Possible Contamination on a Backflush GC System

I started working on a method utilizing this GC which is a custom made GC and started having what looks to be a contamination issue.  The path for this method is a 10 port sample valve, a precolumn which is a shincarbon, and a second 4 port valve leading to a shincarbon column connected to a PDHID.  We are able to backflush and heart cut with this set up.  When I inject helium on the first injection the baseline looks normal, any injection after the first has what looks to be a series of unresolved peaks appears in every run after.  I've tried baking out many times at 180 and up to 250C, I've tried injecting blanks at 250.  It will not go away.  I'm using a simple temperature ramp starting at 50C and increasing to 250C at 10C/min.  It occurs in same area unless some variable like temp ramp rate is changed. I'm thinking it's contamination.  Is there anything else that could be causing this??? If the method is set so that the first valve is set to turn back to off immediately after injecting the baseline disturbance does not appear.  And same goes if the second valve is immediately switched to on so that sample from first valve is directed to vent.  This makes me suspect that the contamination is in the precolumn.  I can provide more information if needed and a diagram of the flow path and valves.

First injection:

Second injection

Was this helpful?