Vector modeling and tunable selectivity strategies for high-speed gas chromatography.

Abstract

High speed gas chromatograms are obtained using short capillary columns with high carrier gas velocities. This reduces resolution and peak capacity, making separations of complex mixtures difficult. A solution is the use of a polar and nonpolar column in series. Adjusting the pressure at the junction tunes the selectivity within limits imposed by the columns. Using two 5.0m long, 0.25mm i.d. columns, pressure steps of 0.1 psi result in 0.007 changes in the fractional contributions from each column. Using electronic pressure control, up to 100 unique retention patterns are obtained reproducibly. Selectivity programming, where tuning pressure is adjusted during analysis, combined with temperature programming separates 30 compounds with a large boiling point range under 2.5 minutes. A model of multiphase separations is developed and used to optimize isothermal and isobaric tandem column separations. Each mixture component occupies a point in two dimensions, where orthogonal axes represent retention on individual columns. A phase fraction axis is defined from the origin of the retention plane. The angle this axis makes with the orthogonal axes is defined by the fractional contributions to the total holdup time from each column. Coordinates along this axis define retention using column combinations. Mixture component points are connected pairwise by separation vectors. Locator vectors connect the origin to the separation vector centers. Relative resolution is the ratio of projections of the separation and locator vectors onto the phase fraction axis. Minimum values of relative resolution considering all nonredundant pairs are plotted versus phase fraction axis rotation. This window diagram identifies the optimum column combination. A model of column efficiency is developed and validated for high speed gas chromatography using ambient pressure air as carrier gas, driven by vacuum outlet. The system is constrained for a 30 second analysis of analytes with retention factors from 0-5. A range of column diameters is considered. The best results are obtained with a 0.1 mm i.d. column, which is predicted to produce up to 25,000 plates under the given constraints.