66
3 CI Troubleshooting
Air Leaks
How do I know if I have an air leak?
Large air leaks can be detected by vacuum symptoms: loud gurgling noise
from the foreline pump, inability of the turbo pump to reach 95% speed, or, in
the case of smaller leaks, high pressure readings on the high vacuum gauge
controller.
The mass flow controller is calibrated for methane, and the high vacuum gauge
controller is calibrated for nitrogen, so measurements are not accurate in
absolute terms.
Familiarize yourself with the measurements on your system under operating
conditions. Watch for changes that may indicate a vacuum or gas flow
problem.
Always look for small air leaks when setting up methane flow. Run the
methane pretune, starting with a good PCI tune file (Figure 3 on page 67). The
abundance of m/z 19 (protonated water) should be less than 50% of m/z 17 for
acceptable PCI performance. For NCI, the abundance of m/z 19 (protonated
water) should be less than 25% that of m/z 17. If the MSD was just pumped
down, look for the abundance of m/z 19 to be decreasing.
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