The obstruction factor in size-exclusion chromatography. 2. The interparticle, intraparticle, and total obstruction factors

"Obstruction factor" is a generic rubric under which are usually gathered the interparticle, intraparticle, stationary phase, and total obstruction factors, γ(e), γ(p), γ(s), and γ(t), respectively. These, in turn, affect longitudinal diffusion and stationary, mobile phase, and stagnant mobile phase mass transfer. We conclude here our investigation into the various obstruction factors operative in size-exclusion chromatography (SEC). Stop-flow experiments were employed to determine either the interparticle (for analytes with K(SEC)=0) or the total (for analytes with K(SEC)>0) obstruction factor, and these results were combined with those from variable-flow-rate experiments which provided the intraparticle obstruction factor. Because of minimal enthalpic interactions between the analytes and stationary phase, in SEC γ(s)≈0, which allows for isolation of the other obstruction factors. A relationship between γ(t), γ(e), and γ(p) was proposed for SEC, based on previous independent work and dependent upon the various column porosities. This relationship was extended to hydrodynamic chromatography, a technique in which, ideally, both γ(s) and γ(p) are equal to zero.

Axial and radial diffusion coefficients in a liquid chromatography column and bed heterogeneity

The axial and transverse diffusion coefficients of a band of iodine in a chromatographic column were measured optically as a function of time. It was found that the axial diffusion coefficient remains constant even when the edges of the sample band get close to the wall. By contrast, the radial diffusion coefficient decreases progressively with increasing time when the edges of the sample band leave the core region and begin to diffuse inside the wall region. The local axial and transverse diffusion coefficients of the band decrease from the column center toward the wall. Hence, the increase in local height equivalent to a theoretical plate observed in the region close to the wall must be explained by increasing mass transfer resistances and degree of heterogeneity of the bed.

A study of the efficiency of polybutadiene-coated zirconia as a reversed-phase chromatographic support

This work describes the dynamic characteristics of columns packed with polybutadiene (PBD)-coated zirconia. Reduced plate height (h) vs reduced velocity (v) data for alkylbenzenes were acquired for three phases coated with different amounts of PBD. Additional data for benzene (k' = 0) on uncoated and four coated phases were also collected. These h vs v plots have a minimum between 4 and 5, indicating good but not excellent column efficiency. By fitting the Knox equation to these data, the A (packing quality), B (axial diffusion), and C (mass transfer resistance) coefficients were determined. The A coefficients were about 2 in all cases, and the B coefficients had values between 2 and 3. The C coefficients varied from 0.05 to 0.2, depending on the polymer load. The data suggest that the quality of packing and the efficiency of mass transfer resistance in the "film" of polymeric stationary phase, not the stagnant mobile phase, are key to further improvements in column performance. The amount of polymer loaded significantly affects the efficiency through the mass transfer resistance in the stationary phase.