Small shape changes at the top of the mass peaks can cause 0.1 m/z shifts in mass assignments. Shifts greater than 0.2 m/z indicate a possible malfunction. Check the following:
Changes in MSD temperature can cause small shifts in mass assignments. If the tune file was acquired before the MSD was at thermal equilibrium, the difference in temperatures may be the cause of shifts in mass assignments. Make sure the MSD is at thermal equilibrium before tuning the MSD or acquiring data. It takes at least 2 hours after all of the heated zones have reached their setpoint for the MSD to reach thermal equilibrium.
Changes in MSD temperature can cause small shifts in mass assignments. A large shift in the temperature in the laboratory is sometimes enough to change the MSD temperature and affect mass assignments.
Changes in the MSD can cause small shifts in mass assignments. If the tune file is very old, conditions in the MSD may have changed enough to cause significant changes in mass assignments. Retune the MSD and use the new tune file.
If the conditions in the tune file do not match the conditions under which data was acquired, there may be shifts in the mass assignments. Make sure the current tune file matches the conditions being used for analyses (make sure the conditions have not been overridden).