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

A pressure-tunable ensemble of two series-coupled capillary columns operate d 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 len gth of polar trifluoropropylmethyl polysiloxane column. Air at an inlet pre ssure 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 ata pressure of 0.3 psi a, Ensemble selectivity is controlled by means of an electronic pressure co ntroller located at the junction paint between the columns. The minimum pre ssure step size is 0.1 psi, and 50 different set-point pressures can be use d, 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 hol dup time of the first column in the ensemble to the total ensemble holdup t ime. A component band trajectory model is used to describe the effects of e nsemble junction-point pressure on the elution patterns generated by the en semble. Ensemble retention times predicted by the model are in good agreeme nt with values obtained from chromatograms. The use of on the-fly set-point pressure changes during a separation (selectivity programming) is demonstr ated and used to improve the quality of the separation of a 19-component te st mixture.