This is a continuation of the Delta source instability saga that Jennifer McKay has been dealing with for a bit too long now (she’s trying hard keep from banging her head against her office wall at the moment….)
Needed to run samples on our EA/Delta in bad way so decided to swap sources with our Delta-Plus used for O18 water work (dual inlet). Source from the O18 MS is working fine w/ the EA spec.
Problematic Source now in the Delta-Plus still having issues. It has been cleaned again, quartz replaced, many other bits as well. Same filament which is less than 1 month old.
Symptoms while troubleshooting problem child source w/ the dual inlet are as follows (Vacuum and spec electronics are OK):
CO2 voltage signal response appears REALLY slowly at first (20 sec! or so before it starts to slowly climb) once the changeover valve is opened (first thing in the morning).
Signal then reaches a fairly normal sensitivity after about 1 minute but then drops pretty quickly to a much lower level (say 30-50%). Tuning looks fairly normal – Except the Extraction Symmetry is mostly maxed at about 10 (uh-oh).
Zero enrichments are acceptable, no isotopic drift, good analysis precision (started after this signal drop and is somewhat stabilized). Haven’t yet tried different CO2 source gases – which will probably tell a different story.
When I close off the changeover for 10-20 seconds or so and switch back to source, the signal jumps back closer to normal, but then decays back in about 30 secs. If I close the CO for longer and reopen, the signal jumps to a higher V and then decays faster. Happens to both bellows.
We have noticed that one of the source flange guide pins has shifted down into the source gas entrance hole (maybe by about 1/3) – which I will try to remedy shortly but doubt this is the issue.
Anyone have any ideas? Gas flow problem? Voltage leak? Charge buildup? Magnets (which are positioned properly)?
Once again my EA-DeltaPlusXL system is giving me problems and I’m not sure why. An iffy power supply is one possibility but I’m not convinced. Here is the sequence of events … note that the source keeps tripping off:
Late November to early December we cleaned the source, changed pump oil, cleaned the Penning gauge, and fixed an EA problem.
Dec. 10 the source tripped off overnight (voltage off, emission on). It turned back on OK but no beams. Played around with the tuning and got my beams back but the sensitivity did not recover until the next day.
Stabilities were fine so I ran some N-only samples for a few days.
Dec. 17 the nitrogen backgrounds and reference gas signals were too low. Decided to rebuilt the source using all new ceramics and replace one of the x-deflection half plates because it had a dent (…not sure how you dent one of these but that’s what it looked like). Made darn sure there were no bits of metal stuck to the source magnets.
Dec 27 sensitivity and stability were OK.
Jan 3 noticed that the water background was really low and beams were a bit jumpy. Started a run of tests any how. Sadly this run failed because the source tripped off (…as it did on Dec. 10 prior to the source clean)
Jan 4 the source turned back on OK. Stabilities looked good even though the beams were jumpy and water was still low (...but not as low as it had been because I had bypassed the He purifier). Restarted the run of test samples and the results looked fine.
Jan 5 and 6 was able to run some N-only samples . No problems.
Jan 7 the sensitivity was low so I retuned. Beams still jumpy.
Jan 12 the stability has gone to “heck”, beams really jumpy. Retuning (increasing Extraction Symmetry and decreasing X-deflection) seems to help but it only lasts a few minutes.
Jan 15 the source tripped off again but turns back on as per the last two times.
Jan 16 same problems as the 15th but in the process of troubleshooting I turned the source off and on and lost my beams. Found them again by changing from CO2 to N2.
My best guess is that I have charging in the source. Anyone have other ideas? If not, I’m pulling the source out again.
Bye for now,
Assistant Professor (Senior Research)
Stable Isotope Lab Manager
CEOAS, Oregon State University
Corvallis, Oregon 97331