Elsevier

Journal of Chromatography A

Volume 629, Issue 2, 22 January 1993, Pages 345-359
Journal of Chromatography A

Influence of linear velocity and multigradient programming in supercritical fluid chromatography

https://doi.org/10.1016/0021-9673(93)87049-RGet rights and content

Abstract

The chromatographic plate height, resolution and time requirements in supercritical fluid chromatography depend on the physical properties of the mobile phase (pressure, mobile phase composition, temperature and linear velocity), on the chemical structure of the analyte, which determines vapour pressure and solubility, and on various properties of the stationary phase. If a given homologous series of compounds, or a test mixture simulating a homologous series, is used with a specific chromatographic column, the chromatographic properties for the compounds in these test mixtures will vary with pressure, mobile phase composition, temperature, velocity and the relative molecular mass of the individual compounds. If in addition to a suitable column a temperature is chosen which, for simplicity, is kept constant, and which is known to lead to good or even optimum resolution, the pressure, composition and relative molecular mass dependence of the Van Deemter plate height minimum, as determined experimentally at this temperature, can be used for predicting an optimum linear velocity programme. This velocity programme can be used either as a stand-alone programme or can be adapted to and superimposed on pressure or composition programmes, or even on combined pressure—composition programmes. The linear velocity dependence of plate height and resolution for a mobile phase composed of a mixture of carbon dioxide and methanol on a column packed with unbonded silica gel is presented. This dependence was measured at different pressures and compositions, employing four condensed aromatics as a test analyte. Each chromatogram of the analyte was therefore measured at constant temperature, velocity, pressure and composition, varying the last three physical properties between chromatograms. The data are presented as Van Deemter plots and as three-dimensional plots showing the dependence of resolution and capacity factor on velocity and either pressure or composition. Based on these data, the change in the linear velocity suitable for pressure programmes, mobile phase composition programmes and for increase in the relative molecular mass of the analyte is discussed. The conclusion is that a pressure (density) programme needs a superimposed negative linear velocity programme for the purpose of decreasing the plate height and increasing the resolution, whereas such a programme is not necessary to a comparable extent for composition programming. If compounds with a wide range of relative molecular masses are separated, the superimposing of a negative linear velocity programme on to a composition programme is also advantageous. For programming the physical properties of a mobile phase, i.e., pressure, density, composition, temperature and velocity, a number of closely related equations are proposed and some corresponding programme curves are shown. Hardware needs for programming are also briefly discussed.

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