Air Leaks

Air leaks are a problem for any instrument that requires a vacuum to operate. Leaks are generally caused by vacuum seals that are damaged or not fastened properly. Symptoms of leaks include:

• Higher than normal vacuum manifold or foreline pressure
• Higher than normal background
• Peaks characteristic of air (m/z 18, 28, 32, and 44 or m/z 14 and 16)
• Poor sensitivity
• Low relative abundance of m/z 502 (this varies with the tune program used)

Leaks can occur in either the GC or the MSD. In the GC, most leaks occur in:

• Injection port septum

• Injection port column nut

• Broken or cracked capillary column

Leaks can occur in many more places in the MSD:

• GC/MSD interface column nut

• GC/MSD interface O-ring

• Side plate O-ring (all the way around)

• Front and rear end cap O-rings

• Triode gauge tube fitting

• Calibration valve

• Vent valve O-ring

• Seal between the high vacuum pump and the vacuum manifold

Leaks are much more likely in seals that are frequently opened and closed such as the column nut, side plate O-ring, or vent valve O-ring. Leaks are much less likely in seals that are rarely or never opened such as the front and rear end cap O-rings or the GC/MSD interface O-ring.

The following procedures will help you confirm the presence of an air leak and locate the leak.

• Confirming the presence of a leak
• Finding an air leak in the GC
• Finding an air leak in the MSD
• Isolating a hard-to-find leak