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Excess ISS variation in calibration

Posted: Tue Sep 13, 2022 10:32 pm
by LALman
I've been seeing too much ISS variation and decided it was time to clean the source: a 5973 inert fast scan with a 6890N GC. Got it clean, no leaks (<2% 18, 28, 32), Repeller ramp peaks at ~35mV. Did an ATUNE and bumped the ion focus to 110mV to get 69/219 of 75%. This has been a good way to get a tune previously.

I put on my calibration and RANDOMLY throughout the run the ISS values all increase or decrease but they are not tracking together like they should. Fluorobenzene goes from ~11 million counts to 26 million counts. This wrecks RSD's because surrogates do not track the internal standards (e.g. toluene-d8 varies from 12 million counts to 24 million counts but not with the same samples). So variation looks like....
23.6% dibromofluoromethane
29.4% 1,2-dichloroethane
17.6% toluene-d8
31.5% 4-bromofluorobenzene

Normally, toluene-d8 variation would be ~0.5% and the other surrogates would vary 2-5%.

It does look roughly like the three surrogates of fluorobenzene all vary in the same direction when plotted in a line series as fluorobenzene increases. With mostly positive deviation from the fluorobenzene 1:1 line.

I did the 5973 stability test and it passes without problem. Trap seems to be holding pressure. Test is dropping ~1-2 psi from 20 psi over 5 minute hold time. Valve body in Tekmar 3000 was reground by Tekmar a couple years ago after 15 years of being stable. So, its either an odd electronics problem or a trap problem.