Ernst Klesper

Separation of oligomers and polymers by supercritical fluid chromatography

Abstract The separation of oligomers and polymers by supercritical fluid chromatography (SFC) is reviewed. Prerequisites for the successful application of SFC to oligomers, such as gradients and the use of suitable mobile and stationary phases, are treated in general terms. The main section covering the separations is largely concerned with oligomers, because the application of SFC to polymers is so far lagging behind. The separations are divided in subsections on hydrocarbon oligomers, vinyl arene oligomers, acrylic acid and acrylate oligomers, oligoethers, condensation type oligomers, miscellaneous oligomers, polymers, and biopolymers. Finally, some problems to which SFC may contribute solutions and problems in the detection of macromolecular substances are discussed.

Chromatography with sub- and supercritical eluents: the influence of temperature, pressure and flow-rate on the behaviour of dimethyl and diethyl ether

Abstract In supercritical fluid chromatography, using dimethyl ether and diethyl ether as eluents, distinct maxima occur in capacity ratios and resolutions at temperatures somewhat above critical when separating polynuclear aromatic hydrocarbons, oligostyrenes or di-alkyl phthalates. They are caused by a decrease in eluent density and the opposing effect of an increase in the substrate vapour pressure. Dimethyl ether allows higher resolutions than diethyl ether. Higher pressures and flow-rates lead to faster analyses but poorer resolution.

Separation of oligomers with UV-absorbing side groups by supercritical fluid chromatography using eluent gradients

Abstract The chromatographic separation of oligomers prepared from vinyl arene compounds was achieved using eluent gradients with the eluent pair n -pentane-l,4-dioxane, which had previously been applied to oligostyrene separations. Separations were found to occur not only with respect to degree of oligomerization, but also between sub-series of oligomeric species. Transition from the supercritical to the liquid state, which may occur during an eluent gradient run if the selected column temperature is relatively little above T c at the beginning of the gradient, does not seem to reduce the resolution considerably. The combination of two gradient techniques, a pressure gradient and a gradient of eluent composition, was applied for the separation of the difficult to elute N-vinylcarbazole oligomers. Supercritical fluid chromatograms showed considerably superior separation compared with high-performance liquid chromatograms obtained at ambient temperature using the same eluent gradient.

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.

Gradient elution in supercritical fluid chromatography

Abstract The applicability to supercritical fluid chromatography (SFC) of elution by gradients in eluent composition is demonstrated, using alkanes and diethyl ether as the primary eluent and alcohols, cyclohexane and dioxane as the secondary component. For the separation of styrene oligomers, the combination of an alkane with dioxane turned out to be the most efficient. The SFC apparatus used was a modified high-performance liquid chromatographic instrument.

Gradient methods in supercritical fluid chromatography

Abstract Single and multiple gradient methods that are employed in supercritical fluid chromatography have been reviewed with a special focus on gradients involving temperature, pressure, density, composition, and velocity.

Effects of temperature, pressure, and density on the chromatographic behavior of supercritical carbon dioxide

Abstract The effects of the physical factors (temperature, pressure, density, and free volume) on the chromatographic parameters (capacity ratio, selectivity, and resolution) are investigated for supercritical fluid chromatography, using carbon dioxide as the mobile phase, unbonded silica gel as the stationary phase in a packed column, and a mixture of polycyclic aromatic hydrocarbons as the analyte. The results are presented as three-dimensional graphs where color is used to exhibit a fourth variable. Two of the axes in the plots represented the physical parameters whereas the third axis and the color represent chromatographic parameters. By selecting temperature as one of the variables and pressure or density or free volume as the other (which together fully characterize the state of the mobile phase), the dependence of two chromatographic parameters on these variables can be shown and the interrelation between these chromatographic parameters can be illustrated. The interrelations between the chromatographic parameters and the dependence on the physical parameters being more varies in supercritical fluid chromatography then in gas and liquid chromatography, such multidimensional graph are particular value in providing an overview.

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.