Chromatography Forum

Hello,

I am using an Agilent 6890N GC connected to an MS. I am having trouble getting the vacuum to purge the MS chamber of atmosphere because of a leak in the inlet (i think...). If I remove the column from the inlet and plug the end of it with one of those blue rubber things that comes with new columns, then I can "air and water test" and the chamber gets emptied and the m/z = 69 is as normal. As soon as I connect the column back into the inlet (cool on column), the chamber is filled with atmosphere and m/z = 69 is not visible on the spectrum, even if I let it pump down overnight.

I have replaced ferrules (85/15 vespel graphite), replaced the septum, cut the column, and cleaned all the surfaces involved (where the ferrule makes contact with the bottom of the inlet, for example). No luck.

Does anyone have any suggestions of what to try next? Unfortunately I cannot afford to pay an Agilent rep to come out to our site and help us...

Thanks in advance,

Luke

When asking for this kind of advice it is best to provide as much details as possible.

Column dimensions?
Flows/pressures?
Are we speaking about working method which ceased to work suddenly?
Are You introducing a new method? New injector or it did work OK before? Who did the instalallation?
Was some GC maintenance/repairs conducted just before problem occured?
Cold-on column injector might be of different manufacturers and differ in construction so more detailed info also might be helpful.
Did You conduct any tests to check if inlet is not leaking?

It is possible you have an air leak in your gas lines. You can use a handheld leak checker to verify or just check all your fittings. Make sure not to overtighten. Leak indicating traps are also very helpful. Another thing to check in the inlet is the large thermal nut that the gold seal housing mates to. Snug that up regularly.

If you have the instrument in splitless mode or a a low split rate and the head pressure is low, you can get air into the GC column by diffusion in through the split vent. Increase the split ratio and see if the air seems to go away. I actually set the flow on the gas saver to give a split flow of about 35 ml/min to avoid air background in the instrument that I run. Injections are run splitless.