Forepump doesn't pump down until enabled on GC?

I inherited an Agilent 7890 with 5975C MSD using Chemstation. I am still trying to figure out what I am doing. The instrument was performing okay about a week ago. Yesterday I tried to do a run and I had high baseline and no peaks. Autotune showed very high water and N2, O2 so I suspected the side door might have a leak (this happened once before). I initiated the vent procedure and, after cooling down the temperatures, opened the side door and cleaned the seals. I closed and sealed everything back up, did the pump down command and left for the evening.


This morning the MSD showed high vacuum was 9.9 X 10 to the neg 9. I was stumped as to why the vacuum was so high. I heated the temps up to their setpoints. While hunting around in Chemstation, I found on the vacuum menu a place to turn vacuum guage on/off so I turned it on. The vacuum quickly showed a better vacuum, approx. 3.65 x 10 to the neg 5. Since then it is slowly getting lower. Now it is around 3.4 X 10 neg 5, still a bit high for my conditions but I understand it can take some time.


I guess I thought that the pump down command in itself was enough to initiate the high vacuum and thought that command in Chemstation was just to show what the vacuum actually was. Apparently I was wrong?


Also, any ideas why my instrument would begin to leak after about a week when I didn't do anything to it? Bad door seal?

  • Hi,


    normally it's not a leak on the door seal if you don't use/open it. If it's leak free it stays leak free until you changed something. 

    But on the transfer line connection it's different. Sometimes the connector and/or the ferrule becomes loose after some days regarding to the high temperature. 

    I suggest using argon (mz 40) or a can of air duster (mz between 40-90, depends on the manufacturer) to search the leak on the MS side (door and transfer line). If you use it on the inlet side, be sure to wait 1-2 minutes to let the gas through the column.




  • Basically, for sudden air problem due to following reason:

    If you changed your carrier gas cylinder or carrier gas cylinder is going to finish so better put new carrier cylinder to see that leak problem still coming or not.

    Second thing if you have connected any filter with carrier line before MS then sometimes, this filter become contaminated with nitrogen so if did not change filter from long time so better to replace it.

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