4 Operating in EI Mode
5975/5977 Series MSD Operating Manual 101
The largest influence on operating pressure in EI mode is the carrier gas
(column) flow. Table 14 lists typical pressures for various helium carrier gas
flows. These pressures are approximate and will vary from instrument to
instrument by as much as 30%.
If the pressure is consistently higher than those listed, refer to the online help in
the Data Acquisition software for information on troubleshooting air leaks and
other vacuum problems.
The vacuum can also be read on the 7890B GC control panel or from the Manual
Tune > Parameters view in OpenLAB.
Table 14 Ion vacuum gauge reading
Column flow
rate, mL/min
Optional Gauge
reading, Torr
Performance turbo
pump
Gauge reading, Torr
Diffusion pump
Foreline reading,
mTorr
Diffusion pump
0.5 3.18E–06 2.18E–05 34.7
0.7 4.42E–06 2.59E–05 39.4
1 6.26E–06 3.66E–05 52.86
1.2 7.33E–06 4.46E–05 60.866
2 1.24E–05 7.33E–05 91.784
3 1.86E–05 1.13E–04 125.76
4 2.48E–05
6 3.75E–05
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