New Emult producing S shape Curve

[oh-wyo]

I just replaced the component of the electron multiplier on my GC-MS (TSQ7000). I decided that it was time to replace the electron multiplier, as the detector was operating around 2400V, and the TIC of a repeated methyl stearate injection had decreased by 10-100 fold over the last month (even when I would re-calculate and use the minimum operating voltage using the GAIN function every morning).

However, now when I inject the same methyl stearate sample, a corresponding peak (to methyl stearate) does not show. Much to my dismay, I see a horrific 'S' shaped curve, with lots of ion at m/z 207 (column bleed?), and a low-mid range TIC of 2E6.

Sometimes the noise test on the detector fails, other times it doesn't. I thought the noise test may depend on the detector voltage, but it is not. Sometimes the noise test will pass or fail when the detector is at 1700V, and sometimes it will pass or fail at 1000V.

I've pumped the detector under vacuum for >24 hours. I have also tried a 5 min 90C manifold bake-out (usually operates at 70C). I've cleaned the gold cup around the multiplier and used clean air to dust the multiplier region.

I did not see an 'S' shape curve before I changed the electron multiplier component, so I am afraid that I have somehow messed up the detector area when changing the component. But I do not understand how getting an 'S' shape curve, especially with lots of suspect column bleed, would be a result of changing the detector. Does anyone have any insight? Thank you so much from a desperate student!

[Yama001]

I would pull the column out, cap the the MS inlet (a blank ferrule is best, otherwise a regular ferrule with a paper clip to plug it will work) and see if a similar baseline is observed. It may be the column has been damaged, with a lot of bleed fouling the MS. If the MS settles down after another bake out, you will know the problem is the other end of the beast.