Pressure-tunable column selectivity for high-speed vacuum-outlet GC

A pressure-tunable ensemble of two series-coupled capillary columns operated at subambient outlet pressure is described. The ensemble consists of a 4.5-m length of nonpolar dimethyl polysiloxane column followed by a 7.5-m length of polar trifluoropropylmethyl polysiloxane column. Air at an inlet pressure of 1.0 atm is used as carrier gas, and a vacuum pump is used to pull the carrier gas and injected samples through the column ensemble. Detection is provided by a photoionization detector operated at a pressure of 0.3 psia. Ensemble selectivity is controlled by means of an electronic pressure controller located at the junction point between the columns. The minimum pressure step size is 0.1 psi, and 50 different set-point pressures can be used, each one producing a different pattern of peaks eluting from the column ensemble. Measured ensemble retention factors for a set of target compounds produce straight lines when plotted versus the ratio of the calculated holdup time of the first column in the ensemble to the total ensemble holdup time. A component band trajectory model is used to describe the effects of ensemble junction-point pressure on the elution patterns generated by the ensemble. Ensemble retention times predicted by the model are in good agreement with values obtained from chromatograms. The use of on-the-fly set-point pressure changes during a separation (selectivity programming) is demonstrated and used to improve the quality of the separation of a 19-component test mixture.