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From YourSITE.com Microfiltration (MF). GE Jun 15, 2005, 01:20
Microfiltration can be defined as the separation of particles of one size from particles of another size in the range of approximately 0.01 m through 20 m. The fluid may be either a liquid or a gas. Microfiltration media are available in a wide variety of materials and methods of manufacture. They can be rated either "absolute" or "nominal" depending upon the percentage of capture of particles of the same size or larger than the retention rating of the media. Membrane filters are generally rated as absolute media. They can be manufactured of various polymeric materials, metals and ceramics. Nominal media includes filters made of glass fibers, polymeric fibers, discrete particles (diatomaceous earth), ceramics, etc. However, even absolute media can be considered absolute only within a finite time span because of the possibility of bacterial grow-through. Microfiltration membranes can be divided into two broad groups based on their pore structure. These are membranes with capillary-type pores, hereafter called screen membranes, and membranes with tortuous-type pores, hereafter called depth membranes. Figure 1 shows a scanning electron micrograph of the surface of a screen, or capillary pore, membrane. This membrane has nearly perfect round cylindrical pores, more or less normal to the surface of the membrane, with even random pore dispersion over the surface. Screen membranes are absolute and are commercially available in thin films of poly-carbonate and polyester. They are manufactured in a two step nuclear track and etch process. They are preferred in a wide variety of applications including optical and electron microscopy, chemotaxis, exfoliative cytology, particulate analyses, aerosol analyses, gravimetric analyses and blood rheology. Figure 2 How polycarbonate screen membrane filters are made: In the first step, thin plastic film is exposed to ionizing radiation forming damage tracks. In the second step, the tracks are preferentially etched out into pores by a strong alkaline solution. Figure 3 is a scanning electron micrograph of the surface of a typical depth, or tortuous pore, membrane. This membrane has a relatively rough surface where there appears to be many openings considerably larger than the rated pore size. Depth membranes are nevertheless absolute, depending upon the random tortuosity of their numerous flow paths to achieve their pore-size rating. Depth membranes are commercially available in pure silver, PVC, PVDF, PTFE, various cellulosic compounds, nylon, polyethersulfone, polypropylene and many other materials. Most depth membranes are manufactured of various polymeric materials using a casting machine. Membranes cast with cellulosic esters are the most widely used membranes. Referring to figure 4, cellulosic membranes are manufactured by dissolving the cellulose esters in a mixture of organic solvents; adding various chemical agents for improved characteristics; and casting the solution as a film approximately 150 m thick onto a moving belt. As solvents are evaporated under controlled conditions, the tortuous pore structure is formed. The resulting open area ranges from 75% to 89%. Membranes of this highly-porous structure with its labyrinth of interconnecting isotropic pores are recommended for general precision filtrations, electrophoresis, sterilization of fluids, culturing of microorganisms and for many other uses. PTFE depth membranes are manufactured by the controlled stretching of a fluorocarbon sheet. Some polypropylene membranes have also been manufactured by this method. The silver membrane is manufactured of pure metallic silver particles that are molecularly bonded to each other to form a uniform porous monolithic structure. A major application for silver membrane filter is inorganic material analyses. With the difference between screen and depth membranes, it is clear that the characteristics of the two types of membranes would allow each to have significant advantages and disadvantages. For optimum results, membrane users should consider all characteristics in selecting which (or both) of the two types of membranes should be used. Particle Retention Particles are captured directly on the surface of the screen membrane. However, screen membranes retain with certainty only those particles the same size or larger than the pore size of the membrane. Except for inertial impaction and diffusion, most particles smaller than the pore size pass unimpeded through the screen membrane. The screen membrane should also be selected if the user wants low non-specific binding (maximum yield of particles or proteins in the filtrate). This is important, for example, when viruses are being separated from a growth solution and the maximum yield of viruses is desired. Binding of proteins in screen membranes has been found to be less than 10 percent that of depth membranes. With depth membranes, most particles are captured within the interstices of the membrane except for relatively large particles. Since depth membranes depend upon the tortuosity of their flow paths for capture, they will trap not only particles of the same size or larger than the rated pore size, but also many particles below that rated pore size. Should the user want maximum removal of all particles and /or a high binding capacity, the depth membrane should be selected. The depth membrane has a much larger available surface area than the screen membrane; therefore it has a much larger particle loading capacity and many more sites where proteins and viruses can bind. Flow Characteristics Screen membranes have no side-to-side flow due to their capillary pores, therefore they are unsuitable for electrophoresis and other applications requiring this characteristic. Depth membranes have excellent side-to-side flow. Flow rates for the two types of membranes are roughly equivalent. Although the depth membrane has more open area, the screen membrane is thinner, 10 m vs. 50 to 120 m. Other Major Characteristics Both membranes are non-migrating. Both types of membranes can be autoclaved. Cryogenic temperatures have little or no effect on either membrane. The screen membrane is completely non-hygroscopic. Many depth membranes absorb moisture from the air and must be dried before use, especially in some critical analytical procedures. Both screen and depth membranes are generally hydrophilic except for PTFE and polypropylene which are inherently hydrophobic. Both types of membranes can be made partially or completely hydrophobic. Chemical resistance of the various kinds of membranes depends upon the material from which they are manufactured. PTFE has the best chemical resistance, followed by PVDF, silver, polyester, polycarbonate, and finally the various cellulosic compounds. Common dyes do not stain or discolor polycarbonate or polyester screen membranes; however this must be considered for membranes manufactured of cellulosic compounds. Major Characteristics of Screen Membranes - Pore size and structure are well defined - Particle size cut-off is sharply defined - No media migration - Smooth, flat surface for SEM, TEM and optical analysis. - Non-hygroscopic - Thin, retains little liquid - Low adsorption, low absorption - Low non-specific binding (3-10 grams/cm) - Non-staining - Strong - Repeatedly autoclavable Major Characteristics of Depth Membranes - Large surface area - High dirt-loading capacity - Long life - No media migration - Good handling characteristics - Repeatedly autoclavable - High binding capacity (100-250 grams/cm2) |
