Molecular weight size exclusion

In contrast to electrodes and oxides with more or less smooth surfaces, the surface of chromatographic material is usually porous with special binding sites or cavities that allow for a separation according to the molecular weight (size-exclusion mechanism) or the chemical affinities. 

An alternative to surface sizing is internal sizing, the addition of a sizing agent to the wet end before the formation of the paper sheet. In this process step, however, exclusively low molecular weight sizing agents like rosin acids or alkyl ketene dimers (AKD) are applied. 

A series of polyvinylpyridine standards of different molecular weight were analyzed by size-exclusion chromatography, yielding the following results. 

An example of a size-exclusion chromatogram is given in Figure 7 for both a bench-scale (23.5 mL column) separation and a large-scale (86,000 mL column) mn. The stationary phase is Sepharose CL-6B, a cross-linked agarose with a nominal molecular weight range of 5000-2 x 10 (see Fig. 6) (31). 

The elution volume, F/, and therefore the partition coefficient, is a function of the size of solute molecule, ie, hydrodynamic radius, and the porosity characteristics of the size-exclusion media. A protein of higher molecular weight is not necessarily larger than one of lower molecular weight. The hydrodynamic radii can be similar, as shown in Table 4 for ovalbumin and a-lactalbumin. The molecular weights of these proteins differ by 317% their radii differ by only 121% (53). 

Fig. 14. Molecular weight characteristics of novolac resins. Shown is the size-exclusion chromatogram for a typical commercial novolac polymer. The unsymmetrical peak shape reflects the multimodal molecular weight distribution of the polymer.

The molecular weight of SAN can be easily determined by either intrinsic viscosity or size-exclusion chromatography (sec). Relationships for both multipoint and single point viscosity methods are available (18,19). Two intrinsic viscosity and molecular weight relationships for azeotropic copolymers have been given (20,21) ... 

Hydroxyl number and molecular weight are normally determined by end-group analysis, by titration with acetic, phthaUc, or pyromellitic anhydride (264). Eor lower molecular weights (higher hydroxyl numbers), E- and C-nmr methods have been developed (265). Molecular weight deterrninations based on coUigative properties, eg, vapor-phase osmometry, or on molecular size, eg, size exclusion chromatography, are less useful because they do not measure the hydroxyl content. 

Size-exclusion chromatography (sec) easily and rapidly gives the complete molecular weight distribution and any desired average (6). Thus, it has become the technique of choice for determining molecular weights despite its relatively high initial cost. 

Except for the high molecular weight range, nearly all substances can be separated by reversed-phase (RP) HPLC. The many different separation mechanisms in RP HPLC, based on hydi ophobic, hydi ophilic and ion-pairing interactions, and size exclusion effects together with the availability of a lai ge number of high quality stationary phases, explain its great populai ity. At present approximately 90% of all HPLC separations are carried out by reversed-phase mode of HPLC, and an estimated 800 different stationai y phases for RP HPLC are manufactured worldwide. 

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