Negative peaks at tail of analytes suppressed IC system

[Winters]

We are seeing negative peaks occurring on our Metrohm suppressed Anion IC system. We have tried a new column, have cleaned and checked our injection system, tried multiple sources of eluent and standards, and cleaned the suppressor system according to metrohm spec.
The negative dip occurs at the tail of most of the analytes we are separating (not SO42-, perhaps because it is a broad peak?). We only integrate for Cl- and SO42- and it is interfering with our Cl- calibration curve. The problem persists in all of our standards/ working samples and gets more pronounced with higher concentrations. Our working samples consist of Denatured Fuel Ethanol (no dilution) and Antifreeze (100:1 dilution). At this point we have tried everything that we can think of and are at a complete loss.
Any suggestions? Thanks!

[Markus Laeubli, Metrohm]

Dear Winters
Could you please attach example chromatograms. Or even better, could you send me such by e-mail (lae@metrohm.com).
Please also let me know your system setup (instrument and software version).
I then will have a closer look to the problem.

[Winters]

With the help of Markus and his colleague we were able to determine that the issues all stemmed from a contamination in our suppressor system. The contamination was traced back to poor quality water in our MP. Issue has been fully resolved and new water source has been obtained.
Thanks to all!