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 correctly. 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:
- GC inlet septum
- GC inlet column nut
- Broken or cracked capillary column
Leaks can also occur in many more places in the MSD:
- GC/MSD interface column nut
- Side plate O-ring (all the way around)
- Vent valve O-ring
- Calibration valve(s)
- GC/MSD interface O-ring (where the interface attaches to the analyzer Chamber)
- Front and rear end plate O-rings
- Diffusion pump KF seal
- Diffusion pump baffle adapter O-ring
GC leaks fall into two distinct categories: large leaks that prevent the Instrument from functioning and smaller leaks that allow the system to operate, but negatively impact chromatography.
Large leaks typically prevent a system from reaching a ready state, leading to an Electronic pressure control (EPC) safety shutdown. These types of leaks can result from a column not being installed in the expected inlet, a column not being connected to the expected detector, a broken column, broken or loose fittings, Broken ferrules, cored septa, or tubing blockage, to call out a few possibilities. The cause of these symptoms typically can be rooted out quickly by visual inspection or review of the method settings.
However, Identifying smaller leaks that allow the system to continue to operate can be more involved. Symptoms of smaller leaks can include constant cycling of actual pressure readings (oscillations greater than 0.02 psi), poor retention time reproducibility, higher than typical background, higher than typical bleed (particularly at temperatures greater than 230 °C), baseline drift, higher than usual inlet activity, tailing peaks, the need for more frequent inlet maintenance, and poor area reproducibility.
Symptoms of air leaks include:
- Loud gurgling noise from the foreline pump (very large leak.)
- Inability of the turbo pump to reach 95% speed
- High foreline pressure in diffusion pump MSDs
- Higher than normal high vacuum gauge controller readings.
- Higher than normal analyzer chamber 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)
The 5975 Series MSD will not pump down successfully unless you press on the side plate (analyzer door) when you turn on the MSD power. Continue to press until the sound from the foreline pump becomes quieter.
Pumpdown failure shutdown
The system will shut down both the high vacuum and the foreline pump if the system fails to pump down correctly. Two conditions that trigger shutdown are:
- For diffusion pump MSDs, shutdown occurs if the foreline pressure is above 300 mTorr after 7 minutes.
- Turbo pump MSDs speed below 80% after 7 minutes.
This is usually because of a large air leak: either the side plate has not sealed correctly or the vent valve is still open. This feature helps prevent the foreline Pump from sucking air through the system, which can damage the analyzer and pump. To restart the MSD, find and correct the air leak, then switch the power off and on. Be sure to press on the side plate when turning on the MSD power to ensure a good seal.
Posted by Hakeem Owolabi