Chromatography Forum

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.

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?

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?

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?

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

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.

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.

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?

Giles,

What is the solvent, what is the column?

Best regards.

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.

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.

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

Thanks