Unknown peak from ammonium sulfate at 260 wavelength

[hahe0104]

I have a unknown peak that absorbs at 260 and is being caused by ammonium sulfate. I am using ACS reagent grade, assay greater then 99%, I am seeing it in both J.T Baker and Sigma ammonium sulfate. Has anyone had this problem before? and does anyone know what could possibly be in the ammonium sulfate or reacting with it to cause this absorption? I am using 10 mM potassium phosphate buffer as my mobile phase pH 3. This peak just so happens to have the exact same retention time as my peak of interest. Any suggestions would help.
Thanks

[tom jupille]

Gradient or isocratic?

[Gerhard Kratz]

Did you run a UV spectrum on a UV photometer of your Ammonium sulfate and your potassium phosphate buffer?