SEC column packing

FIGURE 9.11 For each PSS SEC column, packing pressure and permeability are monitored for the best reproducibility of column performance. 

As in interactive modes of chromatography, reduction in particle diameter reduces mass transfer effects and improves column efficiency in SEC. Column packings with particle diameters of 10 to 12 pm are available for less demanding applications, whereas SEC packings with particle diameters of 4 to 5 pm can be used for applications requiring higher resolution. 

Most synthetic polymers are analyzed in organic solvents, using appropriate SEC column packings in which the only interaction between the macromolecular solute and the packing is steric. Separation of the polymeric species is inversely related to their hydrodynamic volumes because the flow paths of the larger species... 

Retention in SEC. If we consider the separation of a number of monodispersed polystyrene standards of differing molecular size in a given SEC column packing. The retention of the standards can be represented by a graph of molecular size versus retention volume Kr (Figure 9.2). 

Reducing particle size of the SEC column packings reduces the time requirements in SEC because of the increased mass transfer and resultant separation efficiency. Hence, columns can become smaller in dimensions while maintaining resolution. This approach has been used for many years. Column bank lengths dropped from several meters to now typically 60 cm with current SEC column particle sizes of 5 pm as compared with about 100 pm in the early 1960s. During the same period, time requirements dropped from about 6 hr to less than 1 hr. 

Figure 6. Calibration curves for SEC columns packed with CPG (D = 31.0nm Vp= 1.12cm /g dp = 12/rm) (solid line) and silica gel (D = 29.1 nm Vp = 1.09cm /g dp = 10/rm) (dashed line). Column 300 x 6 mm. Polystyrene calibration standards. Mobile phase-tetrahydrofuran.

Only few companies supply SEC columns devised for separation of polar polymers in polar eluents. SEC of basic polymers still represents a challenge because interaction of mactomolecules with the available colunrn packings is rather strong. On the other hand, hydrophilic SEC column packings are well available. Many acidic polymers can be reliably separated also with help of otherwise well interactive silica gel packings. 

The attributes of a SEC column packing material are listed in Table 2. As indicated, the support must be optimized with respect to specific resolution, efficiency, column pressure, and mechanical, chemical, and thermal stability. Recovery of mass and activity is particularly important in the analysis and purification of biopolymers. It also plays a role in the analysis of nonbiochemical synthetic polymers on silica-based SEC columns. In addition to recovery losses by adsorption, the recovery for both groups of polymers can also be reduced by polymer degradation as a result of, for instance, mechanical shear. 

Reducing particle size of the SEC column packings reduces the time requirements in SEC because of the increased mass transfer and resultant separation efficiency. 

Polyacrylamide has been used as a base material for soft SEC column packings. The new products have high stability to the applied column inlet pressure. According to the supplier, this series of columns can be used even with organic solvents such as dimethyl sulfoxide, tetrahydrofuran, and acetone. The announced properties of the columns are listed in Table 8. 

SEC column packing materials are generally represented by a characteristic calibration curve produced using a series of polymer standards. Typical calibration curves for individual pore size columns, as illustrated in Figures 3a and 4 for SEC columns with organic and aqueous eluents respectively, exhibit a... 

In SEC dimensions, the transfer of mixed mobile phases can affect molar mass calibration. In order to obtain reliable molar mass results, the calibration curves have to be measured using the extremes of mobile phase composition and tested for changes in elution behavior and pore-size influence in the SEC column packing. The better the thermodynamic property of the SEC eluent, the less influence is expected on the SEC calibration when the transfer of mobile phase from the previous dimension occurs. It has been shown to be advantageous to use the SEC eluent as one component of the mobile phase in the previous dimension to avoid potential interference and mobile phase incompatibility. 

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