Chromatography Forum

I got a low vacuum after changed the column (DB-5) on 6890/5973.
I removed the column from the injection port, and capped off. The system pumped down very quickly and reached a very good vacuum. So the leak is isolated to GC. I replaced the liner O-ring, septum, and installed the column back to the injection port, then the vacuum went back to ~9*10(-5) torr again.
I did manual tune and find many other ions except 69 and 219. However 28:32 is about 2:1, so it is not an air leak. Do you think there is a contamination of the gas? Does the contamination affect the vacuum? The trap is 2-year old now.

Thanks

Most people look at the water to nitrogen ratio to get an idea of air leaks.

For example in answer 3 at:

http://www.sisweb.com/index/referenc/tip19.htm

"Look at the air/water spectrum between m/z 15 and 50. On a leak tight capillary system manuals say m/z 28 (nitrogen) should be 3 times smaller than m/z 18 (water) but for all practical purposes when 18 and 28 are equal, with the cal gas off, it usually means a pretty tight system. The masses that really mean trouble are m/z 40 (argon) and m/z 44 (CO2) when these are fairly visible it indicates a leak. "

which was found on the following web page plus other additional ideas on solving/detecting air leak problems..

That is a very nice link. I am reading now....
Thank you very much.