Chromatography Forum

I currently have a 10 meter, 0.18mm ID RTX-1 column installed in our Varian GC-MS. When performing air/water checks, I notice the following: At 50C with 10psi pressure, the air/water check looks good. However, at 85C with 2.5psi pressure, the water level goes way up and takes a long time to come down, say 30 minutes, and still doesn't quite reach an acceptible level. At both the lower and higher oven temperatures, the injector split is closed.

Any thoughts as to where the water might be coming from and why I'm only seeing it at higher oven temperatures? The system seems to be free of leaks and the same carrier gas source is also plumbed to another GC-MS system in the lab, which doesn't have an air/water problem. The moisture filter is fairly new on the helium tank.

Thanks for any suggestions.

-Aaron

old septum?

I replaced it this morning, it was already fairly new, changed out last week with fewer than 50 injections... It didn't seem to help the situation. :0(

Here is what I now observe (at 80C oven, 1.0mL/min flow, 250C injector). I changed out the septum and did some basic injector maintenance. I performed an air/water check. Air looks fine, water level is massive. The injector is in splitless mode. I then turned on the injector splitter to 100:1, within 30 seconds the water level fell to "normal". I turned the splitter back off, and within a minute or so, the water level was once again very high. So, the water contamination seems to be a function of the injector split state. Any suggestions?

I did a septum purge calibration this morning. At 10 psi, the Varian manual states the purge flow should be around 4.5mL/min for the 1079 PTV injector. The flow was very close to 4.5mL/min, I did a minimal adjustment and saved the changes.

Why should the split state be affecting the water levels in my system? Any suggestions? I really need to be doing splitless injections...

Thanks,
Aaron

Akthmps,

A guess here; 2.5 psi splitless on a 10 meter column is starving the inlet for flow and you are getting a leak or back diffusion. Might I suggest a longer RTX-1 to see if that has the same problem?

Best regards.

How are you setting your gas flows - as pressure or flow rate ? 2.5 psi sounds very low for a 0.18 mm id column, even if it is only 10 m long with a vacuum at the end. Depending how things are set, the MS vacuum might actually be sucking gas into the column faster than the pneumatics are putting it in, you then have a negative pressure in the inlet that sucks air in through the septum purge. Check the septum purge flow with the pressure at 2.5 psi - but be careful that you do not suck bubble flow soap back into the system !!

If opening or increasing the split decreases air levels there is a leak somewhere - if it is not backflow through the septum purge, and not the septum, you need to check all the upstream plumbing. How are you checking for leaks by the way ?

Peter