etune extractor lens voltages positive and negative. What should they be?

I've been having trouble with one of our GC/MS 5977B tuning lately due to the peak widths being outside the +/- 0.05 range.  Looking more closely at the tunes I noticed that the repeller voltage for this particular instrument was all over the place, anywhere from 0.5 - 4.98.  Two of our other instruments were PM'd at the same time and their repeller voltages were holding steady each day at 0.5.

After switching filaments and the quad voltages (per Agilent tech suggestion), the instrument did reach tune specs that day. 

However the following day, the peak widths were again outside the +/-0.05 range and the repeller voltage changed from  4.49 to 0.5 between 2 back-to-back tunes.  I talked with tech support and they explained that the repeller should always have a positive voltage and the extractor lens should always have a negative voltage because one is pushing ions and the other is pulling them.   He suggested that the extractor lens wire was touching the source body causing the voltage for the extractor lens to fluctuate which causes the repeller voltage to increase due to the push/pull relationship of the two parts. This made perfect sense.   I opened up the source and sure enough the extractor lens wire was touching the source body.  I moved it down so it was no longer touching and re-tuned.  The peak widths were out again and the extractor lens voltage was still positive.

The following day, when tuning all of the other instruments I paid special attention to the repelller and extraction lens voltages on all of our instruments.  What I found was that the repeller voltages stayed around 0.5 for all the instruments, but the extractor lens voltages varied as below.

Instrument #1 - extractor lens always positive

Instrument #2 - extractor lens 50/50 positive and negative

Instrument #5 - extractor lens mostly positive with a couple of negatives

Instrument # 6 - extractor lens all negative

Based on the push/pull relationship of the two parts, which makes sense, I am now very confused. 

  • The etune tune ion peak width target for 69-219-502, Pw50, is 0.6 amu.  If the tune ion peak abundances - 69 between 400-600k, 219 >35% of 69, 502 >2.5% of 69, and the tune ion peak shapes are good - symmetrical, gaussian mostly, not splitting/fronting/tailing, and the isotope peaks at 70-220-503 are nearly baseline separated, that means the quadrupole is working.  The ions themselves are always exact mass - 68.9947, 218.9851, and 501.9706.  The tune ion peak shape is a distribution curve based on the limitations of a quadrupole mass spectrometer.

    One quad related parameter that can affect the tune ion peak shape, tune ion ratios somewhat, and tune ion abundances, is the quadrupole polarity.  One opposing pair of the 'rods' has a positive DC offset applied and one has a negative (or less positive) offset applied.  This Positive or Negative polarity is a setting that is related to your instrument's specific hardware.   A huge preponderance of the time the systems ship with Positive polarity selected but sometimes, over time and use, a Negative polarity setting (or the opposite of what was chosen at shipment) may give slightly better tune ion peak shape, or slightly bettter/different tune ion relative response, or slightly higher ion throughput as seen by a lower EMV - or some combination of all three.  Check out the Quad Polarity document in the Files section of this forum.

    The ion source body/ionization chamber, repeller and extractor lens are working together differently inside an Extractor Ion Source than they do in the Stainless-Steel non-extractor source or the Extractor Ion Source run in Atune instead of Etune. The voltages interact to push/pull the positive ions out of the ionization chamber, but it's not as simple as the repeller pushes and the extractor pulls, since the ion body voltage pushes the ions towards the center of the ionization chamber as well. There is a lot of interaction which depends on cleanliness, insulators, column flow, gas choice, column positioning, and more.  

    That means that there are more things to check. Remove the source and take it apart. Clean the required metal pieces thoroughly, every surface, edge, window, nook, and cranny, inside and outside following the information in the manuals, the guide in the Files section on this forum, and the videos available here, registration required-- Eliminate the Fear of Mass Spec   . Thorough is much more important than how hard you scrub!   I would suggest replacing both repeller insulators, the extractor insulator, the lens stack insulator, and the brown extractor lens wire.  Wipe all surfaces of the repeller heater/sensor block with a solvent wetted lint-free cloth while the repeller is being cleaned, too.   Make sure that the pins on the lenses are cleaned as well, being careful not to bend them.  Verify that the green wire from the ceramic source connector board is connected to the pin at the base of the source radiator - that's the source body connection. I would unplug and plug it back on. Then put it all back together, pump it down, waiting long enough for good vacuum and thermal stability, about two hours or more, and then test it.

    and let us know if that helped.

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