Matrix filters, fibers

Fig. 3 SEM of the random fiber matrix of a depth filter. (Courtesy of Sartorius AG.)...

Another type of disk, SPEC made by Ansys, Inc., uses a glass-fiber matrix rather than Teflon to hold the sorbent particles. This disk has a somewhat more rapid flow rate and is more rigid and thicker than the Teflon disk. There is another disk called the Speedisk made by J. T. Baker that consists of lO-pm packing material that is sandwiched between two glass-fiber filters without any type of Teflon binder. The various types of disks are discussed in detail in Chapter 11. 

However, if a small amount of colloidal silica is added, the negatively charged silica particles flocculate the excess starch as well as the starch coated fibers. The large floes of fibers are readily and efficiently filtered from the water. In addition, the floes of excess starch are trapped within the fiber matrix where they can act as binder. This increased efficiency in filtration has the effect of leaving the water phase much less contaminated with libers and starch which presents much less of a disposal problem. This is illustrated in Figure 12.52 and Figure 12.53. 

Industrial Applications Color filters black matrix liquid crystal displays photoresist conducting polymer films optical fiber pH sensor printed circuit boards inks textiles ... 

Plant material. Weigh 25 g of the chopped and frozen sample into a blender jar. To confirm recoveries, prepare fortiflcation samples by spiking the matrix with the appropriate volume of metabolite standard. Add 200 mL of acetonitrile-water (4 1, v/v) solution to the jar, and blend the mixture at medium speed for 5 min. Filter the extract through a Buchner funnel fitted with a glass-fiber filter pad into a 500-mL round-bottom flask containing 10 drops of Antifoam B and 3mL of 10% aqueous Igepal CO-660 (nonionic surfactant). The flask is connected to the Buchner funnel by... 

Weigh 2.5 or 5 g of crop matrix into a blending vessel. Fortify samples at this point with the appropriate analytical standards. Allow the solvent to evaporate. Add 100 mL of acetone-water (4 1, v/v) and blend the mixture using an Omni mixer equipped with a macro generator for 5 min at 6000-7000 rpm. Filter the sample through a Whatman 934 AFI glass-fiber filter paper on a Buchner funnel/vacuum flask setup. Rinse the blending cup and filter cake with 100 mL of acetone. Transfer the filtrate into a 200-mL TurboVap vessel. 

Filtration can remove fine suspended solids and microorganisms, and microfiltration membranes of cellulose acetate or polyamides are available that have pores 0.1-20 /xm in diameter. Clogging of such fine filters is an ever-present problem, and it is usual to pass the water through a coarser conventional filter first. Ultrafiltration with membranes having pores smaller than 0.1 fim requires application of pressures of a few bars to keep the membrane surface free of deposits, water flows parallel to the membrane surfaces, with only a small fraction passing through the membrane. The membranes typically consist of bundles of hollow cellulose acetate or polyamide fibers set in a plastic matrix. Ultrafiltration bears some resemblance to reverse osmosis technology, described in Section 14.4, with the major difference that reverse osmosis can remove dissolved matter, whereas ultrafiltration cannot. 

Filters can be divided into two types membrane (screen) filters and depth filters. Membrane filters, such as silver membrane filters, physically screen and retain particles on their surfaces. These filters have uniform pore sizes and are rated for absolute retention all particles larger than the pore size are retained. Depth filters, such as glass-fiber filters, consist of a matrix of fibers that form a tortuous maze of flow channels. The particulate fraction becomes entrapped by this matrix. These filters do not have a uniform pore size, and it is not possible to rate them for absolute retention. They are rated according to nominal pore size, which is determined by the particle size that is retained by the filter to a predetermined percentage. This percentage is usually given as 98 retention however, it can be as low as 90. ... 

One efficient removal procedure is to use a 0.45- m filter. There are basically two types of filters depth and screen. Depth filters are randomly oriented fibers that will retain particles throughout the matrix rather than just on the surface. They have a higher load capacity than screen filters. Due to the random nature of the matrix, they have no definite upper-limit cutoff particle size retained. Their porosity is identified as a nominal pore size to indicate this variable. 

Acetylcholineesterase Biosensors were fabricated from filter-supported solventless bilayer lipid membrane (BLM) and used for the analysis of the substrates of hydrolytic enzymes in a flowthrough system. The codeposition of lipid (dipalmitoyl-phosphatidic acid) and protein solutions to form a BLM on a microporous glass fiber or polycarbonate ultra-filtration membrane disc was described. Enzyme was immobilized on the membrane by incorporation of protein solution into the lipid matrix at the air-electrolyte interface before BLM formation. 

A major alternative to direct flow membrane filtration is depth filtration, in which particles are removed throughout the filtration matrix rather than just at the membrane surface, by various mechanisms such as size exclusion, electrostatic, and hydrophobic interactions. Depth filters are typically composed of a bed of cellulose or polypropylene fibers together with an inorganic filter aid such as diatomaceous earth and a binder to form a filter sheet. The filter aid imparts the matrix very high surface areas and plays an important role in increasing both retention and the capacity. Depth filters can also have an electrostatic charge usually associated with the binder polymer. 

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