Separator column bimodal

Zorbax PSM Bimodal and Trimodal columns are packed with mixed pore-size packing to achieve linear size separations over a broad molecular weight range (Table 3.3). Zorbax PSM Bimodal columns are packed with PSM 60 and PSM 1000 particles, and Trimodal columns contain PSM 60, PSM 300, and PSM 3000 particles (Fig. 3.4). Carefully selecting and mixing different pore-size particles in columns provide much better linearity than coupling columns that are each packed with single pore-size particles. 

Whereas in the example just described the sample amount was about 50 mg, a similar procedure developed by another group 129) started with 4 g polyethylene copolymer. The sample was applied as a dilute solution in xylene and precipitated by very slow cooling (1.5 K/h) onto the Chromosorb P packing of a 500 x 127 mm column. The first separation was temperature-rising elution fractionation at a flow-rate of 20 ml/min and a Unear temperature increase by 8 K/h. The MMD of the fractions was measured by SEC at 145 °C in o-dichlorobenzene at 0.7 ml/min flow rate. The column set included a pair of bimodal columns 100 A and 1000 A plus a 4000 A column. The apparatus was equipped with an IR detector. The experimental data is computed to show the distribution of short-chain branching and of molar mass simultaneously. 

Almost all stationary phases used in chromatography have a bimodal pore size distribution. The first mode corresponds to the macropores or throughpores that allow the percolation of the column by the stream of mobile phase. The second distribution corresponds to the mesopores that combine to give the conventional internal porosity distribution described in the previous section. The mesopores are responsible for most of the specific surface area necessary to provide the retention and the saturation capacity that are needed to permit the retention of the mixture components in a good solvent, a condition for chromatographic separation. Nonporous particles have been used with only moderate success because very weak solvents must be used to achieve sufficient retention, which often causes solubility problems, and the saturation capacity of these particles is small. The terms of macro- and meso-pores apply as well to columns made of packed particles and to monolithic columns. 

We believe the most feasible explanation of the bimodal behavior in linewidth to be two distinct molecular weight fractions. The inability to separate the two may be due to an equilibrium between them, or unknown inadequacies in the gel filtration techniques used. The B-polysaccharide sample which we passed through an analytical column (l cm x 90 cm packed with sepharose liB) was diluted to the extent that the precise data required for a histogram analysis was difficult to obtain. 

The potential impact of detector wavelength on the resolution of separation is illustrated in Figure 10.9. Shown are two HDC chromatograms for a bimodal mixture of 38- and 176>nm polystyrene standards which were obtained on the same column, under the same operating conditions, but measured at two... 

The effect of pore size of the packing on the mass transfer kinetics of biopolymeric solutes in HPLC column separations has been a subject of intensive studies (as an example see Ref 27). To optimize the mass transport properties of the supports, the focus was directed to packings with a bimodal pore size distribution having mesopores (diffusional pores) and macropores (transport pores). This has led to a new design of porous packings suitable for fast biopolymer separations at a preparative scale that exhibit a high dynamic capacity for biopolymers [28,29]. 

The separation of PS homopolymers on a C4 bimodal pore diameter reversed phase column has been studied. Separation of PS on a molecular weight basis was achieved using a THF-acrylonitrile system [114]. 

Monolilhic silica columns for high performance liquid chromatography (HPLC) are prepared by a unique sol-gel process utilizing polycondensation of silica and phase separation (Nakanishi, 1991). The monolithic silica columns have a bimodal pore structure, i.e. micropores and mesopores, which contribute to high separation efficiency of the liquid chromatography (Ishizuka, 2000 Lubda, 2002). 

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