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.