baseline shifts in GCMS

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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Has anyone seen anything like this? These are post-maintenance runs with the needle removed to check baseline. Full scan 35-500, with threshold at 100.

System specs:
Agilent 6890N with 5975 iMSD
inlet: split liner (changed every 300 injections) with 20:1 split, new septa, 250'C
column/oven: constant flow @ 1ml, DB-1MS (newly installed and conditioned at 325'C for 2.5 hours after air/water passed), 150'C initial temp, hold 1 min, with 21'C/min ramp to 276'C and hold, post run ramp to 325'C for 3 min
MSD: source 230'C, quad 150'C, transfer line 230'C, baked out for 90 min following column conditioning.

Any ideas on why this is happening would be greatly appreciated!
What ion masses are showing in the spectra of that high background?
The past is there to guide us into the future, not to dwell in.

high background v. drop in background. Everything seems to stay the same and just drop abundance. Unfortunately, I don't know if the 44 m/z abundance is normal for this machine because we usually only scan 50-550 m/z.

I thought maybe was a misaligned column, so cut both ends and replaced gold seal and inlet liner... Now 219 and 502 % abundance are failing on the tune and the dropping baseline problem remains :cry:
44 m/z is from CO2 in the system, the 207 and 281 m/z are from column bleed.

3500 counts is not a very high background at all. The dips could simply be flow pulses during the oven temperature program or some moisture coming from the head of the column and passing through, which is not detected but can suppress the signal( or methanol if that is a solvent you use).
The past is there to guide us into the future, not to dwell in.
Go to manual tune and scan. How many peaks are there?

The drops in baseline were still present when we blocked off the MS with a no-hole ferrule. Cleaning the source seemed to resolve the baseline issue and tune. Thinking it must have oxidized to some extent when the vacuum pump failed.
I asked a similar question a little while ago so you may find this topic useful: viewtopic.php?f=3&t=87454

44 m/z is a common background ion on most systems.
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