Supercritical Fluid Chromatography Applied to the Characterization of a Siloxane-Based Gas Chromatographic Stationary Phase

Abstract

Supercritical fluid chromatography using n-pentane as a mobile phase and Porasil C as a column packing was applied to the characterization of Dow-Corning 710 fluid, a commonly used gas chromatographic stationary phase. Under the experimental conditions, decomposition of the polymer was observed on columns of alumina, and derivatized Porasil packings were found to be unstable. Linear pressure programming variables were studied at 206 and 216°C by normalizing the total analysis time on a fixed length of column. A series of homologous polymer components was used to demonstrate the effects upon resolution of density-isotherm nonlinearity associated with the mobile phase. The results showed that optimal temperature conditions existed for the use of linear pressure programming, with the choice of temperature conditions dependent upon the molecular weight range. Operational guidelines are suggested. Mixed solvent behavior was investigated for this separation. The effect of changing the polarity of the mobile phase by adding 0 to 20% v/v isopropanol to n-pentane was studied under isothermal and isobaric conditions. The alcohol was shown to change adsorption characteristics and alter critical constants. The effect upon the critical constants appeared to conform to Kay's approximation. The results suggest that mixed solvents can be used to speed up separations for mixtures having components of widely different adsorption energies. The pseudocritical concept using Kay's approximation was tested for its qualitative ability to estimate state effects when solvents are mixed. The effect of temperature upon migration behavior was compared for equimolar amounts of isopropanol and methanol, each in n-pentane. Kay's approximation appeared to be useful for qualitative predictions regarding changes in critical pressure. However, an attempt to predict density isotherm behavior for 20 % isopropanol in n-pentane relative to pure n-pentane suggested that Kay's approximation may not be as useful for predicting criticla temperature effects.