In general, if two species have the same hydrodynamic volume then they can not be separated by SEC. If they have a different hydrodynamic volume then increasing resolution in GPC can be accomplished by any of the following methods. I list the associated disadvantage in brackets:
1. Add an additional column (longer run time)
2. Use a smaller particle or more efficient packing material (higher pressure, more danger of shearing the polymer)
3. Change column porosity to give more resolution in the region of interest (best method but may limit separation in other MW ranges)
The last option that you may want to explore is removing the placebo component. If its chemistry can be identified then it may be possible to eliminate it using an appropriate method of sample prep (SPE, L/L). I hope this helps.