GCMS drops in baseline with recovery during no-injection post maintenance

You might try running your mass range 35-500 mz and drop the threshold to see what the baseline looks like

You might try running your mass range 35-500 mz and drop the threshold to see what the baseline looks like

Okay, changed mass range to 35-500 and threshold to 100 (was 150) and just started the sequence to run three times. Will post results in the morning.

Also, ran it three more times with previous settings this afternoon to see if there was any sort of pattern...

results from changing mass range and threshold. Also looked at ions in normal baseline vs dropped baseline and it seems like everything stays in proportion, just drops abundance.

ron_tackett

I thought maybe the column was misaligned, so cut at both ends and reinstalled, and changed the gold seal. Also went to change the inlet liner and the glass wool was pushed out of the top again. It hasn't done that since Herb replaced the EPC module. I let it pump down and air and water check passed a couple hours later. Heated up to operating temps over night. Ran atune this morning and it looked worse--repeller maxed, 219 and 502 % abundance decreased. Decided to bake out MSD again for 0.5 hour and atune--repeller better but 219 and 502 worse. Tried switching filaments and the other was worse, so switched back to #1 and retuned--repeller maxed and 219 and 502 are borderline failing (41% and 2.5%, respectively).

Ran a couple no-injections anyways to recheck baseline and it is still dropping at random points in the run.

Could the baseline problem have something to do with oven temperature, pressure, or flow fluctuations? Not sure how to check if the oven and EPC are functioning properly.

Probably not helpful, but your background is relatively normal except that the 44 peak from CO2 is very high. Argon (40) is present in air at ~1% and C02 should be at ~400ppm (0.04%) a ratio of 25:1  Your ratios vary between ~14:1 and 7:1 the “wrong” way. The 207 siloxane peak only varies by <50% and the 281 siloxane peak seems to vary even less  - The CO2 may be adsorbed on an active surface between runs and desorbed as the system gets hot, so possible places are the injector/liner or injector trap filter. A stupid thought - You don’t have an instrument with liquid CO2 sub-ambient cooling, or another source of CO2 in the lab?

Thanks for the suggestion. It's unfortunate I can't find a scan to compare the CO2 and Argon to, because we usually scan only from 50-550 m/z. I have the injector off and replaced the inlet liner, but will check out the split vent trap. And no sources of CO2 in the lab.