Separation ranges

Ultrafiltration separations range from ca 1 to 100 nm. Above ca 50 nm, the process is often known as microfiltration. Transport through ultrafiltration and microfiltration membranes is described by pore-flow models. Below ca 2 nm, interactions between the membrane material and the solute and solvent become significant. That process, called reverse osmosis or hyperfiltration, is best described by solution—diffusion mechanisms. 

The separator must be stmcturaHy sound to withstand the rigors of battery manufacturing, and chemically inert to the lead—acid cell environment. Numerous materials have been used for separators ranging from wood, paper, and mbber to glass and plastic. The majority of separators used are either nonwoven—bound glass or microporous plastic such as PVC or polyethylene. 

Histogram A diagram of the frequency of occurrences of values of a variable, grouped according to value in a number of separate ranges. 

The third line of development was to increase the selectivity in order to achieve the highest possible resolution to address difficult separations. This may be achieved by a very narrow pore size distribution of the media, e.g., such as achieved by porous silica microspheres (PSM) or by modifying the porous phase by a composite material, e.g., as for Superdex. In practice, this material shows a maximum selectivity over the separation range (e.g., see Fig. 2.2). 

When applied to the SEC column, the calibrated polydisperse polymer solution provides a large number of data points in a single run. Use of a standard with a molecular size distribution that encompasses the full separation range for the column allows the entire separation range to be calibrated in a single run (Fig. 2.4). 

For the purpose of high-resolution fractionation, the gel medium must be tailor made to cope with different separation ranges. The Superdex family is designed for the high resolution of peptides and proteins having a molecular mass of 500 to 100,000. Also, Sephacryl media have found wide applicability as a final polishing step in process scale SEC (see Section III,C). 

The selectivity of a gel, defined by the incremental increase in distribution coefficient for an incremental decrease in solute size, is related to the width of the pore size distribution of the gel. A narrow pore size distribution will typically have a separation range of one decade in solute size, which corresponds to roughly three decades in protein molecular mass (Hagel, 1988). However, the largest selectivity obtainable is the one where the solute of interest is either totally excluded (which is achieved when the solute size is of the same order as the pore size) or totally included (as for a very small solute) and the impurities differ more than a decade in size from the target solute. In this case, a gel of suitable pore size may be found and the separation carried out as a desalting step. This is very favorable from an operational point of view (see later). 

TABLE 4.1 Properties and Separation Ranges for TSK-GEL SW Type Packings"... 

TSK-GEL PW type columns are commonly used for the separation of synthetic water-soluble polymers because they exhibit a much larger separation range, better linearity of calibration curves, and much lower adsorption effects than TSK-GEL SW columns (10). While TSK-GEL SW columns are suitable for separating monodisperse biopolymers, such as proteins, TSK-GEL PW columns are recommended for separating polydisperse compounds, such as polysaccharides and synthetic polymers. 

TABLE 4.6 Properties and Molecular Weight Separation Ranges for TSK-GEL PW Type Packings°... 

Larger particle sizes of each group are for 21.5 mm X 60 cm semipreparative and 55 or 108 mm X 60 cm preparative columns. Maximum separation range determined from estimated exclusion limits. 

Por analytical purposes, TSK-GEL PWxl columns are preferred. Por preparative work, or for other cases in which large amounts of sample must be used, TSK-GEL PW columns are recommended because of their larger loading capacity. To select the proper TSK-GEL PW type column for a particular sample, consult the separation ranges listed in Table 4.6 or the calibration curves in Pig. 4.11. 

Column type Column size (mm) Particle size ( zm) Theoretical plate number Separation range (IcDa) Pore size (A) Flow rate (ml/min) Maximum pressure (kgf/cm ) Maximum temperature pH range Eluent ionic strength (M)... 

Separation Ranges of PSS SDV Columns FIGURE 9.3 Separation characteristics of PSS SDV columns for organic eluents. 

FIGURE 9.5 Comparison of molar mass separation range and separation efficiency of linear mixed-bed (upper curve) and single pore size column (lower curve). 

To select a column for a particular analytical problem, the first step is to make a choice about the pore size(s) to be used for the separation. In general, one cannot expect that a single pore size will fulfill the needs of a separation. In size exclusion chromatography, it is more common that columns of different types are combined with each other to deliver the separation range needed for a particular analysis. Therefore, column banks with different pore sizes are frequently combined with each other to maximize the separation power for... 

Molar Mass Interval. GPC columns are offered for different molar mass intervals for larger intervals it is possible to combine some columns of different pore size types or to combine a few so-called mixed bed or linear columns. Both possibilities have their own special advantages and disadvantages mixed bed columns with a linear separation range of more than four molar mass decades are suitable to quickly get an overall view of a new sample, whereas a column set, carefully selected from different pore size types, often has a much better separation efficiency in a limited mass interval (for details, see Sections in and IV). 

Separation ranges for Sephacryl (S-type) gels are listed in Table 16.3. 

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