Long Chain Hydrocarbon contamination help

[rosemaryandsage47]

I use a Aglient 8890 series GC with MSD, FID, TCD. Spliter from column to MSD & TCD or FID

Multimode Inlet. 30m DB-5 Column. MMI 325C, FID 325C. 10 C/min ramp until 325 and hold for 15 min.

It's 98% used for QC using TCD & MS. However there is an FID/MS method we use occasionally for a different matrix (has since been moved to an instrument devoted to FID)
I ran the FID/MS method a few weeks ago and my colleague noticed some late eluting long chain hydrocarbons (dodecane, decane etc) in the FID chromatogram. Only the first injection is visible in the MS TIC. They decrease as the run progresses.
Today I devoted the day to isolating the contamination source.

1) Ran a non-inject blank after baking column for 30min at 300C: contamination is there, visible via MS, stronger in FID.
2) ran a non-inject blank with a new liner: contamination still there about the same as injection 1
3) Trimmed ~5cm: Contamination very high
4) Dupliate 'Injection' of 3: contamination is reduced significantly
5) Trimmed additional ~30cm and baked column for 30 min at 325C: Contamination still there but reduced significantly compared to the initial 5cm trim. No MS signal.

My conclusion is this: I have some high MW compounds in the front end, likely the column but possibly the MMI as well. Each subsequent injection has progressively lower levels of the compounds.
Do I trim the column even more? Should I also clean the MMI?

[Consumer Products Guy]

When I was working, we'd choose one of the following paths. Keep in mind that the company had provided a decent supplies budget.

1. Install a brand-new column; we always had a spare new column in the drawer anyway.

2. Backflush the column with methylene chloride, then dry out with helium or nitrogen; we had a commercial apparatus for this. Most columns are bonded and can be backflushed, especially DB-5.

[James_Ball]

You may want to add a high capacity hydrocarbon trap to the carrier gas line just before the GC. I have found traces of hydrocarbon in even Ultra High Purity gasses.
If you suspect the MMI then you can cap off the outlet and set to a high split flow and ramp the temperature up to about 350C and see if that knocks it out. Not being able to change the bottom of the inlet in those can lead to more contamination problems, but they heat up fast and if you have good o-rings you can to to 350c for a few minutes without any problems.

[cjm]

If trimming the column helped, trim it some more. Also, if you haven't removed and solvent flushed the split lines, I'd recommend doing that as well.

[DR]

If trimming the column helped, trim it some more. Also, if you haven't removed and solvent flushed the split lines, I'd recommend doing that as well.

I'm thinking that more trimming is a temporary fix - you have to clean and/or add a good trap to cut off the source if you're going to keep using the method.

[rosemaryandsage47]

You may want to add a high capacity hydrocarbon trap to the carrier gas line just before the GC. I have found traces of hydrocarbon in even Ultra High Purity gasses.

We have an Agilent Clean Carrier Gas Filter that was replaced in April. It's the combo filter with water, O2, and charcoal hydrocarbon filter.

I cleaned the MMI yesterday and trimmed a total of 1m of column. The impurities are down but still present.

What would it mean if the hydrocarbons are high when you run a splitless injection? I'm thinking it's possibly the split vent lines. The method I'm using right now is a 20:1 split. I also ran a sample at a lower inlet temp. That reduced the impurities even more.

What's weirder is that there is another GCMS on the same helium tank but doesn't use the MMI that is seeing these peaks via FID. Totally different method; it uses a TD for sample introduction. I'm wondering if you are right about contaminated gas. I've been working on GCMS's for 10 years now and haven't seen contaminated Helium (yet).

I'm going to run my FID method on that GC (it has a MMI, just not used) when it's free next week.

Very frustrating.

[James_Ball]

Hydrocarbon contamination in Helium is present since helium is produced by filtering it from natural gas wells.

Normally it is at a very trace level but can build up in any cold spots in the plumbing or on any adsorbent within the system which can be seals or carbon contamination or other things which can adsorb hydrocarbons. Once saturated these will release the hydrocarbons, sometimes at a steady rate and sometimes with pulses of higher concentration. If you run an MS on helium for years then switch to hydrogen carrier you will see massive amounts of hydrocarbon in the background spectra for days or weeks until the hydrogen cleans all the deposited hydrocarbons out of the plumbing.

The one advantage to the MMI is being able to ramp it up to higher temps to help bake it out before starting a run with it. But if contamination is in the EPC incoming gas line you will still be seeing it later on. If you run high split and it goes away, then that usually means it is coming from the inlet side of things and being flushed out with the split flow. If it stays the same with higher split then possibly it is in the analyzer side.