| Abstract: | Experiments coupled with a systems analysis were conducted on chromatogram dispersion, or zone broadening, in gel permeation chromatography (GPC). Three components of a Waters Associates Model 200 chromatograph, each of which is a potential cause of dispersion, were considered; these are: the packed column, with extraparticle dispersion only; the empty tubing, between pump and columns, columns and detector, etc.; and the detection system, viz., the differential refractometer cell. Toluene solvent was used and the solutes whose dispersion was studied included orthodichlorobenzene (ODCB) and narrow-molecular-weight polystyrene standards having molecular weights of 900, 20,400, 51,000, 97,200, and 160,000. Nonporous glass beads, 50 μ in diameter, were used as column packing. Two diameters, 1 mm and 0.5 mm, of stainless steel tubing were studied. In addition to the usual rectangular pulse sample injection, a step input mode for solute introduction was also used. The empty tubing was found to contribute significantly to the degree of dispersion and to skewness of elution curves. Anomalous bimodal characteristics of the elution curves were also observed which could only be ascribed to the empty tubing. These phenomena depended markedly on parameters such as tube diameter and length, and solute concentration and molecular weight. Dispersion in the packed column, although important, was found to be symmetrical (Gaussian) and less sensitive to these parameters than in the empty tubing, especially with respect to molecular weight. Dispersion in the cell was believed to be insignificant relative to the packed column and empty tubing. |
