Long chain hydrocarbon contamination help

I use a 8890 series GC with MSD, FID, TCD. Splitter 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. 45 min run time

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?

  • It's harder to troubleshoot with the post column splitter installed.  Do you have the dimensions of the two restrictors - one between the splitter and the MS and one between the splitter and the FID?  Is it a purged splitter or an unpurged splitter?

    Dodecane's boiling point is only 216.2°C.  Are you sure that's the late eluting compound? It should be long gone from your system if the inlet, column, and transferline are hotter than that.  Straight chain, or mostly straight chain, hydrocarbons ionize easily and so many bigger molecules may library search as decane or dodecane as the molecular ion is either incredibly small or not there and fragmented to 57, 71, 85, etc.

    usermanual-gc-maintenance-8890-g3540-90015-en-agilent.pdf   starting on page 133.

    I would suggest to cool down the inlet and oven.  Remove the column from the inlet in the oven and cap it by carefully pressing it a few millimeters into the edge of a septum so that the MS is not sucking air.  Clean the MMI following the procedure in the GC manual. Replace the septum, do not overtighten it, replace the liner and use a new O ring.  Cut at least a few more inches off of the column and install it again. The column should be 12-14 mm past the end of the ferrule, at least two to four mm higher than the manual recommends is better.

    After the inlet and column are all back together and the inlet pressure is back to normal, wait at least ten minutes to get all of the oxygen out of the system before heating up the injection port and the oven.  Then heat the inlet and the column up to normal operating parameters.  The inlet is typically run at the same temperature as the final oven temperature, but the O ring is only good to 300°C, so if you need to run it hotter it requires the graphite seal.    Let it run at those temperatures for at least 1/2 hour, and then run a blank solvent injection using your normal run method and see how it goes.  

    Some have to do this level of inlet maintenance once per week or more.

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