Leak and low eV repeller

[mayu]

Hi everybody!

We have a GC-MS 7890A/5975C. In the autotune, it showed high levels of N2, O2 and H2O. Increasing the oven temperatura, this trouble dissapeared; but the eV repeller and the entrance lens fell to 10 and 19 respectively. Obviously, abundance of mass 502, is much lower than currently.

Somebody could help us??
Thankss

Mayu

P/D: sorry my english is very por and i'm learning about GC-MS!

[Yama001]

The leak dissappearing as the temperature rises usually means the MS tranfer line nut is not tight, or is deformed and not sealing well at low temps.

If the autotune was run when you were at the low temp, the leak may cause higher than normal pressure in the ionization chamber. The autotune algorithm may produce unusual results under these conditions.

[James_Ball]

The leak dissappearing as the temperature rises usually means the MS tranfer line nut is not tight, or is deformed and not sealing well at low temps.

If the autotune was run when you were at the low temp, the leak may cause higher than normal pressure in the ionization chamber. The autotune algorithm may produce unusual results under these conditions.

I agree, the most likely cause is the ferrule at the MSD column interface is shrinking due to the heat. One trick I use is I keep some ferrules in a small beaker in the oven and the heat cycles shrink the ferrules so that they do not shrink further once installed on the interface. Try to tighten your interface nut about 1/8 to 1/4 turn and see if the leak goes away at lower oven temperatures.

[Yama001]

Preconditioning the vespel/graphite ferrules works so well that I wonder why the manufacturers do not do it themselves.

When I use a new vespel ferrule that has not gone through a heat/cool cycle, I will raise the oven temp to 150, then cool it down to room temp and give it a final tightening, as James recommended.