You have your protein sample but it has salts and low molecular weight proteins you need to remove. Dialysis tubing and dialysis devices are the answer. Using dialysis membranes you can desalt and remove LMW proteins for volumes from 10µl to 400ml. Molecular weight cut off (MWCO) ranges from 3500 Daltons to 100 kDa.
Dialysis is a simple matter of clamping the bottom of the tubing, placing your sample inside the tubing, removing the air, clamping the top of the tubing and floating the sample in your buffer of choice. Gentle stirring and time will allow the LMW constituents to diffuse through the tubing into the buffer and the buffer will replace the unwanted products. It may be necessary to change the buffer to remove all of the LMW species.
Spectrum Labs carries many dialysis products including tubing, dialysis tubing bags, and dialysis devices which require no clamps or weights. Pierce Protein Research Products have many devices for dialysis including devices for sample volumes down to 10µl, microwell dialysis plates for many samples, cassettes for up to 75ml, and tubing for higher volumes.