Shift in mass for the tune ions in negative mode

Question

Hello, 
 We have an agilent 6545. Today when we are doing a mass calibration check, the masses of the negative tune ions seems to be shifted (For example 1633.9988 is now showing 1639.2109 and so on for the other ions) and mass calibration check seems to be failing. The masses for positive ions seems to be fine and mass calibration check has passed. Any suggestions on what can be done? 
 
 Thanks 
 
 Harsha

Agilent · Answer

Harsha-the first thing that you should try is a negative ion TOF autotune. Depending on what software revision you have, this will be either a Standard Autotune or a System Tune. Be sure the appropriate checkboxes are selected for TOF & negative ion mode.

Shift in mass for the tune ions in negative mode

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