[phth]

Running a c18 column water to methanol: What causes a 215 nm trace to slowly decrease in intensity as the gradient goes from water to methanol? my guess is salt ions?

[anarquimico]

My first thought is if the methanol is HPLC grade and if you're using ultrapure water.

If that's the case, my second guess is that you're simply looking the water absorbance in the UV.