Baseline Drop off during oven ramp

[Matt Spec]

I am troubleshooting a misbehaving GCMS system (Agilent 6890/5973) and I noticed this strange baseline when running a no injection blank (see photo)

Can someone tell me the cause of the steep baseline drop off at the 2 minute mark, and potential reasons why the baseline is slowly decreasing through the entire run.

It seems the drop coincides with my oven program (see below), but why would it drop so suddenly? Additionally I am picking up peaks at m/z 28 and 32, is the problem potentially an air leak? These peaks loose abundance as the run progresses, what would be the cause of that.

If anyone could recommend solution I would be greatly appreciative.

Program: Inlet (splitless) 250C purge flow 50mL/min @ 30 sec, gas saver 2 mL/min @ 1 min. Column: HP-5MS 30m x .25 mm, constant flow at 1.1 mL/min
Oven: Initial 60C hold 2 min, 15C/min to 300C

[James_Ball]

The fact that the baseline is mostly composed of air peaks and is dropping with oven ramp points to the MS inlet nut and ferrule. If you replace the ferrule with a fresh vespel/graphite one, they will shrink over a few oven cycles and begin to leak. This leak can also close off as the oven ramps and swells the ferrule enough to seal the leak. Try giving the interface nut another quarter turn and see if it helps.

Another thing to do for future column changes is keep a few of the ferrules in the oven in a beaker, that way they are pre-shrunk and it doesn't cause the leak as often.

[mathieu_stability]

If the MS interface ferrule is not the issue, then check the GC inlet ferrule as well.
In addition, repeat the intrument blank with a split injection to observe if the step at 2 minutes disappears. This may be caused by the GC inlet pneumatics.