Tuning GCMS too frequently

Recently we have been experiencing the baseline dropping below 0 in our chromatograms. To try to remediate this issue, we have been experimenting with changing the target % for m/z 219 and m/z 502 to 50% and 3% respectively. We have also tried 50% and 5% & 55% and 3% & 55% and 5%. There was an increase in EM volts with the m/z 219 at 50% and m/z 502 at 3% but once we tried that combination again, the EM volts decreased back to it's original EMV. Can tuning the GCMS too frequently cause the EM volts to be stagnant? Can running too many tunes affect the sensitivity/EM volts? 

This was also our last resort. We have tried clipping the column on the GC and MS ends, changing liners, cleaning sources... just about everything. Please help!

  • H aleon,

    What is your application? Water can suppress a baseline and take it below threshold, typically only seen with purge and trap systems running larger than a 0.18 id column . Also please let use know the model number of the GC and MSD you are working with.



  • Hi

    Generally, autotune is best option for tuning.

    Variation in EM volts depends on samples.If sample is with moisture/heavies compounds then frequently source contaminated.

    Once source is contaminted then reading of tune will change that time cleaning of source require.

    More frequent maintenance may affect EM volt.

    If you can share latest tuning report then things will be more clear.

  • Do the tune reports look typical as compared to ones from when you liked the data?  A Total Ion Chromatogram with the baseline dropping below zero could be something as simple as a very clean GCMS with a method threshold set higher than the baseline.  Set your threshold to 50, essentially the lowest that makes sense, and see if there's baseline everywhere.

    There are other things that can cause that symptom, though. Call the 800# local to your location/country and talk to the folks on the phone.

    Tuning -- yes, you can tune too frequently!  PFTBA is a big molecule and the tune exposes every surface inside the MS to PFTBA the whole time it is tuning, so adds to the adhered organic layer caused by your sample residue.  This is the reason for cleaning the ion source.  The MS only needs to be tuned --- when it needs it, a horrible answer!   In Tune, use "Generate Report".  This creates a new tune report with freshly acquired data but using the tune values from the last tune. If the tune report looks good, run samples.  If the tune report doesn't look good, tune it.

    The EM does not get stagnant, but does wear out over time.  If the tune EMV is less than 2400-2600 V and has been creeping up to that value over the last few months, it's still fine but going to need to be replaced soon. This is a consumable part so you'll need to buy it. You can certainly ask an Agilent Field Service Engineer to install it the next time that the instrument is vented and they are around.

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