GC-MS pump down and calibration gas not visible

[Punjabi]

Hello,
I would like someone to give some advice on a couple of issues I am having:
One I am working with a 5972 ms 5890 series II gc--HP. One the system won't pump down. Rough pump is good, transfer line is good and MS o-ring is good. The problem is coming from the poor mans splite in the GC. In other words there is a smaller diameter column stuffed inside a bigger column.

The smaller end is in the inlet. Does anyone have any ideas on this situation. This technique has been used before to pump down the system and it was fine.

The pressure is around 250 Torr. and is not going down.

Second issue:

I cannot see any peaks for the profile scan. The only things that I know of that cause this issue, one the ground wire that connects to the MS circuit board, which is ok, and the insulator for the ion source. Both are good!

Does have anyone have any ideas?

Thanks,
Punjabi.

[littlepeaks]

Are you sure the problem is with your column setup? Check the tubing that runs between the vacuum pump and the MS. Also, you can insert a blank ferrule (a ferrule with no hole) in the transfer-line column nut to seal off the MS, to see if the MS is leaking.

[Don_Hilton]

A smaller column stuffed in a larger column? This sounds like situation that could offer some problems in reproducibility. Are you intentionally using this as a splitter? If not, can you use something that will give a reproducible column connection? If you are using this configuration as a splitter intentionally, any reason for doing this rather than using the split vent in the GC inlet? (A assume you have an inlet with a split vent?

[Peter Apps]

A small column stuffed inside a bigger one might be a flow restrictor allowing a megabore column to be run under partial vacuum, which is reputed to give faster separations. Likely to be susceptible to leaks I would think, and with a megabore column a leak even at the inlet would compromise pumpdown.

So, Punjabi, what is the diameter and length of the column that is connected to the MS, what is the diameter and length of the thinner column, and how is it all connected together ?

Peter