LC/MSMS in negative mode phenomena

Hi everybody! I perform lc/msms analysis of antibiotics(agilent 6430 MS). I have some unexplained issue. When all dmrm in positive polarity, I get good baseline like this:

But when I added negative dmrm (for chloramphenicol), baseline of exctracted positive dmrm changed:

I changed RT window for chloramphenicol to white it all the time, but baseline problems still exist:

Please help me understand this phenomena and maybe someone can give me advice how to avoid it.

All chromatograms wrote without injection. 

Thank you!!!

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