The process of increasing the number of particles per unit volume of a solution, usually by evaporating the liquid. The concentration process relies on a pressure gradient over a membrane, which disallows the analyte molecules. High relative molecular mass substances are generally rejected by ultrafiltration membranes. However, the consistent application of reverse osmosis is that the separation of low relative molecular mass substances from aqueous solutions to treat the water or to centralize the substances which are to be resolved.