Autotune should give an m/z 502 relative abundance greater than 3%. The relative abundance of m/z 502 can, however, vary a great deal depending on column flow, ion source temperature, and other variables. As long as relative abundance is above 3%, the stability of the relative abundance is more important than the absolute value. If you observe significant changes in the relative abundance of m/z 502 for a fixed set of operating parameters, there may be a problem. The View tunes option in the data acquisition software is useful for identifying changes.
Low relative abundance of m/z 502 should not be confused with low absolute abundances at high masses. Sensitivity at high masses can be excellent even if the relative abundance of m/z 502 is near 3%. If your instrument is producing low absolute abundances at high masses, refer to the symptom High mass sensitivity is poor. If the absolute abundances are acceptable but the relative abundance indicates a problem, check the following:
Tune programs other than autotune have different relative abundance targets. The DFTPP and BFB target tune programs adjust the MSD to achieve about a 0.8% ratio of m/z 502/69. Check which tune program was used. If the relative abundance you observe is not appropriate for the tune program used, refer to the other possible causes.
Contaminants that interfere with the relative abundance of m/z 502 may still be present in the instrument. Wait until the MSD has been on with the heated zones at their normal setpoints at least 2 hours, then try autotune again.
Reduce the ion source temperature and see if the relative abundance of m/z 502 improves.
On MSDs equipped with a diffusion pump, column (carrier gas) flow should not exceed 2.0 mL/minute. On MSDs equipped with a turbomolecular pump, column (carrier gas) flow should not exceed 4.0 mL/minute. Reduce the column flow and retune the MSD.
Switch to
the spare filament and when prompted to autotune click
YES.
If sensitivity improves, use the new filament.
Replace the original filament when convenient.
See the MSD maintenance documentation for instructions for replacing a filament.
Switch the dc polarity on the quadrupole mass filter (quad). Retune the MSD. If performance improves, continue to use the new dc polarity.