No peaks

Hi, I've been using GC6890N/MSD5975C with no issue but after I recently performed tune evaluation, suddenly the sensitivity decreased severely and there was no peak detected unless very high concentration like 10% loading. Auto tune was fine every time I tested. I do manual injection, so not relevant to ALS, etc.
So, I followed the troubleshooting guide. These are what was done so far;
- replaced septum, liner, o-ring, gold seal, ferrule in inlet.
- replaced ferrule in GC/MS interface
- removed guard columns, cut off ~1 cm of column at each side and re-installed column (same column) with new ferrules.
- cleaned ion-source
- replaced filament 
While doing these, there was a leak issue but it got fixed, and auto tune passed. 
Surprisingly though, I still have the same problem - poor sensitivity or no peak at all with small loading sample. Tune file was correctly saved and assigned. 
Below is the auto tune report (please note the vacuum reading is not valid because the ion gauge is failing), and the tune views (number 24 is when ion source was cleaned).
Your advice would be very much appreciated. thank you

  • You have a small air leak that should be fixed.

    Did your peak's retention time move or just got really small but stayed in the right place?

    Did you try squirting the sample from the syringe onto a KimWipe to make absolutely sure sample comes out?  Plugged syringes happen all the time as do worn out plungers.

    How far is the column installed in the injection port? Into the MS?  How is your guard column connected to the analytical column? What happens if you install the analytical column direct into the inlet?

  • Thank you very much for your answer. I installed the analytical column direct to inlet, and final tune has passed with no leak.
    Proabbly because I trimmed the column a bit (and removed the guard column), the retention time shifted slightly.
    I actually changed the column to see any difference- under vacuum now,..Let me examine the syringe too. 

  • I found that our syringe plunger failed to withdraw sample. By replacing with a new syringe, the problem solved. So the sensitivity issue is resolved. thank you very much for your tip.
    However, we have come across another issue - unusual features in the auto tune result that we have never had before; low ion focus (72 deg instead of 90.2), double humps in repeller. I also see gain factor and EM voltage increased a bit than early September. I switched to Filament 1 (as this is new) in manual tune parameter window, and ran auto tune, but it continued to tune using filament 2 which is old...During the troubleshooting last week, we had replaced the column with a dirtier one accidentally and got auto tune failed. Then we replaced back with the newer column, cleaned the ion source again and the tune came to normal, exept the two issues mentioned above. Can you please advise me regarding the low ion focus and double humps in repeller?

  • Switch to Filament 1 - then save the tune as atune.u ... and then tune again.

    Sometimes it is necessary to replace both filaments with new ones at the same time. The opposite unused filament is not glowing, but the electrons still go there and are reflected back. The electrons bounce back and forth four or more times. 

    Have the repeller insulators been replaced lately?  Are they pristine white beautiful ceramic?  What about the brown lens insulator, replaced sometime in the last couple of years?  How about the PM on the rough pump - how old is the pump itself?  What is the column flow into the MS during tune?   Pumping/vacuum issues can cause strange tuning and results.

    And just for fun - how many runs are on that system?

    Instrument Status

  • Thank you. I finally switched to Filament 1, and a lot of the tune issues went away - not really expected that they could get better with a new filament!
    But, anyway now I've got lower gain factor by double, and no more two humps in repeller. Ion Focus rose a little, but still significantly lower than 90.2...
    It's good know that all sorts of things can affect the tune results regarding repeller. Thank you for your comments. I'll keep it mind.
    Foreline pump has been refurbished early this year, and column flow remained at 1.0 mL/min, 
    One question is can I sonicate the repeller insulator? I sonicated 4min three times in different solvents during cleaning, which actually made it cleaner. Can that cause  unnoticeable breakage or crack in the insulator? The brown lens insulator is pretty expensive... Please let me know if there is a good way to clean this.
    We've been using the system since 2017, and it's possible that it was a second hand instrument. 

  • The repeller insulators are ceramic. They're quite porous so sonicating in solvent doesn't help all that much. It won't hurt them but doesn't clean them enough.   If your lab has a muffle furnace, you could put them in a ceramic crucible and bake them hotter than 500°C...up to 1000° C is fine for them.  This cleans them well.  Slowly heat up and slowly cool down, too or the thermal shock can break them.

    The brown lens insulator is vespel so maybe some careful use of a cotton tipped swab dipped in solvent will help, but the recommendation is to replace it somewhat regularly.  

    It was originally sold in August 2007.

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