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?