Re: 6890 splitless mode
Posted: Mon Apr 04, 2011 11:31 pm
by chromatonewbie
. In the 6890 the GC goes into forward pressure control mode and the split flow is turned off until the purge valve is turned on. During the splitless time the only flows are the column flow and the septum flow, and it is possible for air to backstream into the system.
Ron, exactly where does the air backstream into the system? Through the split vent or through the septum? Or from somewhere else?
Re: NItrogen ramping in my base line
Posted: Wed Apr 06, 2011 2:10 pm
by AICMM
Couple of comments. 1) Where does the 44 come up? The discussion was originally about nitrogen and then we mentioned 44 which is probably CO2? You would have to have a pretty good leak or a trap to get this one. 2) You mention nitrogen but you say no oxygen. Unless you are eating the oxygen somewhere (column perhaps?) I would probably argue against a leak. So what column are you using and what are your initial conditions? 3) If you don't have a good seal on the split vent valve, I could see air diffusing in this route (but you would normally see the oxygen.) One way around this is to leave a split method (overnight.m) as your GCMS top method while your sequence (analysis.M) is a splitless/split method. When the sequence is done, it should revert back.
If you have access to a squirt bottle and some dry ice, you might use 44 to troubleshoot. Leave yourself in scan mode in tune and spray CO2 gas around your plumbing taking into account the lag time.
Best regards,
AICMM