3 Troubleshooting the MSD
Air leaks
89
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 cor-
rectly. Symptoms of leaks include:
• Higher than normal vacuum manifold pressure 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. The most likely point for an air leak
is a seal you recently opened.
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 (where the interface attaches to the vacuum manifold)
• Side plate O-ring (all the way around)
• Front and rear end plate O-rings
• Triode gauge tube fitting
• Vent valve O-ring
• Calibration valve
• Diffusion pump KF seal
• Diffusion pump baffle adapter O-ring
• Turbomolecular pump O-ring
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