negative baseline during gradient run

[niknosnen]

The last 3 years i am running the same routine analysis with a C-18 column using a gradient from 20% to 100% ACN within 20 min and monitoring at 250 nm, & 440 nm. The last couple of weeks when running after 2-3 min the baseline at 250 nm gets negative progressively. Monitoring spectral changes with the DAD during the run it seems that negative curve in the spectra is observed from 210nm till 300. I have purged the system and cleaned it and use appropriate water grade. Deterium lamp has ~1900 h. Can anyone suggest something?
Nikos

[tom jupille]

If the flow cell has gotten out of alignment, that will exacerbate "RI mismatch" drift, which would be consistent with what your are seeing.

[niknosnen]

If the flow cell has gotten out of alignment, that will exacerbate "RI mismatch" drift, which would be consistent with what your are seeing.

I checked the purity of my mobile phase and then did dark current correction and array calibration and the problem with the baseline was solved as the gradient baseline was then as usual
Thanks
Niko