Chromatography Forum

O.K., riddle me this:

i am running a purge and trap GCMS. After installing a new column I started using a VOC method (USEPA 8260). After the high level of the calibration i got significant carryover of the heavier compounds. I have changed the transfer line from the concentrator to the GC, the transfer line from the autosampler to the concentrator (and flushed with methanol), cleaned the sparge needle of the autosampler, cleaned both valves of the concentrator and i'm still getting carryover.

I've just changed the trap again, and I am getting a lot of alkanes (C12-C16) coming out, which I think happened last time. Could this be a sign of insufficient trap conditioning? (I baked it for an hour at 260c). could insuffiecient conditiong be the cause of the carryover?

Any ideas?

i have'nt tested the new trap for carryover yet as I am still waiting for the alkanes to clear, I'll report back when I have.

cheers

Do you have alkanes in your standard?

Check all the heated zones and then try the following:

1) Bypass P&T by connecting carrier gas directly to GC inlet and
run instrument blank. Set up a sequence and just press GC start button. If you don't have peaks, you rule out GC/MS as the culpit.

2) Connecting carrier gas back to P&T. Run instrument blank without running P&T. Set up a sequence and just press GC start button. If you don't have peaks, you rule out transfer line from P&T to GC as the source.

3)Run instrument blank again while runing P&T without loading any sample. Just go through all the purge-desorb-bake steps without water. If you don't have peaks, the contamination probably comes from autosampler. Otherwise, it points to P&T itself, most likely the trap.

Please give more details about P&T parameters.

It looks like the concentrator is the problem, as I have been using a different concentrator, and there's significantly less carryover. When I get chance i will hook up the old concentrator, i have sonicated all the lines in methanol and if that has'nt fixed it i am going to try purging methanol and heating the zones 20c above usual temps.

cheers

OK, so I sorted out this problem by increasing the flow during bake (I had a leak) however I now have another peak: a constant benzene peak that won't disappear! See my other post for details.