5977B Extraction Source with Repeller voltage too low.

Hello, I have a 5977B (with 7890 GC) Extractor EI Source, one day it tuned to a repeller voltage of 0.5 when the previous tunes had been around 4. I've cleaned the source, and retuned and the Etune still gives me the low repeller voltage. The peaks when I run, have lower response. I've switched to an atune and that has resolved my peak issues, but I would still like a solution for the etune. 

  • Low repeller voltage is a good thing!  0.5V is very typical. What is the extractor voltage setting?   Would you please share a full tune report?

  • Here is the tune. With the 0.5 repeller. 

    I think more of the issue is that it dropped from ~5 where it usually is and took some of my sensitivity with it. Here is an older tune before the plunge. 

  • These are nice looking tune reports. The tune abundances are very similar, amazingly similar.  The Electron multiplier is lower on the newer tune, too.  Do you use absolute EM, relative EM, or GAIN in your acquisition method?  Are you running Scan or SIM?

    The older tune shows air - 11.9 N2 : 3.4 O2 is about the ratio in air. The newer tune has higher nitrogen than oxygen. A trap change? A gas tank change to one with nitrogen in the helium?    It would be nice to get the N2 <5% or lower.

    Why is the transferline only 230?  That's lower than most run it.  It's recommended to have it the same as your final oven temperature or at least the same as your ion source.  The 230 ion source temperature is the default, but a bit higher source temperature, like 265, lengthen the time between source cleanings at the loss of a tiny bit of higher mass ratio. Your system at nearly 10% 502 has high mass to spare.  I would increase it to 265 or 275, save the tune, wait an hour for thermal stabilization, then tune again and run it that way.

  • Hi Paul, SIM. I'm using absolute now, it's a really new system, but would happily change, can you recommend a setting?

    I haven't had much luck getting my N2/O2 lower, I've got a P&T attached - running N2, I'm not sure if that's part of the issue. I think the older tune was after a source clean, so it needed to sit a bit longer. I also have a universal He trap installed. 

    For the transfer line, I'm running EPA 8260, my max Oven temp is 210. Would you still recommend going higher on the transfer line/source? 

    There was a sensitivity drop in area responses when the repeller dropped to 0.5. I'm not seeing the same thing when I use an Atune or force the Etune up to the ~5V. Could there be a current issue causing the drop? I was suspecting maybe cracked insulator on the repeller but it looked ok visually so I put it back in (and promptly ordered back up ceramics).  

  • I'm a fan of GAIN as it compensates for changes.  If you're running nice clean drinking water and don't have to tune very often, because it consistently passes BFB, or clean the source very often, then ABSOLUTE is simple but not necessarily the best.   Set the GAIN so that the resulting EMV setting is about the same as the ABSOLUTE value you are using now to start with.   You can always adjust down if your peaks are big. 

    When the system is not running and only carrier gas is flowing - not N2 purge gas injected  - the air should be low.  10% nitrogen is too high.  Is your GasClean cartridge O2 indicator changing color?  Get an electronic leak checker and test all of the plumbing before the GC.

    Also - at least every couple of weeks, either add a line to your sequence or run a separate run with the MS running in SCAN mode, 10-350, threshold ...oh, 150... so that you can get an idea of any contamination, bleed, peaks, etc - all the stuff you are choosing to ignore running in SIM mode.  Those runs may let you see an issue before it becomes a problem.

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