Chromatography Forum

I'm running a PTV inlet on a 6890 with an FID, and recently had to change from the septum head to the septumless head (due to thread stripping on the septum head, hooray). At the same time I took the opportunity to clean out the vent line (to my knowledge had never been done!) and do all the usual cleaning and maintenance. Now, when I inject, I get a tailing solvent peak, the flat baseline for a few minutes... then a huge jump in baseline that slowly tails away over the next 10-20 minutes, the shape of a shark fin moving to the right. The timing of the jump is ~ the time the gas saver comes on + transfer time for the column (and moving the gas saver time moves the jump accordingly). So, it appears to be solvent that isn't getting flushed out of the inlet during the purge.

In the past, apparently this has shown up (without the septumless head involved) and was cured with increased purge flow. Increasing purge flow now isn't fixing this, though. Increasing purge flow/increasing purge duration doesn't change the occurrence of the baseline jump, just moves it out in time. The peaks themselves look ok.

New liner. System is leak tight.

Injecting in toluene, 60m vf17 column with 2m of retention gap. Helium at 1.1 ml/min. PTV starts at 110, instant-ramps to 320. 100 ml/min purge flow comes on at 2 minutes. Gas saver to 20 ml/min comes on 5 minutes after that. GC oven starts at 120. Lowering or raising the initial oven/PTV temps to be more noticeably above/below the boiling point of toluene hasn't helped.

Am I looking at an obstruction somewhere in the vent system? Might there be an issue at the EPC? The liner is new, but the inlet itself has been through a LOT - not too clean and shiny in there, despite solvent rinsing it. Can oxidization/buildup inside the inlet body hold onto solvent?

If you have any carbonization at all toluene will adsorb to it and then release over time, so there is probably some in there somewhere.

Have you tried removing the bottom fitting and possibly polishing the entire inner surface with a pipe cleaner and something slightly abrasive? or maybe need to backflush from the solvent vent back into the inlet if there are some high boiling contaminates stuck in there.

That makes perfect sense, and there almost certainly carbonization in there. The thing has been through a lot of heavy use. I'm nervous to actually SCRUB though. I don't want to scratch the stainless. All the cleaning instructions I've seen have just been to swab out with solvent, which doesn't get me any noticable color coming out. Any suggestions on actually de-gunking the metal part of an inlet without adding a bunch of permanent active sites to the thing?

That makes perfect sense, and there almost certainly carbonization in there. The thing has been through a lot of heavy use. I'm nervous to actually SCRUB though. I don't want to scratch the stainless. All the cleaning instructions I've seen have just been to swab out with solvent, which doesn't get me any noticable color coming out. Any suggestions on actually de-gunking the metal part of an inlet without adding a bunch of permanent active sites to the thing?

I have cleaned normal split/splitless weldments with jeweler's rouge or Bar Keeper's Friend before, just have to be sure to wash out all the residues. Those are fine enough grit to actually polish the surface. Thinking about the PTV the wall is probably thin to allow the fast heating so probably would not use something like a wire brush, but you could use a pipe cleaner and some of the alumina cleaning powder from the MS cleaning kit.

The tailing though could be from the passage to the split vent line, which you could clean with a pipe cleaner and solvent. Also the bottom of the inlet is replaceable so might want to try another fitting on that too.

Check actual gas flows out through the split vent and the septum purge (if that is still active with a septumless head). It is also worthwhile to check whether the baseline drifts are due to the PTV heating or to the gas saver by doing a run with no heat and a run wth no gas saver. Try changing the retention gap - there might be a bit of crus in there.

Peter