BAND DISPERSION IN HIGH-PERFORMANCE EXCLUSION CHROMATOGRAPHY

The dispersion of polymer samples eluted in the exclusion mode from columns of Hypersil and SG60F (a highly porous silica gel) has been measured for a range of polymer standards over a range of elution velocities and from columns of different lengths. From experiments with columns of different lengths operated at the same velocity, the polydispersities of the polymer standards were obtained and ranged from 1.007 to 1.052. From experiments at different linear velocities, mass transfer coefficients were obtained from which the ratio of the diffusion rates of polymers in bulk eluent to the diffusion rates within the silica gel matrix could be found. For Hypersil these ranged from ca. 6 for PS2K and PS4K (MW 2000 and 4000 respectively) to ca. 17 for PS33K (MW 33,000) indicating fairly severe restrictions to diffusion within the pores of the matrix especially for nearly excluded materials. Measurements with a pure oligomer of polystyrene (MW ≈ 3000) confirmed the validity of the above method of separating spreading effects of polydispersity and kinetics and gave a reduced plate height-velocity curve overlapping that for benzene. Measurements of spreading of excluded polymer samples indicated anomalous behaviour at low flow-rates and with long columns. Measurements of the loading capacity in exclusion chromatography showed that samples should not exceed ca. 1 μg per cm3 of column packing if overloading is to be avoided. It is concluded that the dispersion of bands of polymers eluted in the exclusion mode from silica gels obeys the same kinetic relationships as the dispersion of bands in retentive chromatography. The study confirms, however, that the major contributor to the dispersion of polymer samples eluted in the exclusion mode is their polydispersity. © 1979.