HP 5973 MSD Hardware Manual page 71

Always look for small air leaks when setting up methane flow. Run the methane
pre-tune. The abundance of m/z 19 (protonated water) should be less than that of
m/z 17. If the MSD was just pumped down, look for the abundance of m/z 19 to be
decreasing.
There should not be any peak visible at m/z 32 (O
an air leak.
Special negative CI
Since NCI is so extremely sensitive, air leaks that are not detectable in EI or PCI
notes
can cause sensitivity problems in NCI. To check for this kind of air leak in NCI, inject
OFN. The base peak should be at m/z 272. If the abundance of m/z 238 is much
greater than that of m/z 272, you have an air leak.
 Troubleshooting
). This almost always indicates
2
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