In your case, mjc123:, what you may have wanted was diafiltration, a combination of dialysis and filtration. For example, its known that large biological macro-molecules will form a gel layer on a filter that prevents further filtration, but my maintaining a laminar flow and repeatedly re-filtering, we can remove ions and concentrate macro-molecules.
Now that I think about, this may be what the O.P. wants: to partition the particles into greater than nano-sized, nano-sized and soluble molecules and ions, so they can reject the first and third fractions and concentrate the middle fraction. Maybe diafiltration, or simple dialysis is what's needed.