GC-FID Mineral oil Baseline

[CrisTheChemist]

Hi Chromatographers,
I have a trouble with GC-FID analysis of mineral oils C₁₀-C₄₀ in water, soils, waste (ISO 16703). It is an instrumental problem not related to sample type or extraction.
I have been doing this analysis for many years, but now I have this problem that I have never seen before.

Sometimes the chromatogram of the blank or a std or a sample shows a negative band that goes below the starting level of chromatogram hence I can not integrate the classic large peak at the baseline.

I have just changed the column, the liner, the jet assy of the FID, but some days the problems doesn't appear, whereas other days the problem exist.
Do you have some similar experience?

I can share some chromatograms but I need your email address.

Really thank you