The negative peaks seen in your example chromatogram are typical peaks seen when the TCD or TCD controller are faulty. Agilent certified evaluation and repair would be suggested.
Hello, now the instrument baseline seems ok, just changing the Agilent Gas Clean Filter CP17973
I use Helium as carrier gas and the MOISTURE section was saturated; I was reading online that the TCD sensor performs bad in "wet" conditions
I've ordered also a GENIE 170 FILTER : I won't install it for now, at least until the baseline keeps flowing good