Membrane filters are surface or screen filters. In this type of filtration only the surface is used to filter the solution. Typical pore sizes range from 0.2um (micrometers) to 1.0 um. Membrane filters can be made of cellulose (cellulose acetate, cellulose nitrate), Nylon, Polyethersulfone (PES) PTFE (Teflon*)… Membrane filtration is used very often for sample preparation to ensure that particles above a certain size do not pass through to equipment. Track etched membranes are used to trap cells of interest to be later studied under a micoscope. Teflon* membrane filters are used with acidic or basic samples and samples in strong solvents. PTFE membrane filters are also used to filter air vents where sterility must be mantained such as in fermentation vessels. Membrane filters are also supplied as syringe filters or capsule filters eliminating the need for a filter holder. Whatman, Pall and Nalge Nunc are some of the manufacturers of membrane filters.
*Teflon is a trademark of DuPont Corp.