Trouble Separating Polymeric Analytes

Hello everyone. I've been trying to separate a polymeric Cephalosporins and I'm having trouble achieving separation.  I think most of what makes up the polymer is a dimer, but I suspect there are trimers, tetramers, etc. The sample material is a solid that was isolated from a solution and redissolved in diluent. Previously I've tried an SEC method, but it looked like a very broad peak. Currently I'm trying a reverse phased method with the following parameters:

Column: Hypersil ODS C18, 5 um, 4.6 x 250 mm
Mobile Phase A: 5 mM potassium phosphate, pH 6.5 with 9%ACN
Mobile Phase B: 5 mM potassium phosphate, pH 6.5 with ACN (50:50)
Diluent: Mobile Phase A (Added dilute KOH to completely dissolve sample)
Column Temp: 40C

With the parameters above I have the chromatograms below. There is one sharp peak, but the base line is not flat

Can anyone recommend how I can optimize this method?

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