early Wave shape in GCMS baseline

[Giles]

Please help me troubleshoot this GCMS (EI) chromatography problem.
Zero to one min is flat. Beginning at 1 min (into a 10 min run) the baseline abruptly begins to rise quickly and then it slowly levels off. At 4 min it drops sharply to the original baseline and remains flat until end. This behavior is seen on 'empty' injections. Seen in split and splitless mode. Oven program: start at 30 deg and hold 4 min, then 30 deg/min until 75 deg (5.5 min into run), then 45 deg/min until ending at 275 deg. Extracted mass spectrum of problem area is not revealing (to me). Baking column and ion source for a short time did not help.
Thank you in advance for your advice.

[Giles]

Still working on it. I think its a leak.

The baseline abnormality is higher after replacing inlet liner and septum. Why?

Source pressure 'follows' baseline.

Where is the most likely place to find this type of leak?

[Don_Hilton]

Do you have a purge off time and/or gas saver time set? If so does either of these times match up with the steps you are seeing?

[Giles]

Thank you for the inquiry.
Gas saver interaction was a consideration for me, but I kind of ruled it out because it starts at 3 min (25 ml/min flow). Purge flow is 35 ml/min at 3 min. The septum purge flow is 2 ml/min.

However I did find a leak! at my ion source (by spraying duster). I removed and replacemed ion source, and tightened it up, so I can't see a tetrafluoroethane (leak indicator) signal anymore, but the "early wave" baseline shape is still there, and I believe it was, and still is, an ion signal consisting mostly of air (mass 44 = CO2) and other masses I don't recognize.

So, maybe there is more than one leak?

[GC girl]

Can I ask what do you mean by 'removing and replacing ion source, and tightening it up'; I am asking this because my GC/MS has leak and I observed it in slot of side plate O-ring in the ion source.
I know you was the one who asked question, but maybe your question is my answer!
Thanks

[AICMM]

Giles,

Are you running this instrument with purge and trap? Sounds to me like: 0-1 min filament is off, 2-4 minutes you are getting lots of CO2 from your trap (expected with carbonaceous trap), and 4 minutes or so you are switching from desorb to bake.

Just a guess on my part....

Best regards.

[Ron]

The oven temperature program does look like it is for volatiles, and it does sound like a purge and trap baseline. I have noticed that the water coming over will raise the source pressure and affect the baseline even if you don't scan low enough to see the water.

[Giles]

Hi,
thank you for comments, etc.

Ion inner source was removed after venting instrument. O-ring was inspected, but it was OK. Then inner source was reinserted, and instrument pumped down. This fixed a small leak, but the main problem (elevated baseline between 1.2-4.4 min) is still present in splitless mode.

This method is not purge and trap. It is 1 ul liquid injection into heated injection port. The samples I am running right now have no water in them.

Possibly the elevated baseline is caused by "all material" in splitless liner going into column before septum purge starts at 3 min. 30 m column at 1 ml/min carrier gas flow rate may elute "all material" after ~1.2 min and stop eluting "all material" 1.2 min after septum purge starts at 3 min. I failed to recognize earlier that this problem behavior follows the choice of splitless mode. What is a typical splitless method used to avoid this unsightly chromatogram?

[AICMM]

Giles,

What is the solvent, what is the column?

Best regards.

[Giles]

Column = DB5-MS, 30m, 0.25 mm ID, 0.25 um film
Solvent = none or MeOH.
I can see the ugly baseline behavior on a run with out injecting anything, or with solvent injection. I'm using MeOH for my method with real samples.

[JI2002]

Septum purge flow should be on all the time, no matter it's split or splitless injection. Splitless time should be about 1 min, then split vent flow should be on at 1min. After 3 min, you can then turn on the gas saver.

[chemomet78]

Hi Giles,
Did you find a solution for your problem? I have the same problem.

Thanks