Manfred,
Your mountain sounds very much like a bubble in the flow cell. You might
try changing the wavelength to the visible and look into it with a
magnifying lense. To be certain it is not coming from the type of PVDF,
you might try washing the PVDF with 0.1% triethylamine in MeOH, then with
MeOH and finally with water.
For your old data, on the 610 software, I think you can still use the
filtered data mode which contains a low frequency filter which will help
remove the large "mountain" so you can quantitate the PTH's easier. Be sure
to calibrate the standards with the same filtered mode.
Best of luck,
Gary