Waters GCT Premier Operator's Manual page 369

mass within approximately 15 mDa of the lock mass, it can interfere with the
lock mass and thus shift all the peaks up or down the mass scale. This will
produce mass errors in all the peaks in the spectrum where the lock mass
peak is interfered with.
To verify this, the user should make a RIC (Reconstructed ion chromatogram)
of the lock mass peak. If the lock mass peak intensity apparently increases
when a GC peak elutes, then it has probably been interfered with, and you
should conclude that the accurate mass measurements are invalid for the
interfered with in the chromatogram. To avoid this problem you must repeat
the acquisition, perhaps using a different compound for the lock mass.
Gain drop monitoring G-15