Pore sizes, of filters

Fig. 6.4. Effects of the pore size of filter paper used during fluid sampling on the analytical concentrations reported for aluminum and iron (Kennedy el al., 1974). Samples were acidified, stored for 19 ( ) or 94 ( ) days, and analyzed by standard wet chemical methods. Dotted lines show dissolved concentrations determined by a solvent extraction technique.

Modifications of the method include manual or automated cell feeding in 24-well plates, media modifications, possible coating of filter surfaces, pore size of filter membranes. Other modifications include quality assurance criteria (TEER, permeability values). 

It was pointed out that tbe solids content does not indicate die absolute amount of solids in the oil, because the subtnicron particles of char present after frllration are difficult to measure. However, it was concluded that this analysis is accurate enough for its present purpose. The microscopic analysis of the oils showed a high amount of small particles below 1 pm. The pore size of filter paper may be reduced down to 1 pm. It was also stated diat compounds originated from bark or needles do not dissolve well in alcohols. A more powerful solvent, like methanol-methylene chloride (1 1) can be used. 

A schematic depiction of the sizes of aggregates containing organic carbon in the ocean. The distinction between particulate organic carbon (POC) and dissolved organic carbon (DOC) is operationally determined by the pore size of filters (usually 0.45 pm) used to separate dissolved from particulate material. Constituents of the POM and DOM are indicated, along with methods used to distinguish them. 

Where = elasticity,/= amount of suspension extruded in x min (e.g. 5 or 10 min), r =liposome size after extrusion and rp = pore size of filter membrane. 

Figure 10.1 The sizes of some natural particulate materials found in surface-water and groundwaters, and the pore sizes of filters used to remove them. After W. Stumm and J. J. Morgan. Copyright 1981 by John Wiley Sons. Reprinted by permission of John Wiley Sons, Inc.

It is convenient to compare performances of powder filters on the basis of (/) filtration efficiency, ( ) ease of sterilizing the unit, ( 7) ease of automation, (/v) avoiding beer dilution, (v) ease of spent powder removal and (vi) dryness of the spent powder discharged [12]. Table 20.3 gives details of such a comparison. Flow rates are usually about 4-5 hl/m but slower rates ensure more efficient particle removal. The average pore size of filter sheets is 4-6 (xm, that of leaf septa is 45-70 (xm and the slits in a candle are 50-90 (xm. 

Filtration Depends on size of solid being greater than pore-size of filter Widely used to protect equipment such as pumps and nozzles (for example, burner nozzles) and to recover solid product from liquid (for example, cells from fermentation liquor or waxes during processing of crude oil... 

Filtration Size of solid greater than pore-size of filter Removal of dust... 

Mlcrofiltra.tlon, Various membrane filters have been used to remove viral agents from fluids. In some cases, membranes which have pores larger than the viral particle can be used if the filtration is conducted under conditions which allow for the adsorption of the viral particle to the membrane matrix. These are typically single-pass systems having pore sizes of 0.10—0.22 lm. Under situations which allow optimum adsorption, between 10—10 particles of poHovims (28—30 nm) were removed (34—36). The formation of a cake layer enhanced removal (35). The titer reduction when using 0.10—0.22 p.m membrane filters declined under conditions which minimized adsorption. By removal standards, these filters remove vimses at a rate on the low end of the desired titer reduction and the removal efficiency varies with differences in fluid chemistry and surface chemistry of viral agents (26). 

Spunbonded fabrics are effective filters in that they are layered stmctures of relatively fine fibers, the three-dimensional stmcture of which creates a torturous path. Even relatively thin spunbonded fabrics (eg, 0.2—0.25 mm) present a significant challenge to the passage of soil fines and are suitable for use in some filtration appHcations. The porosity of geotextile fabrics is classified by means of several procedures such as flux (volume flow/area per time) and equivalent opening size (EOS), which is a measure of the apparent pore size of the openings in the fabric. The flux measures the porosity to Hquid water, and the EOS measures the porosity to soHd particles of a known diameter. Literature is available on limitations of particular styles of fabrics within an apphcation (63). 

About 5 ml of sample is withdrawn for every 4-6 hours. The absorbance reading of the sample at 580 nm was measured using a Hitachi U-2000 spectrophotometer. The sample is filtered in a vacuum through Whatman filter paper with a pore size of 2.5 pin and diameter of 47 mm. The dry weight of cells is measured to monitoring microbial cell population and cell density. A plot of optical density reading from the spectrophotometer against cell dry... 

FIGURE 1 Effect of (sequential) extrusion of MLV dispersions through polycarbonate membrane filters (Unipore) with pore sizes of 1.0, 0.6, 0.4, 0.2, and 0.1 ym on the mean liposome diameter. DXR-containing MLV (phosphatidylcholine/phosphatidylserine/ cholesterol 10 1 4) mean diameter of nonextruded dispersion about 2 ym pH 4. Mean particle size determined by dynamic Light scattering (Nanosizer, Coulter Electronics). (From Crommelin and Storm, 1987.)... 

This section specifically concerns itself with filters that must be used that do release fibers. It is noted that an additional filter with a maximum pore size of either 0.2 or 0.45 pm must also be used to finish the filtration. This is an absolute requirement with asbestos filters used because of the total concept of safety and effectiveness of the drug. This section simply sets the time limit for instituting good filtration procedures for parenteral products. 

BDS process. The pore size of the filter (0.2-1.0 xm) is selected such that the liquid phase, which is miscible with the liquid that is used to wet the filter, passes through the filter, while the second liquid phase remains. Thus, an aqueous filter is wet with a liquid, which is miscible with water, but immiscible with oil. The flow rate is chosen so as to prevent solid deposition through the filter. Although, such a separation process can be applied to any oil/water emulsion, it was particularly envisioned as part of a BDS process. One may ask, whether it would be more efficient to break a macroemulsion by filtering than it is by any other means Second, in the case of microemulsions, how efficient would such a filtration process be ... 

As described before, the pore size of porous material ranges widely from atomic size to millimeter order. Different pore sizes are required for different applications of porous materials. Most porous materials do not have uniform pores. Pore size distribution is also an important property. Narrow pore size distribution, i.e., uniform pore size, is required for instance for filters and bioreactor beds. Mercury porosimetry and gas adsorption methods are commonly used to measure pores size and pores distribution. 

The prototype shell-and-tube type cross-flow filtration modules (Pall Corp.) used for filtration tests are welded into a stainless steel shell enclosure. The modules have an inlet (filtrate) and outlet (retentate) port (both at tube sides) with Vi-inch tubing ends, and a permeate port, located near the midpoint of the shell side of the unit. The stainless steel filter membranes have a nominal pore size of 0.1 pm. The surface of the filter media is coated with a proprietary submicron layer of zirconia. 

Cross-flow filtration systems utilize high liquid axial velocities to generate shear at the liquid-membrane interface. Shear is necessary to maintain acceptable permeate fluxes, especially with concentrated catalyst slurries. The degree of catalyst deposition on the filter membrane or membrane fouling is a function of the shear stress at the surface and particle convection with the permeate flow.16 Membrane surface fouling also depends on many application-specific variables, such as particle size in the retentate, viscosity of the permeate, axial velocity, and the transmembrane pressure. All of these variables can influence the degree of deposition of particles within the filter membrane, and thus decrease the effective pore size of the membrane. 

Acidifying the sample causes colloids and fine sediments that passed through the filter to gradually dissolve, yielding abnormally high concentrations of elements such as aluminum, iron, silicon, and titanium when the fluid is analyzed. Figure 6.4, from a study of this problem by Kennedy et al. (1974), shows how the pore size of the filter paper used during sample collection affects the concentrations determined for aluminum and iron. 

Figure 1 Freeze-fracture electron micrographs of egg phosphatidylcholine large unilamellar vesicles prepared by extrusion through polycarbonate filters with pore sizes of (A) 400 nm, (B) 200 run, (Q 100 nm, (D) 50 nm, and (E) 30 nm. The bar in panel (A) represents 150nm. Source From Ref. 7.

The extruder (Northern Lipids) is assembled with two polycarbonate filters (Nuclepore polycarbonate membranes Whatman) with pore size of 0.1 pm and diameter of 25 mM, and connected to a circulating water bath equilibrated at 65°C. The lipid emulsion is extruded 10 times through the filters under a pressure of approximately 400 psi. For larger LUVs (200-400 nm), lower pressures will be adequate (100-200 psi). After each pass, the sample is cycled back to the extruder. It is important to start at a low pressure and gradually increase until each pass takes less than one minute. 

Figure 3 Molecular relaxivities of liposomes with different Gd-containing membranotropic chelators. Liposomes (egg lecithin cholesterol chelator = 72 25 3) were prepared by consecutive extrusion of lipid suspension in HEPES buffered saline, pH 7.4, through the set of polycarbonate filters with pore size of 0.6, 0.4, and 0.2 mm. Liposome final size was between 205 and 225 nm. Gd content determination was performed by Galbraith Laboratories, Inc. The relaxation parameters of all preparations were measured at room temperature using a 5-MHz RADX nuclear magnetic resonance proton spin analyzer. The relaxivity of liposomes with polymeric chelators is noticeably greater because of the larger number of Gd atoms bound to a single lipid residue [16].

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