot. The triode gauge may be dangerously hot.
If the foreline pump is located under the MSD, move it out so you have access to the high vacuum pump.
In Manual Tune>Parameters keep the Cal valve closed.
In the Manual Tune>Acquisition set Acquisition Type to Profile.
Set the masses for all 3 profile ions to 40 m/z.
If you choose to use an inert gas other than argon, enter the main ion of that gas.
Click Start acquiring data.
This starts continual scans of the 3 tune masses. In this case m/z 40 for all 3 masses.
Connect the tubing to the outlet of the cylinder of argon gas.
The tubing makes it easier to direct the gas precisely. Putting a pipette on the end of the tubing further improves your ability to direct the gas precisely.
Immerse the end of the tubing or tip of the pipette in a beaker of water and adjust the gas flow until you achieve a gentle stream of bubbles.
Too much flow will waste argon and flood the outside of the instrument with gas, making it more difficult to precisely locate the leak.
One point at a time, spray argon at the potential leak sites listed below.
Always start with the seal that was most recently open. This is the most likely point for a leak. After spraying each point, observe the peak profile on the data system. If, after 2 to 5 seconds, you see an abrupt and significant increase at m/z 40, there is a leak where you just sprayed. If, after 5 to 20 seconds, you see a gradual increase at m/z 40, there is a leak near, but not at, the point you just sprayed.
GC/MSD interface column nut
GC interface O-ring
Side plate O-ring (all the way around*)
Triode gauge tube fitting
Calibration valve (with the valve closed)
Calibration valve (after changing the PFTBA field to OPEN)
Vent valve O-ring
High vacuum pump where it joins the vacuum manifold
WARNING! Do not touch the high vacuum pump. It is very hot.
After you have located and repaired a leak, test that point again to make sure it is repaired.
If repairing a leak requires shutting down or venting the MSD, check the remaining points before correcting the first leak. There may be more than one leak and it is more efficient to correct them all at once.
Click Stop acquiring data when the testing is completed.
*An extension of the side plate O-ring groove is machined into the lower right hand corner of the vacuum manifold. The end of this extension extends just past the lower right hand corner of the side plate. If the leak check gas is sprayed on this point, it will proceed around the O-ring and check the O-ring for leaks. This is much more convenient than trying to spray all the way around the side plate.