Baseline rise with T-programming

[Roxanne42]

So I can't seem to find a clear answer about this, but from searches I gather that baseline drift with temperature-rise in GC-MS is normal and attributed to column bleed? Can anyone clarify this for me? I mean, the column's upper temperature is 300 degC, why would it bleed so? What are the mechanisms that cause this to occur...?

And how much is actually "normal"? I've put up a screenshot of the chromatogram (TIC) we collected by injecting 1 uL of methanol [inlet = 250; split injection 60:1; flow 1.2 mL/min; T-program = 50 for 1 minute, then ramp (20 degC/min) to 100 (hold 0 mins) and ramp to 275 (7.5 degC/min) and hold 5 mins with a post-run of 2 mins at 300 degC.]

Screenshot can be seen here :

http://aix1.uottawa.ca/~rclemen2/MeOHinj.jpg

Thanks!

Roxanne.

[AICMM]

Roxanne42,

I would have to say that some column bleed at higher temperatures is to be expected. How much is "normal" depends on the manufacturer, the column type, system cleanliness and the upper operating temperature. Basically, there is no real "normal". It becomes more of an issue of what you can deal with.

Oxygen and water in the carrier gas are two of the biggest culprits. Even at very low levels these constituents can eat away at phase and, in my experience, they are always present at some level. Remember, too, that it does not take much to give a rise in baseline. Best thing you can do for yourself is to keep the system as dry as possible and as leak free as possible and use an oxygen scrubber just before the instrument.

Best regards.