Pore-Size Effects on Column Performance in Gas-Liquid Chromatography

Abstract

A series of porous glasses with uniformly controlled pore size have been used to experimentally determine the effect of pore size and liquid loading on column performance in gas-liquid chromatography. The HETP values for columns prepared from these glasses are compared to results obtained with two common diatomite supports. Similar retention times were obtained in all cases by adjusting substrate loading to equalize the amounts of liquid contained in each column. Heptane, a nonpolar molecule, was employed at a column temperature of 100°C to minimize interaction of the test compound with the column packings. For glasses with pore diameters greater than 600 angstrom and for diatomite materials the elution peaks were symmetrical, and minimum plate heights of less than 1.0 mm were obtained. At low substrate loadings the glass packing materials (pore diameters 1200–4000 angstrom) showed efficiencies which were essentially independent of pore size. At any given liquid loading the minimum HETP values obtained with these glasses were comparable to those obtained for Chromosorb P (pore diameter range 10.0–0.1 μ) and less than those for Chromosorb W (pore diameter range 20.0–2.0 μ). This comparative increase in efficiency is attributed to the higher surface areas of the smaller-pored materials. This results in a thinner liquid film on the support and a consequent increase in column performance. The greater surface area of the small-pore materials also enables effective distribution of the liquid phase to occur at high loadings. Good efficiencies were thus observed at these high loadings together with increased capacity. “Apparent equilibrium constants,” K, have been calculated for all columns and these values compared with the calculated liquid film thickness. If the film thickness is greater than approximately 33 angstrom, K is essentially constant. Below this value, however, “apparent K values” increase rapidly as the liquid film becomes thinner and adsorption forces, therefore, become more evident. When the pore diameter of the glass packing material was less than about 600 angstrom, the elution peaks for heptane at 100°C were no longer symmetrical. This asymmetry can be ascribed either to the presence of nonspecific wall interactions, surface area effects, or most likely to discontinuities in the distribution of the liquid substrate. Increasing the liquid loading diminishes these effects, and the elution peaks become more symmetrical.