Carbon Dioxide peak and elevated baseline for 8260 Volatile

[Darber99]

Greetings,
I'm having an issue with a very large carbon dioxide peak and an elevated baseline following it. I'm running 8260 Volatiles with a Tekmar 3100 Purge and Trap, 6890+GC and a 5973 MSD. I'm pretty sure I've isolated the issue to the concentrator because I did a blank run with just the GC and there was nothing but noise with no disernable peaks. Performing a desorb only run shows a little bit of CO2 and baseline rise but no where near the abundances seen when a purge takes place. I've attached a picture of the chromatogram. Thanks in advance.

[Darber99]

Sorry, I should've included my instrument conditions as well...

I'm using a Restek RTX-VMS (30x0.25x1.4) Helium carrier gas, injector is in split mode:
Inlet:225C
Starting pressure:7.8 psi
Flow rate: 1.0mL/min
Total flow:23.9 mL per minute

P&T conditions:
25mL purge
11 minute purge time
2 minute dry purge
4 minute desorb
10 minute bake

Trap is 40C ambient/255C desorb.

[cleh]

What trap are you using? Has it been replaced? I've often found the fitting at the bottom of the trap will loosen over time so check that. Have you leak checked the P&T? Please let us know what you find.

[Darber99]

I'm using a Supelco Purge Trap K for a Tekmar concentrator and it's brand new. The trap has seen maybe 10-20 samples. We used the trap to develop a 25mL purge method but the computer crashed before we could finish (no carbon dioxide or water seen during method development). Unfortunately the version of ChemStation on the new computer is newer than the other versions we are running so we had to build our method from scratch rather than copying it over. The GC inlet was having pressure flutuations for a couple of days (setpoint 7.4psi, inlet was pumping to 7.6, losing pressure to 7.2 and then pumping up again) b/c it was in splitless mode and the concentrator and GC/MS were turned off numerous times during this time frame. After switching it to split, we were able to start runs again but the carbon dioxide and water appeared. I'm baking the trap out now and will be switching it out for another if the bake out doesn't help. Is it possible the trap got "fouled up" by having the pressure flutuations in the GC and having the concentrator turned on and off? Also leaked tested the P&T by closing off the vent and purging. The SPC is set to 20psi and the TPC got up to about 16psi when the vent was closed. A slight leak but probably not enough to cause such a large carbon dioxide peak and water baseline?

[James_Ball]

By chance is the raised portion equal in time to that of your bake cycle?

I had a similar problem using an old PTA30 once, and it was gas from the bake out getting into the carrier stream through a leaking 6 port valve. It can come from CO2 and water crossing through the leak into carrier gas while the trap is baking. The easy way to test it is to run a 2 minute bake and maybe a 10 minute or 15 minute bake and see if that hump changes length with the changing bake times. If it does you need to replace at least the rotor in the valve in the P&T over or possible the whole valve. I can't remember if the 3100 drops the back pressure on the trap during bake mode or not, if not that is helping to push the gasses over making the leak even worse.

[Darber99]

@James-I don't think the bake cycle has anything to do with it. The elevated baseline drops down around 6.5-7 minutes and we desorb for 4 and bake out for 10.

[James_Ball]

@James-I don't think the bake cycle has anything to do with it. The elevated baseline drops down around 6.5-7 minutes and we desorb for 4 and bake out for 10.

If the elevated baseline is about 4 minutes in duration (difficult to tell, I can't see the time scale on my computer here) then it can be from water being desorbed from the trap. Since you do not scan down to 18 the water will show up as other masses since it will overwhelm the mass filter.

What trap are you using?

[Darber99]

@James-See the post right above your first one. It's a Supelco Trap K and I explain what's happened to the instrument in the past week.

[Bigbear]

You will see CO2 in water samples. The only thing to do is stop scanning @ 35 , that way all you will see is 44 from CO2. I have always seen it using almost the same system as yours.

[cleh]

What is your trap bake temp? What is the temp you use for conditioning a new trap and how long do you condition it? What He grade are you using? He purifier(s) could be bad. Do you have the carbon filter (usually a green canister) installed on the He line to the P&T? If that is old, it may not be working.

[James_Ball]

Ahh I missed the post that you made about the same time I sent mine. K Trap is the VOCarb 3000 so it shouldn't retain too much water the way one with silica gel or charcoal would, that is good.

With that trap you can get by with a shorter desorb time than 4 minutes, probably as little as 1.5 minutes, that will reduce the amount of moisture and CO2 transferred to the column. Can you post a pic of the spectra you are getting during the high baseline? I want to see if it compares to what we have had in the past. It could still be a lot of water coming across, especially when using the 25ml purge.

[cleh]

Forgot to ask about your moisture control settings.