HP 5973 MSD Hardware Manual page 55

 CI operation
7R VHW XS PHWKDQH UHDJHQW JDV IORZ
Methane pre-tune after more than a day of baking out. Note the low abundance of m/z 19 and
absence of any visible peak at m/z 32. Your MSD will probably show more water, but the abundance
of m/z 19 should still be less than that of m/z 17.
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