Column Sizing

Figure 6.5 shows a chromatogram of Epikote 1004 using the KE-600 series and SYSTEM-24H (a newly developed GPC system used with the KF-600 series for full utilization of the efficiency of the downsized columns). Compared with conventional-sized columns such as the KF-800 series, the time of analysis is shortened to one-half and the chromatographie resolution is improved. 

FIGURE 9.4 Selecting columns according to molar mass range of sample the small pore size column has a separation volume of 1.3 ml (and partial exclusion above 200 kDa) and the other column is well selected with a separation volume of 2.7 ml. 

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). 

Figure 9.7 shows separations under identical conditions using PSS SDV columns with 3 (Fig. 9.7a)-, 5 (Fig. 9.7b)- and 10 (Fig. 9.7c)-/i,m particle size columns. A polystyrene oligomer standard was injected and all analyses were performed in THF as the eluent. The much higher efficiencies of small particle size columns are obvious, which is important in the SEC separation of low molecular weight compounds such as additives, by-products, and resins. The reader should note that all chromatograms are area normalized and have the same Y axis to show the differences in peak width and height. 

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.)... 

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... 

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... 

FIGURE 19.4 Elution of cross-linked polymer from 5- and 20-/u.m particle size columns. 

Tables 21.3 and 21.4 show the results of our evaluation on a column set that we felt performed very well. These tables address criteria 1 through 5 described previously. We judged the values listed to be very acceptable for high temperature GPC applications. For room temperature applications, where a smaller particle size column could be used, better values would be expected.

The larger macromolecules can be separated using larger particle size columns. However, the flow rate should be watched carefully. As the effective hydrodynamic size of the macromolecules may be reduced due to the deformation by shear (23). Figure 22.8 shows that the effective hydrodynamic size of a 12-15 X 10 MW polyacrylamide sample will not reach its maximum, or the size without shear, unless the flow rate is reduced to 0.01 ml/min. A... 

The HdC calibration curves of different particle sizes, as shown in Fig. 22.12 (30), are similar to the calibration curves of different pore size columns the separation ranges of MW due to hydrodynamic chromatography depend on particle size. The larger the particle size, the higher the MW ranges. Stegeman et al. (30) proposed that a smooth calibration curve may be achieved by proper ratio of the particle diameter to the pore diameter. 

MW separation factor Pore size Column i.d. Interstitial void size Thermal gradient... 

In analytical SEC, the optimal pore size for a given polydisperse polymer is approximately 1/4 < IRJd — 1/2 when a single-pore size column is used (20). In contrast, the pore size needs to be much smaller in HOPC. Small pores are deliberately used in HOPC to exclude nearly all MW components at low concentrations but to allow the entry of low MW components only at high concentrations by the high osmotic pressure (2). Use of the same pore size as used in SEC results in a poor separation. The latter is essentially an overloading in SEC. 

It is better to use smaller packing (less than 1 in.) in this diameter column (16-in. I.D.), such as 5 in. if recalculated above. Packing of 14 in. or % in. are better in this size column however the effects of packing factor should be calculated if changed from % in. packing. 

Figures 9-33A and B illustrate the fit of data taken by FRI on a commercial size column for hydrocarbon systems, using No. 2.5 Nutter rings at three different pressures, and comparing the latest Nutter proprietary correlation previously presented.

Commercial size columns using Series X or Series S packing will operate at full efficiency with liquid loads as low as 5 gal/hr/ft column area, and sometimes less liquid rate. They can operate satisfactorily exceeding liquid rates of 2,650 gal/hr/ft. See Figure 9-49. 

Figure 4. SEC calibration curve for silica sol separation (hard sphere particles, single pore size column) (1), Column PSM-1500 (8.9 fim), 30 X 0.78 cm mobile phase O.IU Na.HPO -NaH.PO, pH 8.0...

For analysis, the HPLC mobile phase gradient started at 3 17 acetonitrile-0.15% acetic acid in water and ended at 9 1 acetonitrile-0.15% acetic acid in water (0-32min). The HPLC column was a Zorbax RX-C8, 2.1-mm i.d. x 150mm, 5-p.m particle size column with a flow rate of 0.2mLmin and a 100-p.L injection volume. 

The collection efficiencies shown in column 4 of the table were read from Figure 10.45a at the scaled particle size, column 3. The overall collection efficiency satisfies the specified solids recovery. The proposed design with dimension in the proportions given in Figure 10.44a is shown in Figure 10.48. 

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