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Stable Isotope Geochemistry

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Fri, 25 Oct 2019 10:40:40 -0400
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Hello all,

Background: We have been working on our delta V for some time now trying to fix a air leak into our system. We ended up making our own inlet manifold ( single port instead of the triple fork) using 316 cleaned stainless steel tubing, swagelok fittings, etc. We also figured to clean the source and replace old worn out parts. We pumped down( E^-7), turned the filament on, and performed a leak test with Ar. No leak, yaay! Our backgrounds were still suspiciously high so we decided to turn on the analyzer and source heater. for 8hrs. This was doing wonders initially as our background m40 went from ~150 to 9mV on our sensitive cup and vacuum went from 7.3E-7 to 5.9E-7. However...

Issue(s): With the inlet open, analyzer and source heater on, we swapped back to our main gas configuration ( m44/45/46) and all of a sudden our vac jumped from 5.9E-7 to 9.2E-7 and the backgrounds went back to being elevated ( m40 back to 150mV). It was as if we didn't just have great numbers a second ago. I decided to just let it continue to bake out with the heaters on and hopefully it would return to the nice values we just had. This morning I found our HV to be at 0.33 kV with a poor box and trap ( ~0.85 / 0.55 -> was just 0.75/0.75). It seemed to fail at night. I turned off the filament and the heaters, waited a minute, and turned it back on and its back to its happy self 3.04 kV , 0.75 Box and trap. After turning on the filament m40 = 340mV, vac 8.4E-7 , and rapidly over the course of 10 minutes, fell to m40=140mV vac=8.0E-7 and again to m40=70mV vac= 7.8E-7 over some more minutes. Curious i swapped to N2O again to get a sense of background and back to Ar and the m40 shot back up to 130mV. Ug!

Question: What would cause backgrounds and vacuum to spontaneously rise after switching gas configurations (magnet setting stayed the same)? Why would the HV fail and be steady after turning off and on the filament?  Any insight is helpful. I just want these background to remain low :(

Cheers,
Peter R

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