losing sensitivity after changing septum

Discussions about GC and other "gas phase" separation techniques.

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I'm running a Thermo Trace GC (30 meter DB-35 column) connected to a Quantum triple quad detector. I was running some samples in MSTFA and was noticing that my internal standard area counts were slowly going down. I wanted to change the septum thinking that it may be slowly leaking. The inlet was set at 190, oven at 40, transfer-line at 280, and source at 250. Since I was in a hurry I didn't wait for the inlet to cool down, I turned the He flow off, and then changed the septum in about 15 seconds. Now I get no signal. This is the second time that I have lost my sensitivity after changing my septa with a hot injector. Next, I cooled everything down, replaced the liner, the septum, and the metal injector port outlet at the bottom of the injector. I cut 10" off the inlet side of the column. I heated everything back up and I still have a small signal signal. I'm injecting only about 0.32 pg of sample onto the column. Can anyone explain what is happening? Thanks for your time.
Most likely related/caused by what you just recently did.

I didn't have experience with that brand, but will add that I used a new septum EVERY day of use. For both GC and GCMS. And I used trimethylsilyl derivatizing agents a lot.
Are you doing split or splitless injections?

When I was doing a split injection I noticed my area counts increased by over double and it dropped back to normal after changing a leaking septa. What was happening was the septa was leaking and I was not getting the full split flow so instead of a 5:1 it was closer to 2.5:1, replacing the septa actually increased the split ratio back to normal.
The past is there to guide us into the future, not to dwell in.
I am running pulsed splitless. I was suspecting that a leaking septa was causing my slow loss of sensitivity. When I changed the septa quickly (15 seconds) while the system was hot, I then lost most of my sensitivity. I'm thinking that by letting in a small amount of air (oxygen) that either the column died or maybe some hot metal surface (inlet, ion source, ??) oxidized and either is adsorbing my compound or ??

Anyone experience this at trace levels or have an explanation?

You said the oven was at 40c and you removed about 10" of column so I wouldn't think it was in the column since only the first few inches would have been at a temperature that should cause problems with air and heat interacting, since it would have been heated by the injection port.

I don't know about Thermo equipment, only used Agilent, but if something gets on the inlet that can't be removed using solvent and a cotton swab, we use a bronze wire brush from a 32 caliber gun cleaning kit to brush the inner walls of the inlet. That was a tip given to me a long time ago by an Agilent service tech and it seems to work very well. It works best if you can remove the seal at the bottom of the inlet, which it seems your has also.

The only other place to look would be the source, if a little air on the hot source might have oxidized it enough to lose sensitivity, or possibly it damaged the very end of the column that is in the heated zone at the entrance to the MS.
The past is there to guide us into the future, not to dwell in.
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