Recently did full maintenance on GC/MS; cleaned the source, replaced the gold seal, inlet liner, septa as well as eventually installed a new column with a new guard column (we were starting to observe lots of ghost peaks and bleed since it had been in use for now a while). I'm observing a contamination peak with 129 and 147 main ions, which seems to correspond to Bis(2-ethylhexyl) adipate - https://imgur.com/us01aBd - both in split and splitless mode. Pretty confident it's from the GC side since if I do a run while keeping the temperatures of the inlet and oven low (50-60), I do not observe this peak (plus the peak is consistent in its retention time).
Interestingly enough, when running a method with a higher starting oven temperature, the abundance of the peak decreases until it pretty much disappears if the starting temperature is above ~175-200 degC. The peak also seems to be absent if I run while heating the oven, but keeping the inlet temperature off. So, thinking this might be an inlet issue, I then replaced the split vent trap, cleaned the inlet itself, as well as the vent line, and replaced both septa and liner again - also baked out the column some before re-installing. I am still seeing this contamination peak! Note that all runs were conducted without actual injections.
At this point, I feel I have replaced everything what am I missing? Anyone know what this peak could be characteristic of?