Individual pore size

FIGURE I l.l Resolution of a single pore size column and a mixed-bed column. Individual pore size columns deliver a significantly higher resolution than mixed-bed columns. The theoretical resolution of a single pore size column is compared to the resolution obtainable with a mixed-bed column. (Courtesy of Waters Corp.)... 

FIGURE 11.2 Selectivity of several individual pore size columns. This graph shows the resolution capability of Styragel HT 3, HT 4, HT S, and HT 6 columns. The selectivity [Eq. (I)] of these columns vs the molecular weight of polystyrene is plotted. (Courtesy of Waters Corp.)... 

For organic SEC separations the use of polystyrene/divinylbenzene (PS/ DVB) particles is almost universal throughout the industry. Polymer Laboratories PS/DVB material, PLgel, which is produced in a series of individual pore sizes, formed the basis for the original product line of SEC columns. Developments in the refinement of particle sizing introduced the benefits of smaller particle size and more efficient columns, which significantly reduced SEC analysis time through a reduction in the number of columns required for... 

A range of individual pore size PLgel packing materials is produced and their pore size distribution is conveniently represented by a SEC calibration curve as illustrated in Fig. 12.1. It should be pointed out that the descriptors used for the different pore sizes, 50 A, 100 A, and so on, are not the actual pore sizes of the beads but relate to the size of a polystyrene molecule just excluded from the packing material. This nomenclature comes from the original work carried out by Moore (3) and should only be viewed in the context of differentiating... 

FIGURE 12.1 SEC calibration curves for PLgel individual pore size columns (300 X 7.5 mm), eluent THE at 1.0 ml/min, polystyrene calibrants. 

Exclusion limits and practical operating ranges specified for PLgel individual pore size columns are summarized in Table 12.1. Table 12.1 illustrates the versatility of this family of columns. Pore volume can also be expressed as... 

TABLE 12.1 Operating Ranges for PLgel Individual Pore Size Columns... 

Each of the PLgel individual pore sizes is produced hy suspension polymerization, which yields a fairly diverse range of particle sizes. For optimum performance in a chromatographic column the particle size distribution of the beads should be narrow this is achieved by air classification after the cross-linked beads have been washed and dried thoroughly. Similarly, for consistent column performance, the particle size distribution is critical and is another quality control aspect where both the median particle size and the width of the distribution are specified. The efficiency of the packed column is extremely sensitive to the median particle size, as predicted by the van Deemter equation (4), whereas the width of the particle size distribution can affect column operating pressure and packed bed stability. 

Individual pore size columns have variable pore volume, and because the column dimensions are fixed, the combination of different columns must result in variable slope of the overall calibration curve and hence variable degrees of resolution as a function of molecular weight. 

The user must have a good understanding of the individual calibration curves for each individual pore size column in order to avoid mismatching them, which can result in artifacts in polymer distributions. 

The use of mixed gel technology has become widespread since the late 1980s. If the column is well designed it should not he necessary to supplement resolution in certain areas of the operating range (particularly the extremes) hy the addition of individual pore size columns. This practice is not necessary or recommended for PLgel mixed gel columns, although it is a common practice for other commercial products. 

The main criterium for column selection is pore size distribution as it is desirable to have maximum pore volume for separation in the molecular weight range of interest. Having determined the upper molecular weight limit required, a column with a suitable exclusion limit should be selected. In the case of individual pore size columns, it is then a question of selecting other columns with complementary calibration curves to comprise a column set covering the re-... 

These problems can be associated with the fact that various pore size SEC packings, either in a bank of individual pore size columns or in a mixed gel column set, respond differently in this extremely polar solvent. In addition the... 

Application to polymer analysis Combination of individual pore sizes (columns or gels) to cover required molecular weight range Molecular weight range covered by single pore size... 

Table 1 Commercial PS/DVB Packings for Organic SEC Individual Pore Size Packings...

The first criterion for column selection is the molecular weight of the sample to be analyzed. For some applications in which resolution is required over a relatively narrow molecular weight range, individual pore size packings are suitable. This is particularly the case for small-molecule separations, as shown in Figure 5. For polymer analyses in which resolution is required to cover several decades of molecular weight, mixed gel or linear columns are widely applicable. 

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... 

Fig. 3. Calibration curves for columns of PLgel for organic SEC at ambient temperature (a) a range of individual pore size columns designated 50 A to 10E6 A, (b) a range of mixed gel columns designated A to E. Eluent THF, flow rate 1 mL/min, calibrants polystyrene standards.

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