Moisture hump in voa run

[sllimbri65]

We are experiencing an unusually large moisture hump at the start of a voa run. we use a Tekmar 3100 with an mcs loop. The hump is not there when doing a direct inject on to the gc. i don't have much experience diagnosing p&t concentrator issues but am good with a wrench. any help would be appreciated.

[Johnny Rod]

Is this a new problem, or have you always had it? Same types of samples?

[AICMM]

sllimbri65,

Do you really need to scan down to 18? That does not solve the problem, I know but still..... If you don't scan down to 18 then, in theory, it is not stable in the quad and you will minimize the impact.

What is your split ratio?

Best regards,

AICMM

[sllimbri65]

We scan to 35. The hump is showing ion 44 from the disolved co2 in the water. We use a 1:50 split for water samples. We have been using the same method for years.

[cleh]

How old is your trap and which one are you using. What are your p&t parameters?

[sllimbri65]

These are the concentrator parameters. The trap is a new Vocarb 3000

[Johnny Rod]

I think these have to be conditioned before use?

[cleh]

[quoteThese are the concentrator parameters. The trap is a new Vocarb 3000

Did you leak check the purge and trap after installing the trap? The ferrule on the bottom of the trap is usually graphite. I like to use graphite/vespel because it holds up better to the 250 desorb and 260 bake temps required for the Vocarb 3000 trap. If you leak check the system and you have a leak, this ferrule is usually the problem. It's hard to seat it because the bottom of the trap line is moves around. The Vocarb 3000 does need to be baked out as Johnny Rod stated. Once you have a leak tight system, you should bake out the Vocarb 3000 at 270C for at least an hour (I like to do four hours). Leak check after heating it up to 270 for ten minutes and cooling it down to make sure the ferrule has settled (you may have to tighten it a bit) then bake it out for one to four hours. Leak check again after the bake-out and you should be good to go.

[Bigbear]

I sometimes see some wierd humps in the early part of my chromatograms on a new Vocarb 3000. They go away after a few runs. Spoke with supelco a while ago and they claim some traps need a higher temp bakeout. As I remember they said 340-360C ( check with them first!). Seems the packing can go to 400 but the glass wool packing would get activated.

[sllimbri65]

Called Supelco and they said we could bake at 375 for 2 hours. Will try that and see if it helps.

We use graphite ferrules also. It has been changed a few times already. All leak test have passed.

[jenniferd]

Hi sllimbri65,

I believe I have the same problem, how did you fix this?

Thanks!

[sllimbri65]

It took a long time. We just kept running blanks and it went away by itself. Sorry, no magic bullet.

[jenniferd]

OK, thank you!