Dagmar Leyendecker

Chromatography with sub- and supercritical eluents: influence of temperature, pressure and flow-rate on the behaviour of lower alkanes

Abstract The chromatographic behaviour of n-pentane, n-butane, isobutane and propane was studied in the liquid, gaseous and supercritical gaseous states by running chromatograms of a mixture containing four polynuclear aromatic hydrocarbons. Both the capacity ratios and the resolutions reach maxima at temperatures above the boiling or the critical temperature at a given constant pressure. The maxima are the more pronounced the lower the alkane and the higher the molecular weight of the substrate. This temperature-dependent course of resolution at constant pressure is influenced mostly by changes in plate numbers. On increasing the pressure, the capacity ratios and resolution decrease.

Chromatographic behaviour of various eluents and eluent mixtures in the liquid and in the supercritical state

Abstract The influence of temperature, pressure, and density, as well as type and composition of the mobile phase on capacity factors and on resolution was studied. .In general, a shorter analysis time and lower value for the resolutioon were observed when increasing either pressure, density or modifier content or when working at temperatures just below the critical point. However, there are combinations of distinct values for temperature, pressure, and composition, where capacity factors and resolutions reach maximum values. Knowledge of these combinations of values opes a variety of possibilities to optimize separations.

Comparison of Eluents in Supercritical Fluid Chromatography

Abstract The chromatographic behavior of a number of low boiling eluents has been compared in supercritical fluid chromatography (SFC) with packed columns. Unmodified silica gel has been used as the stationary phase and a test mixture of polycyclic aromatic hydrocarbons (PAHs) as the substrate. Carbon dioxide, nitrous oxide, trifluoromethane, chlorotrifluoromethane, n-pentane, n-butane, i-butane, propane, ethane, diethylether, and dimethylether have been investigated as mobile phases.

Influence of density on the chromatographic behaviour of lower alkanes as mobile phases in supercritical fluid chromatography

Abstract In supercritical fluid chromatography with pentane and propane as the mobile phases, the capacity factors of polycyclic aromatic hydrocarbons on unmodified silica gel decrease with temperature at constant density, i.e. , under isochorous (isodense) conditions. Equal density leads also to equal linear velocity of the mobile phase and to equal dead time, independent of temperature. At lower densities the dependence of the resolution on temperture passes through a maximum, while at higher densities the resolution decreases monotonously, like the capacity factors. Varying the density at constant temperature results in a decrease in resolution with increasing density in pentane, while in propane a resolution maximum may be found if the temperature is kept just above the critical temperature. A comparison between propane and pentane at the same reduced density and reduced temperature shows that propane leads to higher retention and higher resolution than does pentane.

Possibilities for optimization oligomer separation with supercritical fluid chromatography

Abstract Based on the dependence of elution on the pressure and temperature under supercritical conditions, strategies are developed for improving oligomer separation by supercritical fluid chromatography. Temperature programming is shown to be applicable if the appropriate temperature region is chosen. Optimization strategies are also given, both for pressure-programmed separations and for separations with gradient elution. In particular, multiple gradient techniques are demonstrated to be capable of enhancing the efficiency of oligomer separations.

Three-dimensional network plots in supercritical fluid chromatography with n-pentane 1,4-dioxane

Abstract Using n -pentane-1,4-dioxane as a binary eluent system in supercritical fluid chromatography (SFC) and silica as the stationary phase, three-dimensional network plots of capacity ratios, k ′, selectivities, α, effective plate numbers, N , resolutions, R , of four polycyclic aromatic hydrocarbons versus column temperature eluent composition at constant column pressure were obtained. Although the effects of the physical parameters temperature, composition pressure on the chromatographic properties k ′, α, N or R of the binary eluent are found to be complex in detail, general information can be derived from the three-dimensional plots. The individual shapes of all three-dimensional surfaces are similar inasmuch as they are characterized by elevations located above the boiling temperature, T b , when the pressure is subcritical, or above the critical temperature, T c , at supercritical pressures of the eluent mixtures. Differences exist between k ′, α, N and R with respect to the individual shapes and details of the elevations. Whereas above T b or T c the selectivities, α, for instance, remain high relatively independent of composition, the other chromatographic parameters change considerably with composition. An increase in pressure always reduces the overall height of the elevations.

Supercritical fluid chromatography using mixtures of carbon dioxide or ethane with 1,4-dioxane as eluents

Abstract In supercritical fluid chromatography (SFC), the permanent chemical modification of a silica stationary phase which occurs when eluent mixtures containing 1,4-dioxane are used leads to decreased retention with ethane and carbon dioxide as the mobile phases. Contrary to ethane, with carbon dioxide improved resolution is obtained on the dioxane-modified silica stationary phase. The chromatographic parameters capacity factor, selectivity, and mean resolution for a test mixture of aromatic compounds showed maxima as a function of temperature. The intensity of these maxima decreased with increasing pressure and increasing amount of 1,4-dioxane in mixtures with carbon dioxide or ethane. Furthermore, their positions were shifted to higher temperatures with increasing pressure and with increasing dioxane content, the latter effect being due to the much higher critical temperature of dioxane compared to carbon dioxide and ethane. The data presented can be used for optimizing SFC separations with mixed mobile phases.

Influences of temperature, pressure and eluent composition on the behavior of binary eluents at sub- and supercritical conditions. Pentane and diethyleneglycol dimethyl ether

SummaryIn supercritical fluid chromatography (SFC), studies on binary eluents composed of pentane and diethyleneglycol dimethylether were performed in the concentration rage of 5 to 15% of the ether compound and at pressures of 20 to 50 bar. Influences of temperature, pressure and eluent composition on the behavior of the mobile phase were found to closely resemble those for pentane-dioxane mixtures. Resolution depended on variations in capacity ratios, selectivities, and plate numbers. The critical data for diethyleneglycol dimethylether were estimated by means of different group contribution methods.

Influence of temperature, pressure and eluent composition on the behaviour of binary eluents under sub- and supercritical conditions : Pentane and 1,4-dioxane

Abstract In supercritical fluid chromatography, the temperature- and pressure-dependent behaviour of the chromatographic parameters for binary eluent mixtures, containing pentane as the primary component and 1,4-dioxane as a secondary component, is similar to that or pure eluents. The isobars show maxima above the critical temperature for capacity factors and for resolution. With increasing content of the secondary component in the mixture, the critical temperatures rise and the maxima move to higher temperatures. The maxima are more pronounced at lower coloumn pressures. Maxima appear also when plotting versus eluent composition at constant temperature and pressure. All maxima occur at both sub- and supercritical pressure. Temperature—pressure—composition triple points can be found where the capacity factors or the resolution reach maximum values. This can be used for optimizing separations.

Elution behavior of styrene oligomer fractions in supercritical fluid chromatography

SummaryIn supercritical fluid chromatography (SFC), the elution behavior of styrene oligomers in pentane depends greatly on the degree of oligomerization, n. The influence of capacity ratio, k′, on the lower oligomers up to about n=11 resembles the behavior of other substrates of low molecular weight, like aromatic hydrocarbons. Thus, with pentane of 100 bar and below, minima are found in plots of k′ versus temperature. In contrast, the k′ of oligomers of higher molecular weight (n>11) increase monotonously with increasing temperature. The studies were performed using oligostyrene fractions obtained by semipreparative SFC fractionation.